Intermittent Fasting

Diet Reference Archive: an eating pattern defined by timing

Intermittent Fasting

A family of eating patterns that set when a person eats, or on which days they eat very little, rather than which foods they eat. Common versions are time-restricted eating, such as 16:8, the 5:2 pattern and alternate-day fasting, and they are promoted mainly for weight loss and blood sugar control.

Family: Fasting and meal timing Promoted by: diet book authors, app makers and research groups First appeared: as a popular diet, the 2010s CINeMA, a GRADE-based method: Moderate for measured body weight changing about as much with time-restricted eating, such as 16:8, and with whole-day fasting, a category including 5:2 and other weekly schedules, as with daily calorie restriction, in adults mostly with overweight or obesity in trials of 3 to 52 weeks; published review (Semnani-Azad et al., 2025)

ON THIS PAGE

1. In brief
2. Overview
3. In practice
4. For the coach
5. The evidence (opens on request)
6. Mechanisms (opens on request)
7. Risks, contraindications and myths (opens on request)
8. Key terms
9. Review questions
10. References
11. Evidence rating and review record

The summary and sections 1 to 3 are the short version and are open when the page loads. Sections 4 to 6 hold the research detail, the biology, and the cautions. Each of those opens when you select its heading, and you can read the page without opening them. Review questions 2 and 4 draw on sections 4 and 6. Back to the Diet Reference Archive index.

In brief

What is it? A name for several eating patterns built around time rather than food. In 16:8 eating, a person eats within the same 8 hours each day and does not eat for the other 16. In the 5:2 pattern, the best-known form of whole-day fasting, a person eats very little, about 500 to 600 calories, on 2 days a week and eats as usual on the other 5. In alternate-day fasting, low-calorie days and ordinary days take turns.

Does it work? In randomized trials, mostly in adults with overweight or obesity and mostly lasting weeks to a year, people following these patterns lost about as much weight as people asked to eat a little less every day. A large 2025 review rated that finding moderate in certainty for time-restricted eating, such as 16:8, and for whole-day fasting, a category that includes 5:2 and other weekly schedules, not 5:2 alone. Alternate-day fasting produced slightly more weight loss than daily calorie cutting, about 1.3 kilograms more, below what the review counted as important. For long-term blood sugar, the review did not detect a difference from daily calorie cutting for any of the 3. In most trials read for this page, the benefits tracked eating less rather than the clock; whether timing adds anything is not settled. No trial identified for this page measured heart attacks, strokes or length of life.

Is it safe? For most healthy adults in the trials, side effects were mild: hunger, headaches, tiredness, dizziness and constipation. The main cautions are for people taking insulin or sulfonylurea medicines for diabetes, whose blood sugar can drop too low while fasting, and for people with a history of disordered eating, for whom fixed rules about when to eat can become harmful.

Who should not do it without talking to a doctor or dietitian first? Anyone taking medicine for diabetes, especially insulin or a sulfonylurea; anyone who is pregnant, breastfeeding or trying to become pregnant; anyone with a history of an eating disorder; children and teenagers; older adults who are frail or losing muscle; anyone with chronic kidney disease; and anyone taking medicine for blood pressure or a blood thinner who plans a large change in eating.

Related pages: Mediterranean Diet. This archive also lists pages on time-restricted eating, alternate-day fasting, prolonged fasting, the fasting-mimicking diet and eating one meal a day. Full list on the archive index.

1. Overview

What you do

Intermittent fasting is not one diet. It is a family of eating patterns that share one idea: periods of eating alternate with periods of eating nothing, or very little, on a regular schedule. A 2025 review of the trials sorted the patterns into 3 groups, and this page uses the same 3 (Semnani-Azad et al., 2025).

• Time-restricted eating, often called 16:8. All eating happens within a set window each day, commonly 8 hours, such as noon to 8 p.m., and the person fasts for the rest of the 24 hours. Researchers describe windows of 6 to 10 hours a day, and many of the trials set no limit on calories or food choices inside the window (Lin et al., 2023; Pavlou et al., 2023).
• Whole-day fasting, best known as the 5:2 pattern and also called intermittent energy restriction. In the 5:2 pattern, on 2 days a week, back to back or not, the person eats about 500 to 600 calories; on the other 5 days they eat as usual (American Diabetes Association Professional Practice Committee for Diabetes, 2026; Carter et al., 2018). The 2025 review's whole-day fasting category also includes trials with 1 or 3 fast days a week, so its results describe the whole category, not 5:2 alone (Semnani-Azad et al., 2025).
• Alternate-day fasting. Fast days and ordinary days take turns. In many research versions, called modified alternate-day fasting, the fast day allows about a quarter of usual energy needs, often as one small meal, and the following day allows more than usual (Trepanowski et al., 2017).

During the fasting period, the time-restricted trials read for this page allowed water and drinks without calories, such as black coffee and plain tea (Lin et al., 2023; Lowe et al., 2020). A 2026 diabetes guideline notes that the principles of healthy eating still apply during the eating times; the pattern sets the clock or the calendar, and the food choices are a separate matter (American Diabetes Association Professional Practice Committee for Diabetes, 2026).

What you do not do

The rule is about timing. In time-restricted eating, a person does not eat or drink anything with calories outside the window. On a 5:2 fast day or an alternate-day fast day, a person keeps to the low-calorie limit. Whether the limit is a hard rule depends on the plan; in the trials, the fast-day limit was set in calories or as a share of the person's usual needs (Carter et al., 2018; Trepanowski et al., 2017). Most versions name no foods to avoid. Nothing in the pattern itself requires cutting out a food group.

Where it came from

Going without food for set periods is old. A 2026 diabetes guideline notes that religious fasting has been practiced for thousands of years and is part of many faith traditions (American Diabetes Association Professional Practice Committee for Diabetes, 2026). That kind of fasting is a practice in its own right, not a diet, and section 3 describes how a coach meets it as part of a client's life.

Intermittent fasting as a weight-loss diet is much newer. Researchers tested intermittent and continuous calorie restriction against each other in women with overweight in the United Kingdom in a trial published in 2011 (Harvie et al., 2011). A 2017 scientific statement from the American Heart Association described intermittent fasting diets as having grown in popularity over the previous decade (St-Onge et al., 2017). The authors of a 2017 trial named best-selling diet books from 2012 and 2013 that promoted fasting on some days of the week, including The Fast Diet, and stated that more than 1 million copies of such books had sold in the United States and the United Kingdom (Trepanowski et al., 2017).

The research case for the newer patterns grew out of animal studies. Experiments in rodents found that restricting the hours of feeding prevented weight gain on a high-fat diet, and that fasting changed how the body uses fat and sugar; the authors of a 2025 review note that these metabolic changes have not been confirmed in people (Lowe et al., 2020; Semnani-Azad et al., 2025). Researchers who study time-restricted eating credit its rise in popularity to its simplicity, because it does not require counting calories (Lin et al., 2023).

What it claims to do

The central claim is weight loss without the daily effort of counting calories. Researchers who study time-restricted eating describe its appeal this way: it lets people "watch the clock" instead of monitoring calories (Lin et al., 2023). Promoters and some researchers go further and propose that fasting improves blood sugar, insulin sensitivity, blood pressure and cholesterol beyond what weight loss alone would do, and that it switches the body into a fat-burning state (Semnani-Azad et al., 2025; Sutton et al., 2018).

One review describes this as a metabolic switch, typically beyond 12 hours without food, and proposes that it may improve body composition and slow aging and disease processes (Anton et al., 2018). Clients also bring claims about mental clarity, energy, longevity and cell repair; that list is offered from observation, not from a study. Section 4 reports what the trials show for each kind of claim, and section 5 separates mechanisms shown in people from those proposed.

What it gets right

Two things hold up. The first is that a simple structure can help some people eat less. In trials where people ate freely within an 8-hour window and counted nothing, they ate about 300 to 425 fewer calories a day, and the trials did not detect a difference in weight change from people who counted calories (Lin et al., 2023; Pavlou et al., 2023). Some participants find an eating window easier to follow, and in one trial some reported finding it easier than calorie counting (Pavlou et al., 2023). Ease is one consideration alongside the person's needs and clinical circumstances.

The second is that the pattern is flexible. A 2026 diabetes guideline notes that time-restricted eating can be adapted to any eating pattern (American Diabetes Association Professional Practice Committee for Diabetes, 2026). A person can follow 16:8 with Mediterranean-style meals, with the foods of their own culture, or with whatever they already eat. That flexibility also means the pattern says nothing about food quality, which is where the American Heart Association's 2026 guidance on vegetables, fruits, whole grains, healthy proteins, unsaturated fats and less added sugar and sodium still applies (Lichtenstein et al., 2026).

How clients describe it, and what it gets confused with

Clients say "I do 16:8," "I only eat between noon and 8," "I skip breakfast now," "I'm doing 5:2," "Mondays and Thursdays are my fast days," "I eat one meal a day," or "I'm trying to stay in a fasted state." These phrases are offered from observation, not from a study. A client who says "I skip breakfast" may be following a time-restricted plan or may simply be busy; the coach asks before assuming.

Intermittent fasting is often confused with 4 other things. Religious fasting, such as fasting from dawn to sunset during Ramadan, is a practice with its own meaning and rules, and the research on it is separate (American Diabetes Association Professional Practice Committee for Diabetes, 2026). Prolonged or water-only fasting for more than a day, the fasting-mimicking diet sold as a product, and eating one meal a day are each listed as separate pages on the archive index. And writing about fasting often discusses ketones (Anton et al., 2018), which can lead a client to mix it up with the ketogenic diet; the ketogenic diet changes what a person eats, while intermittent fasting changes when. The Mediterranean Diet page describes one food pattern a person can follow within an eating window.

2. In practice

A typical day of eating

A person following 16:8 with an eating window from noon to 8 p.m. might start the morning with water and black coffee or plain tea. Lunch at noon is a bowl of bean and vegetable soup with whole-grain bread and a piece of fruit. An afternoon snack is yogurt with a handful of nuts. Dinner around 7 p.m. is baked fish, chicken or tofu with roasted vegetables and brown rice, and the kitchen closes at 8. Nothing about the food is special; what defines the day is the window. The same window could hold fast food and soda, which is why the food choices inside it matter separately.

A person following the 5:2 pattern eats ordinary meals on 5 days. On each of the 2 fast days, they might have a boiled egg and a piece of fruit in the morning and a large bowl of vegetable soup with some lean protein in the evening, staying within roughly 500 to 600 calories, and drink water, tea or coffee through the day. A person on alternate-day fasting eats a similar small meal on every other day. These sample days are illustrations, not plans.

Cost, products, and who profits

The pattern itself is free. The authors of a 2023 trial point out that time-restricted eating does not require buying special food products, lets people keep eating familiar foods, and leaves out calorie tracking (Lin et al., 2023). A client can follow any of the 3 patterns with the groceries they already buy, and eating fewer meals or snacks may lower a food bill, although the searches for this page found no study that measured cost.

Money enters through books, apps and products. The authors of a 2017 trial note best-selling fasting diet books from 2012 and 2013, with more than 1 million copies sold, and the trial's senior researcher disclosed an advance for her own book on alternate-day fasting (Trepanowski et al., 2017).

The authors of one app study write that millions of people use fasting apps; 2 free tracking apps from that company recorded 2.5 million downloads from 2018 to 2020, and the study describing their users was written mostly by that company's employees (Torres et al., 2022). A review of 35 fasting apps in Chinese app stores found that nearly all offered a fasting timer and weight tracking, and the reviewers noted limited clinical input (Fang et al., 2025). Some trials supplied products, such as a daily protein shake in one 12-month trial (Liu et al., 2022); a client does not need them to follow the pattern.

For someone with limited money, the pattern adds no cost. For someone with limited time, a shorter eating window can mean fewer meals to prepare. For someone who works night shifts or irregular hours, the trials offer little guidance: 2 of the time-restricted trials this page relies on excluded night-shift workers (Lin et al., 2023; Pavlou et al., 2023).

Environmental footprint

The searches for this page found no study estimating the environmental footprint of intermittent fasting itself. Because the pattern sets timing rather than food choices, the footprint depends mostly on which foods a person eats within it.

Cultural fit

Timing is cultural. Many households and communities build their days around shared meals, and an eating window that closes before the family dinner, or opens after the work lunch, can mean eating apart from other people. The authors of a 12-month trial in China chose an 8 a.m. to 4 p.m. window because, they write, many people in China eat their largest meal in the middle of the day (Liu et al., 2022). The authors of a trial in Chicago chose noon to 8 p.m. and note that American adults who practice time-restricted eating tend to place their window later in the day so that they can keep eating dinner with family and friends (Lin et al., 2023). A window that suits one household may not suit another, and the pattern can be shifted to fit.

Fasting also has meaning in many faith traditions, including Ramadan and Yom Kippur (American Diabetes Association Professional Practice Committee for Diabetes, 2026). A client who fasts for religious reasons is following a practice, not a diet, and the coach meets it with respect and curiosity rather than evaluation. Section 3 describes the health questions that can arise for people with diabetes who fast for religious reasons, which belong with their clinicians.

What makes people stop

The trial records show several patterns. In a 1-year trial of alternate-day fasting, 38 percent of the fasting group dropped out, compared with 29 percent of the daily calorie restriction group and 26 percent of the control group, and more people left the fasting group than the daily restriction group because they found the diet hard to follow. People in that group also ate more than planned on fast days and less than planned on the other days, turning the pattern into something close to daily calorie restriction (Trepanowski et al., 2017).

In a 6-month trial of the 5:2 pattern in women, 21 percent of the fasting group withdrew, compared with 13 percent of the daily restriction group; 15 percent of the fasting group reported hunger, and some reported lack of energy, headaches, feeling cold, lack of concentration, bad temper or preoccupation with food (Harvie et al., 2011). In a 12-month trial of the 5:2 pattern in adults with type 2 diabetes, compliance was above 90 percent in both groups over the first 3 months and fell to about 44 to 49 percent after that (Carter et al., 2018). In a 12-month trial of a 9-hour eating window, more people left because of the intervention in the time-restricted group than in the dietitian-advice group between months 4 and 12, 13 against 7 (Parr et al., 2026).

Outside trials, the numbers are lower still. Among nearly 800,000 users of 2 free fasting apps, 16.7 percent were still recording fasts at 13 weeks, 6.9 percent at 26 weeks and 2.1 percent at 52 weeks (Torres et al., 2022). App records are not the same as a person's eating, but they suggest that many people try the pattern and do not keep it. Not every result points the same way: in 2 time-restricted trials, people reported keeping to their eating window on about 6 days a week, and some found it easier than counting calories (Lin et al., 2023; Pavlou et al., 2023).

What happens when you stop

Few studies followed people after a fasting program ended. In the 5:2 trial in adults with type 2 diabetes, participants were seen again 12 months after the 12-month program finished: both groups kept off about 3.9 kilograms on average, but long-term blood sugar, measured as HbA1c, had risen back above where it started in both groups (Carter et al., 2019). In a trial in which adults followed an 8-hour eating window for 12 weeks, the early-window and late-window groups had kept more of their weight loss than the usual-care group 12 months later, and about a quarter reported practicing time-restricted eating during that year (Camacho-Cardenosa et al., 2026).

Within longer trials, effects that appeared early often faded. In a trial that added 20-hour fasts on 3 days a week, an early advantage in blood sugar response over daily calorie restriction at 6 months was no longer present at 18 months, after support from the researchers had ended; at 18 months, 42 percent of the fasting group who completed the trial reported still fasting 2 or 3 days a week (Teong et al., 2023). In the alternate-day fasting trial, weight regain in the second 6 months was not significantly different between the fasting group and the daily calorie restriction group (Trepanowski et al., 2017). A 2025 review found only 5 trials that lasted a year or more, and in trials of at least 24 weeks, none of the fasting patterns differed from daily calorie restriction in weight change (Semnani-Azad et al., 2025).

Common variations

Within time-restricted eating, the window can be early in the day, such as 8 a.m. to 4 p.m. (Liu et al., 2022), late, such as noon to 8 p.m. (Lin et al., 2023), or chosen by the person, such as a 9-hour window ending by 7 p.m. (Parr et al., 2026). Some programs combine an eating window with a calorie target (Liu et al., 2022), and others combine fasting days with an early window (Teong et al., 2023). A version with only 6 hours of eating, ending in the afternoon, has been tested in a small, closely controlled study (Sutton et al., 2018).

Whole-day fasting does not always mean 5:2. Some versions use 1 fast day a week, and the 2 fast days of a 5:2 plan can be back to back or apart (American Diabetes Association Professional Practice Committee for Diabetes, 2026; St-Onge et al., 2017). The trials the 2025 review grouped as whole-day fasting used 1, 2 or 3 fast days a week, some allowing only water on fast days (Semnani-Azad et al., 2025). A 12-month trial published after that review tested a 4:3 pattern: about a fifth of usual energy needs on 3 nonconsecutive days a week and no limit on the other 4 (Catenacci et al., 2025). Alternate-day fasting in the trials read for this page allowed a small amount of food on fast days, about a quarter of usual needs (St-Onge et al., 2017; Trepanowski et al., 2017).

Clients may also mention eating one meal a day, water-only fasting for more than a day, or the fasting-mimicking diet, a commercial product; each is listed as a separate page on the archive index. Separate pages on time-restricted eating and alternate-day fasting are also listed there.

3. For the coach

SCOPE OF PRACTICE

A health and wellness coach does not prescribe, adjust, endorse or discourage intermittent fasting or any other way of eating, and does not diagnose or treat. The coach does not set an eating window, a fast-day calorie limit or a schedule for a client. The coach helps the client understand what they have chosen, notice how it is going, and decide what to do next.

Four points belong to someone else. A client who takes insulin, a sulfonylurea or another medicine that lowers blood sugar talks with the prescriber before starting, because fasting while taking these medicines can bring blood sugar too low and the doses may need to change. A client who is pregnant, breastfeeding or trying to become pregnant plans any change in eating with the obstetric team. A client with a history of an eating disorder makes any plan with their clinician under their existing care plan. And a client with chronic kidney disease involves the kidney care team, whose advice depends on the stage of the disease, lab results and treatment.

Who does what

The physician

Decides whether fasting is medically safe for this person and adjusts any medication it affects. For diabetes, that means deciding whether doses or timing of insulin, sulfonylureas or other glucose-lowering medicines should change on fasting days and how blood sugar will be monitored; a 2026 diabetes guideline states that people taking insulin or medicines that make the pancreas release insulin should be medically monitored during fasting (American Diabetes Association Professional Practice Committee for Diabetes, 2026). Orders and reads the tests that matter, such as HbA1c and kidney function. Evaluates symptoms such as dizziness, fainting or weight loss the person did not intend.

The registered dietitian

Designs and monitors an individual eating plan that fits the window or the fast days: enough protein and fiber across fewer meals, fast-day menus, and adaptations for diabetes, kidney disease, pregnancy, older age or heavy training. For a person with diabetes who fasts for religious reasons, the 2026 diabetes guideline calls for individual advice on fluids and meals (American Diabetes Association Professional Practice Committee for Diabetes, 2026). Screens for a history of dieting and disordered eating, which the same guideline recommends during nutrition care for people with diabetes.

The health and wellness coach

Helps the client see their own reasons for choosing a fasting pattern and test it against the life they lead: work hours, family meals, sleep, exercise, social plans and how hunger affects their mood. Helps the client design and adjust a routine they own, notice what the experience is like from week to week, and keep their prescriber informed. With the client's permission, the coach can share general information from recognized authorities, such as what a diabetes guideline says about fasting with diabetes medicines, and explore how the client wants to use it. The coach notices whether the pattern is widening or narrowing the client's life and holds the referral line. In one 12-month trial, trained health coaches delivered the dietary counseling under the research protocol (Liu et al., 2022); in this program, setting calorie targets and eating windows is not the coach's role.

What a client bringing this diet needs

Clients arrive with intermittent fasting for different reasons: a friend lost weight with it, a video promised it would reset their metabolism, they are tired of counting calories, they already skip breakfast and want a name for it, or a clinician mentioned it. Some arrive frustrated after several diets. A client who has diabetes may want something that helps their blood sugar; a client who works night shifts may want a structure that fits irregular days. The reason shapes the conversation.

What the coaching conversation offers that a search engine cannot is a careful look at fit. A fasting pattern asks a person to be hungry at predictable times and to arrange eating around a clock, which can feel freeing to one person and punishing to another. With permission, the coach can share what the research shows, that most trials found results similar to eating a little less every day, and then hand the decision back. The useful questions are about this person's days: when they are hungriest, who they eat with, what happens on hard days, and what they will do if the plan is not working.

What to ask

• "What drew you to intermittent fasting, and what are you hoping will be different?"
• "Walk me through a day on your plan. Where does it fit easily, and where does it pinch?"
• "Who else needs to know about this, like the people you eat with or the doctor who prescribes your medicines?"

Referral red flags

A client who takes insulin, a sulfonylurea or another blood sugar lowering medicine and is starting or has started fasting without telling the prescriber needs to talk with the prescriber before going further. In a trial of the 5:2 pattern in type 2 diabetes, 35 percent of participants who took sulfonylureas or insulin had episodes of high or low blood sugar in the first 2 weeks, even with a medication protocol supervised by an endocrinologist, and the protocol was tightened partway through because of low blood sugar (Carter et al., 2018). The federal diabetes institute lists fasting while continuing glucose-lowering medicines as a cause of low blood sugar (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.).

If a client describes what sounds like a low blood sugar episode, such as feeling shaky, dizzy or confused during a fast, the first step is the client's own diabetes treatment plan; severe symptoms, or an episode the person cannot manage, call for emergency help. Telling the prescriber is the separate step that follows. The coach does not diagnose the cause.

A client who is pregnant, breastfeeding or trying to become pregnant, a teenager, or a client with chronic kidney disease, is a referral before any fasting plan. So is a client who reports fainting, persistent dizziness, new heart palpitations, or weight loss they did not intend; each is a reason to see a clinician, who works out the cause.

A client whose fasting comes with bingeing when the window opens, distress about breaking the rules, a list of rules that keeps growing, or withdrawal from meals with other people is worth asking about directly: what the fasting is for, whether it causes distress, whether it is narrowing what they eat, and whether it gets in the way of daily life. There are ordinary reasons to skip a meal. Where the answers raise a real concern about disordered eating, the coach follows their own referral process, or involves the client's clinician under an existing care plan.

Special populations

Pregnancy and breastfeeding

Several large trials and the 2025 review of intermittent fasting excluded pregnancy (Carter et al., 2018; Lin et al., 2023; Liu et al., 2022; Trepanowski et al., 2017; Semnani-Azad et al., 2025). Two randomized reports now add evidence specific to eating windows. In a 5-week feasibility trial, 32 pregnant participants at increased risk of gestational diabetes were randomized to a maximum 10-hour eating window or usual care; 30 completed it. The window shortened, but the trial did not detect improvements in glucose measures or other cardiometabolic outcomes (Skarstad et al., 2024). A second trial randomized 167 people contemplating pregnancy to exercise plus time-restricted eating or standard care; 111 became pregnant. The combined intervention did not clearly improve the late-pregnancy glucose outcome and cannot isolate the effect of an eating window (Sujan et al., 2025). These studies do not establish safety across pregnancy or breastfeeding. Ramadan studies concern a different exposure: the 2024 review reported uncertain observational associations, often without knowing whether the mother actually fasted (Pradella et al., 2024). For pregnant people with diabetes, the 2026 diabetes guideline places pregnancy in a high-risk religious-fasting category (American Diabetes Association Professional Practice Committee for Diabetes, 2026). Changes in eating during pregnancy and breastfeeding are planned with the obstetric team.

Children and adolescents

Most trials in young people found for this page were small and short. In adolescents with obesity, an 8-hour eating window over 12 weeks did not change energy intake or diet quality compared with a control group (Bakhsh et al., 2024). In 27 adolescents and young adults with newly diagnosed type 2 diabetes taking metformin, an 8-hour window was feasible and no adverse events were reported, with no clear difference from the control group after 12 weeks (Hegedus et al., 2024). In a Canadian survey of 2,762 adolescents and young adults, between about 38 and 52 percent, depending on gender, reported intermittent fasting in the previous year, and fasting was associated with more eating disorder symptoms (Ganson et al., 2022). A young person's eating belongs to the family and the pediatrician, and a coach works with the adult in front of them.

One longer trial, Fast Track to Health, randomized 141 adolescents aged 13 to 17 with obesity and related health problems at 2 children's hospitals in Australia, after screening for eating disorders and depression. Everyone began with 4 weeks of a very low energy diet using meal replacements, then followed either intermittent energy restriction, about 600 to 700 calories on 3 days a week, or daily energy restriction, within an intensive program run by dietitians, pediatricians and psychologists. After 52 weeks, the trial did not detect a difference between the 2 groups in body composition or cardiometabolic health. Eight serious adverse events occurred in 6 participants; 2 were judged possibly related to the program, 1 of them a case of atypical anorexia nervosa in the intermittent group (Lister et al., 2024). This was a closely supervised clinical program, not daily 16:8, and it is not evidence that young people can fast safely on their own.

Older adults

The evidence in older adults is limited and short. In a 9-month pilot trial in 90 adults aged 60 and older, an 8-hour eating window was feasible, and average weight change was about 1 kilogram, smaller than in the calorie restriction groups, which lost both fat and lean tissue (Houston et al., 2026). A 2026 review of 20 trials in adults aged 45 and older found small differences in lean mass over mostly 12 weeks or less and concluded that the evidence does not allow firm conclusions about muscle in older people over the long term (Santero et al., 2026). Losing muscle matters more with age. An older adult considering fasting, especially one who is frail, has a small appetite or takes several medicines, needs the physician and a dietitian involved.

Athletes and people with high activity

A 2025 review of 35 trials that combined exercise with intermittent fasting or calorie restriction found no significant effect on most measures of physical performance compared with exercise alone, while weight, fat and also fat-free mass fell (Kazeminasab et al., 2025). In 40 resistance-trained women eating the same calories and protein, an 8-hour window did not reduce gains in lean mass or strength over 8 weeks (Tinsley et al., 2019). An earlier review found aerobic capacity lower during Ramadan fasting (Correia et al., 2020), when people also go without water from dawn to dusk (American Diabetes Association Professional Practice Committee for Diabetes, 2026). Energy and protein needs at high activity are individual and belong with a sports dietitian.

People taking medication for diabetes, blood pressure, or blood thinning

Diabetes medicines matter most. Insulin and sulfonylureas carry the highest risk of low blood sugar during fasting, and the 2026 diabetes guideline describes dose and timing changes that clinicians may make for fasting (American Diabetes Association Professional Practice Committee for Diabetes, 2026). In trials, these changes were made under medical supervision with a written protocol (Carter et al., 2018; Pavlou et al., 2023). For a client taking insulin, a sulfonylurea, blood pressure medicine or warfarin, a substantial change in eating is something the client discusses with the prescriber before making it. The searches did not identify a study specifically testing these nonreligious schedules in warfarin users; studies during Ramadan address a different fasting context (Lai et al., 2014). A small observational study of 58 adults whose high blood pressure was controlled with medicine found no change in blood pressure during Ramadan fasting, while blood glucose was higher (Hammoud et al., 2021); it does not show how other fasting patterns interact with blood pressure medicine. The rule to keep the prescriber informed applies either way.

People with a history of disordered eating

Trials often screen out people with a history of eating disorders (for example, Pavlou et al., 2023), so the trials say little about them. In community samples, intermittent fasting has been associated with more eating disorder symptoms: in Canadian adolescents and young adults (Ganson et al., 2022), in 64 adults who fasted about 16 hours a day, nearly a third of whom scored at or above a clinical cutoff (Cuccolo et al., 2022), and in Chinese adults followed for 8 months (He et al., 2025). These studies cannot show whether fasting causes symptoms or whether people with symptoms are drawn to fasting. A diabetes guideline notes that rigid meal plans can be contraindicated for people at risk of disordered eating (American Diabetes Association Professional Practice Committee for Diabetes, 2026). For a client with this history, any plan is made with their clinician.

People who fast for religious reasons

Fasting during Ramadan, on Yom Kippur and in other traditions is part of a client's faith and life, and the coach approaches it with respect, without evaluating it as a diet. The health question that can arise is practical: for a client with diabetes, the 2026 diabetes guideline recommends a risk assessment and a review of medicines well before the fast, with education to reduce the risk of low or high blood sugar and dehydration (American Diabetes Association Professional Practice Committee for Diabetes, 2026). The coach can encourage that conversation with the clinician. The searches for this page found a small study in which eating disorder symptoms fell during Ramadan among 56 Muslim women in the United States (Ain & Christensen Pacella, 2026); religious fasting and fasting for weight are different experiences.

Client misconceptions

Three come up often. The first is that fasting burns fat in a way ordinary dieting does not. In trials, weight loss on fasting patterns was similar to daily calorie restriction, and in a 12-month trial the authors concluded that eating fewer calories explained most of the benefit (Liu et al., 2022; Semnani-Azad et al., 2025). The second is that anything goes inside the window. People who lost weight with an eating window did so mainly by eating less overall, and in a 12-week trial comparing noon-to-8-p.m. eating instructions with instructions for structured mealtimes, weight loss was not significantly different between the groups (Lowe et al., 2020).

The third is that fasting is proven to slow aging or extend life. That idea comes mostly from animal research and proposed mechanisms, and no trial identified for this page measured length of life or disease outcomes in people. A related worry, that 16:8 eating raises the risk of dying from heart disease, comes from an observational study of usual eating habits whose authors note that other factors may explain the finding (Chen et al., 2025). Section 6 gives each myth with its origin.

🤝 COACHING CONNECTION

Denise, 54, tells her coach she is starting 16:8 on Monday because her sister lost weight with it. She has type 2 diabetes and takes metformin and glipizide. The coach asks what she is hoping for. Denise says she wants fewer evening snacks and a lower A1C at her next checkup. The coach asks what her doctor knows about the plan. She says she has not mentioned it, because it is only skipping breakfast.

The coach asks whether she would like to hear what a diabetes guideline says about fasting and diabetes medicines. She says yes, and the coach shares that the guideline advises medical monitoring for people taking medicines like glipizide while fasting. Denise decides to call her doctor's office before she starts. Then she talks about her evenings, when she snacks in front of the television, and decides to try closing the kitchen after dinner as her first step. The coach did not recommend fasting, set a window or advise on her medicine.

4. The evidence

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The trials of intermittent fasting are numerous but mostly short and small, and nearly all compare a fasting pattern either with eating as usual or with daily calorie restriction, which means eating a little less every day. Against eating as usual, all 3 patterns led to weight loss, small in size; the 2025 review classed the loss with time-restricted eating as trivial. Against daily calorie restriction, which is the comparison that matters for a person choosing between approaches, time-restricted eating, such as 16:8, and whole-day fasting, a category including 5:2, produced about the same weight change, and alternate-day fasting a slightly larger one; one later 12-month trial of a 4:3 pattern found a small advantage (Catenacci et al., 2025). No trial identified for this page measured heart attacks, strokes or deaths, and the observational studies of eating timing describe people's usual habits, not intentional fasting.

Each pattern is reported separately below, and every row of the rating table at the foot of this page names the review-defined category or the specific trial schedule it rates. The categories are not pooled into one rating, because they ask different things of a person and were tested in different trials.

Evidence from randomized controlled trials

A trial assigns people to the diet by chance and compares them with people assigned to something else, which is the strongest way to test whether the diet itself causes a result. A 2025 systematic review with network meta-analysis gathered 99 trials of intermittent fasting with 6,582 adults; 92 percent of the trials compared parallel groups. The median trial had 46 participants and lasted 12 weeks, the longest lasted 52 weeks, and only 5 lasted a year or more. Two thirds of the participants were women, the median age was 45, and most had overweight or obesity; trials in children, in pregnancy and of religious fasting were excluded (Semnani-Azad et al., 2025).

A network meta-analysis combines direct comparisons from trials that tested 2 diets head to head with indirect comparisons through a shared comparison diet, so it can estimate how each pattern compares with every other. The review rated its certainty with a GRADE-based method for network analyses and judged a difference in body weight of 2 kilograms to be the smallest that matters; it called smaller differences trivial (Semnani-Azad et al., 2025).

Cardiometabolic outcomes

For long-term blood sugar, measured as HbA1c, the 2025 review did not detect a difference between daily calorie restriction and any of its 3 fasting categories, including whole-day fasting, each comparison judged separately and rated moderate in certainty (Semnani-Azad et al., 2025). The trials in people with type 2 diabetes point the same way. In a 12-month trial in 137 Australian adults, HbA1c fell by 0.3 percentage points with the 5:2 pattern and 0.5 with daily calorie restriction, a difference that met the trial's test for equivalence (Carter et al., 2018). In a 6-month trial in 75 adults in Chicago, mostly Black and Hispanic, HbA1c fell by about 0.9 percentage points more than in a control group with both an 8-hour eating window and daily calorie restriction, with no significant difference between the two (Pavlou et al., 2023).

In 247 Australian adults at risk of type 2 diabetes, a self-chosen 9-hour eating window was not worse than individual dietitian advice for HbA1c after 4 months, but by 12 months the trial could no longer conclude that it was not worse; the changes in both groups were small (Parr et al., 2026). In a trial that combined 3 fasting days a week with an early eating window, blood sugar after a test meal improved more than with daily calorie restriction at 6 months, and the difference was gone at 18 months (Teong et al., 2023).

For blood pressure and blood fats, most comparisons with daily calorie restriction found little difference, with some exceptions. The 2025 review reported that alternate-day fasting was the only fasting pattern to show added improvements over daily calorie restriction, in body measurements, some blood fats and systolic blood pressure, which it classed as trivial to moderate in size (Semnani-Azad et al., 2025). In the 1-year alternate-day fasting trial, LDL cholesterol was 11.5 mg/dL higher at 12 months in the fasting group than in the daily calorie restriction group (Trepanowski et al., 2017). In a 12-month trial of an 8 a.m. to 4 p.m. window combined with a calorie target, blood pressure, glucose and blood fats changed about as much as with the calorie target alone (Liu et al., 2022).

Evidence about earlier eating windows now includes a 2026 network meta-analysis. It estimated about 1.15 kilograms more weight reduction with early than late windows, with an interval from 0.45 to 1.86 kilograms, and rated that comparison High using CINeMA. Only 2 trials compared early and late windows directly; their combined interval included no difference, and the network added indirect comparisons. The review did not set an importance threshold; the difference is below the 2-kilogram threshold used in the 2025 fasting review, and the authors cautioned that their certainty ratings might be optimistic (Y.-E. Chen et al., 2026; Semnani-Azad et al., 2025).

This does not show that timing itself produces lasting clinical benefit independently of calories or weight change. Evidence for that narrower mechanism remains limited. In a controlled crossover study of 8 men with prediabetes, an early 6-hour window with enough food to keep weight steady improved some insulin and blood-pressure measures over 5 weeks, while morning triglycerides rose. Tests followed different fasting durations, and possible carryover complicated interpretation (Sutton et al., 2018). These findings are reasons for further research, not a basis for a coach to prescribe an earlier window.

No trial identified for this page measured heart attacks or strokes, and a 2021 review of trials published through 2019 found that none of the trials it included reported them (Allaf et al., 2021).

Cancer

The searches for this page identified no trial of intermittent fasting that measured whether people developed cancer. A 2025 review of time-restricted eating in people who already had cancer found a small number of completed studies; its authors described all of the findings as preliminary, including feasibility, quality of life and early reports of lower recurrence and tumor markers (Stringer et al., 2025). What the studies reviewed do not establish is whether any fasting pattern prevents cancer or changes its course.

Body weight, as measured

This archive reports body weight as something a study measured, in that study's own numbers, and not as a goal or as a measure of whether an eating pattern worked; see the standing rule in the review record at the foot of this page. The one exception is weight a person loses without meaning to, which can signal a health problem and is a reason to involve a clinician, never a sign that a diet worked.

Compared with eating as usual, all 3 patterns reduced weight in the 2025 review: alternate-day fasting by 3.40 kilograms, whole-day fasting by 2.36, and time-restricted eating by 1.72, while daily calorie restriction reduced it by 2.11 (Semnani-Azad et al., 2025). Compared with daily calorie restriction, the review found no clear difference for time-restricted eating, a difference of 0.39 kilograms favoring calorie restriction with an interval from 1.02 favoring calorie restriction to 0.23 favoring time-restricted eating, and none for whole-day fasting, 0.25 kilograms favoring whole-day fasting with an interval from 0.38 favoring calorie restriction to 0.88 favoring whole-day fasting. It rated both comparisons moderate in certainty. The whole-day fasting category includes 5:2 trials and trials with 1 or 3 fast days a week, and the review gives no separate result for 5:2 alone.

Alternate-day fasting reduced weight by 1.29 kilograms more than daily calorie restriction, from 0.59 to 1.99 more, also rated moderate; the review classed that difference as trivial because it fell below 2 kilograms. In the 17 trials lasting at least 24 weeks, the review did not detect a difference between any fasting pattern and daily calorie restriction (Semnani-Azad et al., 2025). The results varied a great deal from trial to trial, which is the main reason the ratings are moderate rather than high.

Two Cochrane reviews each combine the fasting patterns into a single estimate, so neither can supply the pattern-specific ratings used on this page. A 2021 review, read for this page as its abstract, found the effect of intermittent fasting compared with continuous energy restriction uncertain at 3 months and possibly absent at 3 to 12 months (Allaf et al., 2021).

A 2026 review, read in full, included 22 trials lasting at least 6 months in adults with overweight or obesity. Compared with regular dietary advice, mainly involving calorie restriction, intermittent fasting may make little or no difference to the percentage of body weight lost over up to 12 months, rated low in certainty. Compared with no intervention or a waiting list, people who fasted lost 3.42 percentage points more of their starting weight, from 1.90 to 4.95 more, rated moderate in certainty. That difference is statistically clear, but the review described it as little or no difference because it is below the 5 percent change in body weight that the review set in advance as the smallest important change (Garegnani et al., 2026).

The threshold is the review's way of interpreting a measured trial outcome, not a goal for a client. The review's published ratings are listed in the review record at the foot of this page.

The individual trials fill in the picture. In the 1-year alternate-day fasting trial in 100 adults with obesity, weight fell by 6.0 percent with fasting and 5.3 percent with daily calorie restriction, each compared with the control group, with no significant difference between them (Trepanowski et al., 2017). In 90 adults with obesity in Chicago, an 8-hour window without calorie counting, widened to 10 hours for the second 6 months, reduced weight by 4.61 kilograms over 12 months compared with a control group, and daily calorie restriction by 5.42, with no significant difference between them (Lin et al., 2023). In 139 adults with obesity in China, an 8-hour window combined with a calorie target reduced weight by 8.0 kilograms over 12 months, against 6.3 with the calorie target alone, a difference that was not statistically significant (Liu et al., 2022).

One longer trial was published after the 2025 review's search, which ended in November 2024. In 165 adults with overweight or obesity in Denver, a 4:3 pattern, eating about a fifth of usual energy needs on 3 nonconsecutive days a week, reduced weight by 2.89 kilograms more than daily calorie restriction over 12 months, with an interval from 0.14 to 5.65 kilograms; both groups received an intensive group behavioral program, and 125 people completed the trial (Catenacci et al., 2025). It tested its own schedule, separately identified, and it does not change the review's published estimates for time-restricted eating, whole-day fasting or alternate-day fasting.

When the comparison group had structured mealtime instructions, not a calorie target, results were smaller. In a 12-week trial run through a phone app in 116 adults, people told to eat only between noon and 8 p.m. lost 0.94 kilograms and people told to eat 3 meals a day lost 0.68, a difference too small to distinguish from chance (Lowe et al., 2020). In the 6-month trial of the 5:2 pattern in 107 women in the United Kingdom, weight fell by 6.4 kilograms with fasting and 5.6 with daily restriction, a difference that was not statistically significant (Harvie et al., 2011). In 75 adults with type 2 diabetes, the 8-hour window group lost 3.56 percent more weight than the control group; the calorie restriction group lost 1.78 percent more, which was not statistically significant, and the 2 active groups did not differ significantly from each other (Pavlou et al., 2023).

Lean mass, the weight of everything except fat, deserves attention because muscle matters for health at every age. In the app-based trial, among the 50 people measured in person, the eating-window group lost more lean mass in the arms and legs than the 3-meals group (Lowe et al., 2020). In the 12-month trial in China, both groups lost some lean mass, with no significant difference between them (Liu et al., 2022). A 2026 review of 20 trials in adults aged 45 and older found small differences in lean mass, mostly over 12 weeks or less, and concluded that the long-term effect on muscle in older people is uncertain (Santero et al., 2026).

Mortality

No trial identified for this page measured deaths as an outcome. The 2021 Cochrane review of intermittent fasting for preventing cardiovascular disease found that none of its included trials reported deaths from any cause or from cardiovascular disease (Allaf et al., 2021). The trials were mostly too short and too small to do so. The observational studies of eating timing and death are described under the cohort evidence below.

Eating behavior and wellbeing

In trials, which usually screen people carefully, fasting patterns have not clearly worsened measured eating behavior. A 2023 review of 16 studies of time-restricted eating found that scores on disordered eating questionnaires did not change, although hunger at bedtime was higher, and interviews described fear of hunger and eating when not hungry (Vizthum et al., 2023). In an 8-week alternate-day fasting study in 59 adults with obesity, binge eating and depression scores fell, while restrictive eating rose (Hoddy et al., 2015). In the 12-month 4:3 trial, which excluded people with binge eating disorder, binge eating scores improved in both groups and more with fasting (Catenacci et al., 2025). These results come from people who were screened and supported; the community studies described in section 3 point in a less reassuring direction, and the 2 kinds of evidence describe different groups of people.

For quality of life, the 12-month trial in China found similar physical and mental scores in both groups (Liu et al., 2022), and the 2026 Cochrane review reported that intermittent fasting may make little or no difference compared with regular dietary advice, at low certainty (Garegnani et al., 2026). For side effects, the 2025 review found that most trials reporting them described mild effects such as constipation, nausea, hunger, diarrhea and dizziness, with one serious event, a fall during low blood sugar (Semnani-Azad et al., 2025). One sentence in that review describes the adverse events as severe, which contradicts its own supplementary table; this page relies on the table. A 2024 review of 15 trials found no clear difference from control diets in fatigue, headache or dropout (Zhong et al., 2024).

Evidence from long-term prospective cohort studies

A cohort study follows people who already eat a certain way and records what happens to them. It can show that a pattern goes with an outcome; it cannot by itself show that the pattern caused it.

The searches for this page did not identify a years-long cohort specifically following intentional adoption of the 3 nonreligious schedules described here. Studies of periodic religious fasting and usual eating windows ask different questions. One cohort of adults having heart catheterization, for example, followed people who reported routine fasting over decades and found higher survival among them (Bartholomew et al., 2022); that describes a long-standing way of life in the people who chose it, not a test of these schedules, and it cannot separate fasting from the other features of their lives.

The longest follow-up of a health outcome found was 8 months, for eating disorder symptoms (He et al., 2025); an analysis of app records followed users for up to 52 weeks, but only for app use and self-reported weight (Torres et al., 2022). The cohort studies that exist estimate a person's usual eating window, or overnight fast, from a few days of recalled eating, often 1 or 2, and then follow health outcomes. A short eating window in those studies describes a habit, which may come from illness, poverty, work schedules or a deliberate plan; the studies cannot tell these apart.

Cardiometabolic outcomes

The searches for this page did not identify a cohort study following people who intentionally adopted the 3 nonreligious schedules described here for new cases of heart disease, stroke or diabetes. The cohort evidence on eating timing and cardiovascular death is described under mortality below.

Cancer

In 2,413 women with early breast cancer followed for about 7 years, those whose usual overnight fast was shorter than 13 hours had a higher rate of breast cancer recurrence than those who fasted 13 hours or longer, a hazard ratio of 1.36, from 1.05 to 1.76. Differences in death from breast cancer or any cause were not statistically significant (Marinac et al., 2016). This is one observational study of overnight fasting in women who had already had cancer, not a test of a fasting diet.

Body weight, as measured

Of 800 Chinese adults surveyed, 491 were followed up 8 months later; among them, those who had practiced intermittent fasting in the past had a higher self-reported body mass index at follow-up than those who never had, after accounting for their starting body mass index, while current fasters did not differ clearly (He et al., 2025). Among app users who recorded fasts, weight change was larger in people who started at a higher body mass index and who fasted longer, but those users chose themselves, most stopped recording, and the study was written mainly by the company that made the apps (Torres et al., 2022). Neither study can show what fasting does to weight.

Mortality

In 19,831 United States adults followed for a median of 8.1 years, a usual eating window shorter than 8 hours, measured from 2 days of recalled eating, was associated with a higher rate of death from cardiovascular disease than a window of 12 to 14 hours, a hazard ratio of 2.35, from 1.39 to 3.98. It was not associated with death from any cause or from cancer, and the authors wrote that further research is needed to know whether the risk comes from the short eating window itself or from other factors linked to it (Chen et al., 2025). Only 383 people were in the shortest-window group, 33 of whom died of cardiovascular disease, and when the short window had to appear on both recalled days, the association was no longer statistically clear (Chen et al., 2025).

A second analysis of the same national survey, in 33,052 adults, found the lowest death rates with eating windows of about 11 to 12 hours and higher rates with both shorter and longer windows (Mao et al., 2025). A third found that eating the first meal later in the day and the last meal very early or very late went with higher death rates (Hu et al., 2026). These studies describe how people usually ate on 1 or 2 days and cannot separate a fasting plan from the circumstances that shorten a person's eating day.

Highlighted studies

These 4 carry the most weight for this page: the review that compares all 3 patterns with daily calorie restriction, and one long trial of each pattern. Each is described so that a reader can judge it without opening the paper.

On a narrow screen, scroll the table sideways to see every column.

Highlighted studies for Intermittent Fasting

Study	Design	Sample size	Duration	Finding	The limitation that matters
Semnani-Azad et al., 2025	Systematic review and network meta-analysis of randomized trials, certainty rated with a GRADE-based method	99 trials, 6,582 adults, mostly with overweight or obesity	Trials of 3 to 52 weeks, median 12	Compared with daily calorie restriction, no clear difference in weight for time-restricted eating or for whole-day fasting, a category including 5:2 and other weekly schedules (moderate certainty); 1.29 kilograms more weight loss with alternate-day fasting (moderate), below the review's 2-kilogram threshold for importance. No difference in HbA1c detected for any pattern	Results varied widely between trials, and only 5 trials lasted a year or more; in the 17 trials of at least 24 weeks, no pattern differed from daily calorie restriction
Trepanowski et al., 2017	Randomized controlled trial: alternate-day fasting, daily calorie restriction, or no intervention	100 adults with obesity, 86 women	1 year: 6 months of weight loss, 6 months of maintenance	Weight fell 6.0 percent with alternate-day fasting and 5.3 percent with daily restriction, each compared with control, with no significant difference between them. LDL cholesterol was 11.5 mg/dL higher with fasting at 12 months	38 percent of the fasting group dropped out, against 29 percent with daily restriction, which reduced the trial's power to detect a difference
Lin et al., 2023	Randomized controlled trial: 8-hour eating window without calorie counting (widened to 10 hours after 6 months), daily calorie restriction, or control	90 adults with obesity, mostly Black and Hispanic women	12 months	Compared with control, weight fell 4.61 kilograms with the eating window and 5.42 with calorie restriction; no significant difference between them. People ate about 425 and 405 fewer calories a day	Small and unblinded, and designed to detect only large differences between the 2 active approaches
Carter et al., 2018	Randomized noninferiority trial: 5:2 pattern against daily calorie restriction	137 adults with type 2 diabetes	12 months	HbA1c fell 0.3 percentage points with 5:2 and 0.5 with daily restriction, meeting the trial's test for equivalence. Weight fell 6.8 kilograms with 5:2 and 5.0 with daily restriction, a difference that did not meet the test for equivalence	Diabetes medicines were reduced under an endocrinologist's protocol, more so for insulin in the fasting group, which can affect HbA1c; 29 percent of participants did not finish

Head to head

Few trials have compared fasting patterns with each other directly; the 2025 review found only 3. Combining direct and indirect evidence, it estimated that alternate-day fasting reduced weight by 1.69 kilograms more than time-restricted eating and 1.05 more than whole-day fasting, both rated moderate in certainty and both below its 2-kilogram threshold for importance, and it did not detect a difference between time-restricted eating and whole-day fasting (Semnani-Azad et al., 2025).

Against daily calorie restriction, the comparisons are covered above. The searches for this page did not identify a trial comparing an intermittent fasting pattern head to head with a named food pattern such as the Mediterranean Diet that this page could use; some trials combine an eating window with a named food pattern instead of comparing them (Camacho-Cardenosa et al., 2026). What the studies reviewed do not establish is whether timing adds anything to a healthy food pattern over the long term.

Where the evidence is absent

• The trials identified for this page did not measure heart attacks, strokes or deaths, and did not establish effects on cancer incidence, progression or survival. Whether any fasting pattern changes those outcomes is not established by the studies reviewed.
• Few trials lasted a year or more, and the searches found only a few studies that followed people for a year or more after a program ended, read for this page as abstracts except the 18-month trial, read in full (Camacho-Cardenosa et al., 2026; Carter et al., 2019; Teong et al., 2023).
• Most trials were in adults with overweight or obesity, two thirds of them women. Evidence in children and adolescents is limited to small trials and one supervised 52-week trial; in pregnancy, older adults with frailty, athletes and people with eating disorders, it is limited to small trials, observational studies or none.
• The cohort studies of eating timing describe usual habits recorded on a few days of recalled eating, not intentional fasting, and cannot separate a fasting plan from illness, work schedules or poverty.
• The searches did not identify a study specifically testing these nonreligious schedules in warfarin users; studies during Ramadan address a different fasting context. Nor did they identify a study measuring cost or an estimate of the environmental footprint.

5. Mechanisms

(optional reading; select the heading to open or close this section)

This section explains how the diet is thought to work in the body. You can skip it. Nothing in the coaching sections depends on it.

Established mechanisms

Reduced energy intake explains much of the weight change observed in these trials. Participants assigned to an eating-window program reported about 425 fewer calories per day, about the same as people who counted calories; the window was 8 hours during the first 6 months and 10 hours during maintenance (Lin et al., 2023), and adults with type 2 diabetes on the same window ate about 313 fewer (Pavlou et al., 2023). In the 1-year alternate-day fasting trial, measurements of energy use with a laboratory method called doubly labeled water showed that the fasting group and the daily restriction group cut their energy intake by similar amounts over the first 6 months, about 21 and 24 percent (Trepanowski et al., 2017). The authors of the 12-month trial in China concluded that calorie restriction explained most of the benefits of their eating window (Liu et al., 2022).

This matters for how the evidence reads. Most of the improvements in blood sugar, blood pressure and blood fats seen with fasting patterns are the improvements that come with eating less and losing weight by any route, which is why trials comparing fasting with daily calorie restriction find so little difference between them (Semnani-Azad et al., 2025). Small controlled studies also suggest effects of eating time independent of weight change (Sutton et al., 2018); whether those effects produce lasting clinical benefits remains uncertain.

Proposed mechanisms

Two further explanations are widely repeated, and neither supports a claim in section 4. The first is the metabolic switch. After roughly 12 hours without food, the liver's store of sugar runs low and the body shifts toward burning fat and making ketones; one review proposes that repeatedly crossing this point improves body composition and turns on signaling pathways that slow aging and disease (Anton et al., 2018). The authors of the 2025 review note that such changes come mainly from animal studies and have not been confirmed in people (Semnani-Azad et al., 2025).

The second is timing with the body clock. The body handles sugar differently across the day, and the 2026 diabetes guideline describes the study of meal timing, called chrononutrition, as an emerging field that currently lacks conclusive evidence (American Diabetes Association Professional Practice Committee for Diabetes, 2026). A small, closely controlled study found better insulin sensitivity and blood pressure with an early 6-hour eating window without weight loss (Sutton et al., 2018), and a larger trial found an early advantage in blood sugar response that faded by 18 months (Teong et al., 2023). Claims about cell repair, autophagy and longevity belong in this proposed category as well. A mechanism shown in animals, or in a handful of people over a few weeks, is a reason to study a pattern, not evidence that it prevents disease.

6. Risks, contraindications and myths

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Warnings from professional or regulatory bodies

The searches for this page found no regulatory warning about intermittent fasting itself. The warnings that exist concern diabetes medicines. The American Diabetes Association's 2026 Standards of Care state: "People with diabetes who are taking insulin and/or secretagogues should be medically monitored during the fasting period" (American Diabetes Association Professional Practice Committee for Diabetes, 2026). Secretagogues are medicines, such as sulfonylureas, that make the pancreas release more insulin. The National Institute of Diabetes and Digestive and Kidney Diseases lists fasting among the causes of low blood glucose: "Continuing to take medicines that lower your glucose level while fasting for a medical procedure or for other purposes can also increase the risk for low blood glucose" (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.).

The American Heart Association's 2026 dietary guidance does not list meal timing or fasting among its features of a heart-healthy pattern; it notes in general that some popular weight-loss diets may bring short-term benefits with uncertain long-term effects on the heart and can worsen cardiovascular risk factors (Lichtenstein et al., 2026).

Nutrient gaps

The pattern removes no food, so it creates no built-in gap. The studies read for this page did not report nutrient deficiencies. In one 6-month trial, the makeup of what people ate did not change with an eating window (Pavlou et al., 2023). The open questions are about fitting enough into fewer meals. The authors of the app-based trial wrote that protein intake may have changed in their eating-window group, which they did not measure, and linked that possibility to the lean mass loss they observed (Lowe et al., 2020). Protein, fiber and overall food quality across a shorter eating day, especially for older adults, people with high activity and people with diabetes, are questions for a registered dietitian.

Who should not follow it without medical supervision

• People taking insulin, a sulfonylurea or another medicine that lowers blood sugar, because fasting while taking them raises the risk of low blood sugar and doses may need to change (American Diabetes Association Professional Practice Committee for Diabetes, 2026; National Institute of Diabetes and Digestive and Kidney Diseases, n.d.).
• People who are pregnant, breastfeeding or trying to become pregnant. Pregnancy evidence now includes a small feasibility trial and a combined lifestyle trial, as described under Special populations; it does not establish safety for self-directed fasting. Eating changes belong with the obstetric team.
• People with a history of an eating disorder, who were often excluded from trials and for whom community studies link fasting with more eating disorder symptoms (Ganson et al., 2022; He et al., 2025).
• Children and adolescents, in whom most trials found for this page were small and short, and the one year-long trial was a closely supervised clinical program that recorded a possibly related case of atypical anorexia nervosa (Bakhsh et al., 2024; Hegedus et al., 2024; Lister et al., 2024).
• Older adults who are frail or losing muscle, because the effect of fasting patterns on muscle over the long term is uncertain (Santero et al., 2026).
• People with chronic kidney disease, whose needs depend on the stage of the disease, lab results and treatment; studies of Ramadan fasting in kidney disease report inconsistent effects on kidney function and come with important limitations (Bello et al., 2024). The kidney care team decides.

Medication interactions

A coach never adjusts medication. The point of knowing these is to know when to send the client back to the prescriber. Insulin and sulfonylureas carry the highest risk of low blood sugar during fasting. In the 2 diabetes trials read for this page, their doses were reduced or stopped under a written protocol set by each person's HbA1c, in one trial only on fasting days for some people, and in that trial the protocol had to be made stricter because of low blood sugar (Carter et al., 2018; Pavlou et al., 2023).

In its section on religious fasting, the 2026 diabetes guideline lists other diabetes medicines, such as metformin, SGLT2 inhibitors and GLP-1 receptor agonists, as lower in risk, usually with changes to timing rather than dose, and it says treatment before and after fasting should be culturally sensitive and individualized (American Diabetes Association Professional Practice Committee for Diabetes, 2026).

For warfarin, the searches did not identify a study specifically testing these nonreligious schedules in warfarin users; studies during Ramadan address a different fasting context (Lai et al., 2014). For blood pressure medicines, the only study identified that looked at people taking them was a small observational study during Ramadan fasting (Hammoud et al., 2021). Under the archive's standing rule, a client taking insulin, a sulfonylurea, blood pressure medicine or warfarin who plans a substantial change in eating keeps the prescriber informed before making it, rather than asking permission for every meal.

Signs it is going wrong

Physical signs a coach or a family member might notice include shakiness, dizziness, confusion or a racing heart during a fast in someone taking diabetes medicine, which can mean low blood sugar (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.); fainting; persistent headaches, tiredness, feeling cold or constipation (Harvie et al., 2011; Liu et al., 2022); and weight loss the person did not intend. Each is a reason to involve a clinician, not a diagnosis. A possible low blood sugar episode is treated first under the person's own diabetes plan, with emergency help for severe symptoms or an episode the person cannot manage; telling the prescriber follows.

Behavior matters as much and has to be read with care. Skipping breakfast or eating within set hours can come from work schedules, religious practice, appetite or a clinician's plan, so on its own it tells a coach little. Eating a great deal as soon as the window opens, feeling guilt or anxiety after eating outside the window, preoccupation with food, rules that keep tightening, or turning down meals with other people carries more meaning and is worth asking about directly. The questions are the same in every case: what the behavior is for, whether it causes distress, whether it is narrowing what the person eats, and whether it is getting in the way of daily life. Where the answers raise a real concern about disordered eating, the coach follows their own referral process.

Myths and where they came from

Myth: "Fasting burns fat in a way that normal dieting doesn't."

Where it came from: animal studies of fasting and the idea of a metabolic switch into fat burning after about 12 hours without food (Anton et al., 2018). What is true: in trials comparing fasting with daily calorie restriction, weight change was about the same, or different by amounts the 2025 review called trivial (Semnani-Azad et al., 2025); one later trial found a modest advantage for fasting on 3 days a week (Catenacci et al., 2025). Section 4 gives each result. In a 1-year trial, people fasting on alternate days and people cutting calories daily reduced their energy intake by similar amounts (Trepanowski et al., 2017).

Myth: "On 16:8 you can eat whatever you want, as long as it's in the window."

Where it came from: plans and trials that set no calorie limit inside the window. What is true: in those trials, people who lost weight ate about 300 to 425 fewer calories a day (Lin et al., 2023; Pavlou et al., 2023), and in a 12-week trial comparing noon-to-8-p.m. eating instructions with instructions for structured mealtimes, weight loss was not significantly different between the groups (Lowe et al., 2020). A diabetes guideline adds that the principles of healthy eating still apply during the eating times (American Diabetes Association Professional Practice Committee for Diabetes, 2026).

Myth: "Fasting slows aging and helps you live longer."

Where it came from: animal research and reviews proposing that fasting turns on pathways that slow aging (Anton et al., 2018). What is true: no trial identified for this page measured length of life in people, and the authors of the 2025 review note that the metabolic changes seen in animals have not been confirmed in people (Semnani-Azad et al., 2025).

Myth: "16:8 eating doubles your risk of dying from heart disease."

Where it came from: an observational study in which United States adults whose usual eating window was shorter than 8 hours, on 2 recalled days, had a higher rate of death from cardiovascular disease (Chen et al., 2025). What is true: the study measured habits, not a fasting plan, and its authors wrote that the finding may reflect other factors linked to a short eating day. The trials identified for this page did not measure deaths. The finding is a reason for caution and research, not a settled risk.

7. Key terms

Intermittent fasting

A family of eating patterns in which periods of eating alternate with periods of eating nothing or very little, on a regular schedule.

Time-restricted eating

Eating all food within a set window each day, such as 8 hours, and fasting for the rest; 16:8 is the common name for an 8-hour window.

Early time-restricted eating

A version in which the eating window falls early in the day, ending in the afternoon or early evening.

5:2 pattern

Eating about 500 to 600 calories on 2 days a week and as usual on the other 5; the best-known form of whole-day fasting, also called intermittent energy restriction.

Whole-day fasting

A category used by the 2025 review on this page for schedules with 1 or more fast or very low calorie days a week; it includes 5:2 and other weekly schedules, so its results are not results for 5:2 alone.

Alternate-day fasting

Taking turns between fast days, usually allowing about a quarter of usual energy needs, and ordinary or larger eating days.

Daily calorie restriction

Eating a little less every day, also called continuous energy restriction; the main comparison in the trials on this page.

Ad libitum

Eating freely, without a set limit on amount.

Network meta-analysis

A method that combines direct comparisons from trials with indirect comparisons through a shared comparison diet, so that several diets can be compared at once.

HbA1c

A blood test that reflects average blood sugar over about the previous 3 months, used to monitor diabetes.

Sulfonylurea

A diabetes medicine, such as glipizide, that makes the pancreas release more insulin and can cause low blood sugar if a person eats less than usual.

Hypoglycemia

Blood sugar that drops below what is healthy for a person, also called low blood sugar.

Lean mass

Everything in the body except fat, including muscle, organs, bone and water.

Metabolic switch

A proposed shift, after roughly 12 hours without food, from using mainly sugar to using mainly fat and ketones for energy.

Equivalence trial

A trial designed to show that 2 treatments differ by no more than a set margin, rather than that one is better.

Confidence interval

The range of results compatible with the data; when it includes no difference, the study did not detect one, which is not the same as showing there is none.

8. Review questions

1. Describe time-restricted eating, the 5:2 pattern and alternate-day fasting in one sentence each. Then explain why this page treats intermittent fasting as a family of patterns rather than one diet.
2. Using section 4, explain what the 2025 network meta-analysis compared each fasting pattern with, what it found for body weight in each comparison, and why the choice of comparison diet matters when a client says fasting works.
3. A client who takes insulin for type 2 diabetes tells you she will start the 5:2 pattern next month and has not told her doctor. What does the coach explore, what can the coach share with permission, and where exactly is the referral line?
4. Using sections 4 and 6, explain where the claim that 16:8 eating raises the risk of dying from heart disease came from, what kind of study produced it, and what that study cannot show. Write the 2 sentences you would use with a client who raises it.

9. References

Ain, H. U., & Christensen Pacella, K. A. (2026). Eating disorder symptom severity decreases in fasting Muslim women in the United States during Ramadan: A preliminary longitudinal study. International Journal of Eating Disorders, 59(7), 1619-1629. https://doi.org/10.1002/eat.70089

Allaf, M., Elghazaly, H., Mohamed, O. G., Fareen, M. F. K., Zaman, S., Salmasi, A. M., Tsilidis, K., & Dehghan, A. (2021). Intermittent fasting for the prevention of cardiovascular disease. Cochrane Database of Systematic Reviews, 2021(1), Article CD013496. https://doi.org/10.1002/14651858.CD013496.pub2

American Diabetes Association Professional Practice Committee for Diabetes. (2026). 5. Facilitating positive health behaviors and well-being to improve health outcomes: Standards of care in diabetes-2026. Diabetes Care, 49(Suppl. 1), S89-S131. https://doi.org/10.2337/dc26-S005

Anton, S. D., Moehl, K., Donahoo, W. T., Marosi, K., Lee, S. A., Mainous, A. G., Leeuwenburgh, C., & Mattson, M. P. (2018). Flipping the metabolic switch: Understanding and applying the health benefits of fasting. Obesity, 26(2), 254-268. https://doi.org/10.1002/oby.22065

Bakhsh, J. A., Vu, M. H., Salvy, S. J., Goran, M. I., & Vidmar, A. P. (2024). Effects of 8-h time-restricted eating on energy intake, dietary composition and quality in adolescents with obesity. Pediatric Obesity, 19(11), Article e13165. https://doi.org/10.1111/ijpo.13165

Bartholomew, C. L., Muhlestein, J. B., Anderson, J. L., May, H. T., Knowlton, K. U., Bair, T. L., Le, V. T., Bailey, B. W., & Horne, B. D. (2022). Association of periodic fasting lifestyles with survival and incident major adverse cardiovascular events in patients undergoing cardiac catheterization. European Journal of Preventive Cardiology, 28(16), 1774-1781. https://doi.org/10.1093/eurjpc/zwaa050

Bello, A. K., Lloyd, A., Osman, M. A., Kurzawa, J., Chambers, T., Habib, S., Olah, M. E., Habeeb, Z., Qarni, U., Shojai, S., & Pauly, R. P. (2024). Impact of Ramadan fasting on kidney function and related outcomes in chronic kidney disease and kidney transplant recipients: A systematic review and meta-analysis. BMJ Open, 14(11), Article e085329. https://doi.org/10.1136/bmjopen-2024-085329

Camacho-Cardenosa, A., Merchán-Ramírez, E., Clavero-Jimeno, A., De-la-O, A., Romero-Noguera, J. M., Martín-Rodríguez, J. L., Muñoz-Torres, M., Labayen, I., Ruiz, J. R., & Dote-Montero, M. (2026). Effects of an early, late, and self-selected time-restricted eating intervention on weight loss maintenance in adults with overweight or obesity: A 12-month follow-up of a randomized controlled trial. Clinical Nutrition, 63, Article 106706. https://doi.org/10.1016/j.clnu.2026.106706

Carter, S., Clifton, P. M., & Keogh, J. B. (2018). Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: A randomized noninferiority trial. JAMA Network Open, 1(3), Article e180756. https://doi.org/10.1001/jamanetworkopen.2018.0756

Carter, S., Clifton, P. M., & Keogh, J. B. (2019). The effect of intermittent compared with continuous energy restriction on glycaemic control in patients with type 2 diabetes: 24-month follow-up of a randomised noninferiority trial. Diabetes Research and Clinical Practice, 151, 11-19. https://doi.org/10.1016/j.diabres.2019.03.022

Catenacci, V. A., Ostendorf, D. M., Pan, Z., Kaizer, L. K., Creasy, S. A., Zaman, A., Caldwell, A. E., Dahle, J., Swanson, B., Breit, M. J., Bing, K., Wayland, L. T., Panter, S. L., Scorsone, J. J., Bessesen, D. H., MacLean, P., & Melanson, E. L. (2025). The effect of 4:3 intermittent fasting on weight loss at 12 months: A randomized clinical trial. Annals of Internal Medicine, 178(5), 634-644. https://doi.org/10.7326/ANNALS-24-01631

Chen, M., Xu, L., Van Horn, L., Manson, J. E., Tucker, K. L., Du, X., Feng, N., Rong, S., & Zhong, V. W. (2025). Association of eating duration less than 8 h with all-cause, cardiovascular, and cancer mortality. Diabetes & Metabolic Syndrome: Clinical Research & Reviews, 19(7), Article 103278. https://doi.org/10.1016/j.dsx.2025.103278

Chen, Y.-E., Tsai, H.-L., Tu, Y.-K., & Chen, L.-W. (2026). Effects of timing and eating duration of time restricted eating on metabolic outcomes: Systematic review and network meta-analysis. BMJ Medicine, 5(1), Article e001071. https://doi.org/10.1136/bmjmed-2024-001071

Correia, J. M., Santos, I., Pezarat-Correia, P., Minderico, C., & Mendonca, G. V. (2020). Effects of intermittent fasting on specific exercise performance outcomes: A systematic review including meta-analysis. Nutrients, 12(5), Article 1390. https://doi.org/10.3390/nu12051390

Cuccolo, K., Kramer, R., Petros, T., & Thoennes, M. (2022). Intermittent fasting implementation and association with eating disorder symptomatology. Eating Disorders, 30(5), 471-491. https://doi.org/10.1080/10640266.2021.1922145

Fang, L., Huang, C., Lin, B., Lei, K., Zhou, J., Zhong, X., Liu, Y., & Liu, J. (2025). Evaluation of mobile intermittent fasting applications in Chinese app stores: Quality evaluations and content analysis. JMIR mHealth and uHealth, 13, Article e66339. https://doi.org/10.2196/66339

Ganson, K. T., Cuccolo, K., Hallward, L., & Nagata, J. M. (2022). Intermittent fasting: Describing engagement and associations with eating disorder behaviors and psychopathology among Canadian adolescents and young adults. Eating Behaviors, 47, Article 101681. https://doi.org/10.1016/j.eatbeh.2022.101681

Garegnani, L. I., Oltra, G., Ivaldi, D., Burgos, M. A., Andrenacci, P. J., Rico, S., Boyd, M., Radler, D., Escobar Liquitay, C. M., & Madrid, E. (2026). Intermittent fasting for adults with overweight or obesity. Cochrane Database of Systematic Reviews, 2026(2), Article CD015610. https://doi.org/10.1002/14651858.CD015610.pub2

Hammoud, S., Saad, I., Karam, R., Abou Jaoude, F., van den Bemt, B. J. F., & Kurdi, M. (2021). Impact of Ramadan intermittent fasting on the heart rate variability and cardiovascular parameters of patients with controlled hypertension. Journal of Nutrition and Metabolism, 2021, Article 6610455. https://doi.org/10.1155/2021/6610455

Harvie, M. N., Pegington, M., Mattson, M. P., Frystyk, J., Dillon, B., Evans, G., Cuzick, J., Jebb, S. A., Martin, B., Cutler, R. G., Son, T. G., Maudsley, S., Carlson, O. D., Egan, J. M., Flyvbjerg, A., & Howell, A. (2011). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: A randomized trial in young overweight women. International Journal of Obesity, 35(5), 714-727. https://doi.org/10.1038/ijo.2010.171

He, J., Chen, X., Cui, T., Xiao, Y., Barnhart, W. R., Wang, Y., Yi, S., & Nagata, J. M. (2025). Engagement in intermittent fasting is prospectively associated with higher body mass index, higher eating disorder psychopathology, and lower intuitive eating in Chinese adults. International Journal of Eating Disorders, 58(1), 225-237. https://doi.org/10.1002/eat.24322

Hegedus, E., Vu, M. H., Salvy, S. J., Bakhsh, J., Goran, M. I., Raymond, J. K., Espinoza, J. C., & Vidmar, A. P. (2024). Randomized controlled feasibility trial of late 8-hour time-restricted eating for adolescents with type 2 diabetes. Journal of the Academy of Nutrition and Dietetics, 124(8), 1014-1028. https://doi.org/10.1016/j.jand.2023.10.012

Hoddy, K. K., Kroeger, C. M., Trepanowski, J. F., Barnosky, A. R., Bhutani, S., & Varady, K. A. (2015). Safety of alternate day fasting and effect on disordered eating behaviors. Nutrition Journal, 14, Article 44. https://doi.org/10.1186/s12937-015-0029-9

Houston, D. K., Fanning, J., Nicklas, B. J., Delany, J. P., Hsu, F. C., Chen, S. H., Walkup, M., Neiberg, R. H., Stowe, C. L., Kennedy, K., Espeland, M. A., Ard, J. D., Miller, M. E., Rejeski, W. J., & Kritchevsky, S. B. (2026). Caloric restriction and time-restricted eating in older adults with overweight or obesity: The Health, Aging, and Later-Life Outcomes Pilot Study. The Journals of Gerontology: Series A, 81(4), Article glag061. https://doi.org/10.1093/gerona/glag061

Hu, W., Wang, M., Liu, S., Wu, Q., Di, H., Qin, T., Zhou, S., Guo, Y., Cheng, X., Wang, C., Liu, L., & Shan, Z. (2026). Meal timing and eating window with all-cause and cardiovascular mortality and life expectancy: Population-based cohort study. European Journal of Clinical Nutrition. Advance online publication. https://doi.org/10.1038/s41430-026-01796-1

Kazeminasab, F., Sharafifard, F., Bahrami Kerchi, A., Bagheri, R., Carteri, R. B., Kirwan, R., Santos, H. O., & Dutheil, F. (2025). Effects of intermittent fasting and calorie restriction on exercise performance: A systematic review and meta-analysis. Nutrients, 17(12), Article 1992. https://doi.org/10.3390/nu17121992

Lai, Y. F., Cheen, M. H. H., Lim, S. H., Yeo, F. H. I., Nah, S. C., Kong, M. C., Mya, D., Lee, L. H., & Ng, H. J. (2014). The effects of fasting in Muslim patients taking warfarin. Journal of Thrombosis and Haemostasis, 12(3), 349-354. https://doi.org/10.1111/jth.12496

Lichtenstein, A. H., Khera, A., Anderson, C. A. M., Appel, L. J., DeSilva, D. M., Gardner, C., Hu, F. B., Jones, D. W., Petersen, K. S., & American Heart Association. (2026). 2026 dietary guidance to improve cardiovascular health: A scientific statement from the American Heart Association. Circulation, 153(18), e1285-e1295. https://doi.org/10.1161/CIR.0000000000001435

Lin, S., Cienfuegos, S., Ezpeleta, M., Gabel, K., Pavlou, V., Mulas, A., Chakos, K., McStay, M., Wu, J., Tussing-Humphreys, L., Alexandria, S. J., Sanchez, J., Unterman, T., & Varady, K. A. (2023). Time-restricted eating without calorie counting for weight loss in a racially diverse population: A randomized controlled trial. Annals of Internal Medicine, 176(7), 885-895. https://doi.org/10.7326/M23-0052

Lister, N. B., Baur, L. A., House, E. T., Alexander, S., Brown, J., Collins, C. E., Cowell, C. T., Day, K., Garnett, S. P., Gow, M. L., Grunseit, A. M., Henderson, M., Inkster, M.-K., Kwok, C., Lang, S., Paxton, S. J., Truby, H., Varady, K. A., & Jebeile, H. (2024). Intermittent energy restriction for adolescents with obesity: The Fast Track to Health randomized clinical trial. JAMA Pediatrics, 178(10), 1006-1016. https://doi.org/10.1001/jamapediatrics.2024.2869

Liu, D., Huang, Y., Huang, C., Yang, S., Wei, X., Zhang, P., Guo, D., Lin, J., Xu, B., Li, C., He, H., He, J., Liu, S., Shi, L., Xue, Y., & Zhang, H. (2022). Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine, 386(16), 1495-1504. https://doi.org/10.1056/NEJMoa2114833

Lowe, D. A., Wu, N., Rohdin-Bibby, L., Moore, A. H., Kelly, N., Liu, Y. E., Philip, E., Vittinghoff, E., Heymsfield, S. B., Olgin, J. E., Shepherd, J. A., & Weiss, E. J. (2020). Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: The TREAT randomized clinical trial. JAMA Internal Medicine, 180(11), 1491-1499. https://doi.org/10.1001/jamainternmed.2020.4153

Mao, Z., Grant, H., Kritchevsky, S. B., Newman, A. B., & Farsijani, S. (2025). Association of eating window with mortality among US adults: Insights from a nationally representative study. Aging Cell, 24(11), Article e70230. https://doi.org/10.1111/acel.70230

Marinac, C. R., Nelson, S. H., Breen, C. I., Hartman, S. J., Natarajan, L., Pierce, J. P., Flatt, S. W., Sears, D. D., & Patterson, R. E. (2016). Prolonged nightly fasting and breast cancer prognosis. JAMA Oncology, 2(8), 1049-1055. https://doi.org/10.1001/jamaoncol.2016.0164

National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.). Low blood glucose (hypoglycemia). National Institutes of Health. niddk.nih.gov/health-information/diabetes/overview/preventing-problems/low-blood-glucose-hypoglycemia

Parr, E. B., Charrouf, R., Hutchison, A. T., Flint, S. A., Teong, X. T., Vincent, A. D., Bravo-Garcia, A. P., Siviour, Z., Wittert, G. A., Brennan, L., Devlin, B. L., Hawley, J. A., & Heilbronn, L. K. (2026). Time-restricted eating versus dietetic guidance on glycaemic outcomes in adults at risk of type 2 diabetes: A non-inferiority randomised clinical trial. Diabetologia, 69(9), 2458-2471. https://doi.org/10.1007/s00125-026-06762-x

Pavlou, V., Cienfuegos, S., Lin, S., Ezpeleta, M., Ready, K., Corapi, S., Wu, J., Lopez, J., Gabel, K., Tussing-Humphreys, L., Oddo, V. M., Alexandria, S. J., Sanchez, J., Unterman, T., Chow, L. S., Vidmar, A. P., & Varady, K. A. (2023). Effect of time-restricted eating on weight loss in adults with type 2 diabetes: A randomized clinical trial. JAMA Network Open, 6(10), Article e2339337. https://doi.org/10.1001/jamanetworkopen.2023.39337

Pradella, F., Witte, P., & van Ewijk, R. (2024). Ramadan during pregnancy and offspring health outcomes over the life course: A systematic review and meta-analysis. Human Reproduction Update, 30(6), 789-812. https://doi.org/10.1093/humupd/dmae026

Santero, S., Blagojević, B. D., Merzah, M., Celada Guerrero, J. A., Cena, H., Ordovas, J. M., & Ruiz, L. D. (2026). Intermittent fasting and fat-free mass outcomes in middle-aged and older adults: A scoping review. Advances in Nutrition, 17(7), Article 100663. https://doi.org/10.1016/j.advnut.2026.100663

Semnani-Azad, Z., Khan, T. A., Chiavaroli, L., Chen, V., Bhatt, H. A., Chen, A., Chiang, N., Oguntala, J., Kabisch, S., Lau, D. C., Wharton, S., Sharma, A. M., Harris, L., Leiter, L. A., Hill, J. O., Hu, F. B., Lean, M. E., Kahleová, H., Rahelic, D., . . . Sievenpiper, J. L. (2025). Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: Systematic review and network meta-analysis of randomised clinical trials. BMJ, 389, Article e082007. https://doi.org/10.1136/bmj-2024-082007

Skarstad, H. M. S., Haganes, K. L., Sujan, M. A. J., Gellein, T. M., Johansen, M. K., Salvesen, K. Å., Hawley, J. A., & Moholdt, T. (2024). A randomized feasibility trial of time-restricted eating during pregnancy in people with increased risk of gestational diabetes. Scientific Reports, 14(1), Article 22476. https://doi.org/10.1038/s41598-024-72913-y

St-Onge, M.-P., Ard, J., Baskin, M. L., Chiuve, S. E., Johnson, H. M., Kris-Etherton, P., Varady, K., & American Heart Association Obesity Committee of the Council on Lifestyle and Cardiometabolic Health; Council on Cardiovascular Disease in the Young; Council on Clinical Cardiology; and Stroke Council. (2017). Meal timing and frequency: Implications for cardiovascular disease prevention: A scientific statement from the American Heart Association. Circulation, 135(9), e96-e121. https://doi.org/10.1161/CIR.0000000000000476

Stringer, E. J., Cloke, R. W. G., Van der Meer, L., Murphy, R. A., Macpherson, N. A., & Lum, J. J. (2025). The clinical impact of time-restricted eating on cancer: A systematic review. Nutrition Reviews, 83(7), e1660-e1676. https://doi.org/10.1093/nutrit/nuae105

Sujan, M. J., Skarstad, H. M., Rosvold, G., Fougner, S. L., Follestad, T., Salvesen, K. Å., & Moholdt, T. (2025). Time restricted eating and exercise training before and during pregnancy for people with increased risk of gestational diabetes: Single centre randomised controlled trial (BEFORE THE BEGINNING). BMJ, 390, Article e083398. https://doi.org/10.1136/bmj-2024-083398

Sutton, E. F., Beyl, R., Early, K. S., Cefalu, W. T., Ravussin, E., & Peterson, C. M. (2018). Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metabolism, 27(6), 1212-1221.e3. https://doi.org/10.1016/j.cmet.2018.04.010

Teong, X. T., Liu, K., Vincent, A. D., Bensalem, J., Liu, B., Hattersley, K. J., Zhao, L., Feinle-Bisset, C., Sargeant, T. J., Wittert, G. A., Hutchison, A. T., & Heilbronn, L. K. (2023). Intermittent fasting plus early time-restricted eating versus calorie restriction and standard care in adults at risk of type 2 diabetes: A randomized controlled trial. Nature Medicine, 29(4), 963-972. https://doi.org/10.1038/s41591-023-02287-7

Tinsley, G. M., Moore, M. L., Graybeal, A. J., Paoli, A., Kim, Y., Gonzales, J. U., Harry, J. R., VanDusseldorp, T. A., Kennedy, D. N., & Cruz, M. R. (2019). Time-restricted feeding plus resistance training in active females: A randomized trial. The American Journal of Clinical Nutrition, 110(3), 628-640. https://doi.org/10.1093/ajcn/nqz126

Torres, L., Lee, J. L., Park, S., Di Lorenzo, R. C., Branam, J. P., Fraser, S. A., & Salisbury, B. A. (2022). Retention, fasting patterns, and weight loss with an intermittent fasting app: Large-scale, 52-week observational study. JMIR mHealth and uHealth, 10(10), Article e35896. https://doi.org/10.2196/35896

Trepanowski, J. F., Kroeger, C. M., Barnosky, A., Klempel, M. C., Bhutani, S., Hoddy, K. K., Gabel, K., Freels, S., Rigdon, J., Rood, J., Ravussin, E., & Varady, K. A. (2017). Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: A randomized clinical trial. JAMA Internal Medicine, 177(7), 930-938. https://doi.org/10.1001/jamainternmed.2017.0936

Vizthum, D., Katz, S. E., & Pacanowski, C. R. (2023). The impact of time restricted eating on appetite and disordered eating in adults: A mixed methods systematic review. Appetite, 183, Article 106452. https://doi.org/10.1016/j.appet.2023.106452

Zhong, F., Zhu, T., Jin, X., Chen, X., Wu, R., Shao, L., & Wang, S. (2024). Adverse events profile associated with intermittent fasting in adults with overweight or obesity: A systematic review and meta-analysis of randomized controlled trials. Nutrition Journal, 23(1), Article 72. https://doi.org/10.1186/s12937-024-00975-9

Evidence rating and review record

Each row identifies the method, source, outcome or recommendation, and comparison being rated. Ratings from different tools are not interchangeable.

The ratings on this page use GRADE, the 4-level scale used in the clinical guidelines you will meet in practice, or CINeMA, a GRADE-based method for comparing several approaches at once. High means further research is very unlikely to change the conclusion. Moderate means further research may change it. Low means further research is likely to change it. Very low means the conclusion is uncertain. Both rate each outcome separately, so a diet can be high for one outcome and very low for another. Each row below also says whether the rating comes from a named published review or is this page's own judgment. The rating in the banner is for this diet's main claimed outcome. A high rating means the research is solid; it does not mean the diet is right for everyone, and it does not mean a coach should recommend it.

The banner's rating comes from a review that used CINeMA, a GRADE-based method for comparing several approaches at once, so the banner names that tool.

Three things about this table matter before reading it. Every row names the category or the specific trial schedule it rates: one of the 2025 review's 3 categories (time-restricted eating, whole-day fasting and alternate-day fasting), or the schedule a single trial tested. The review's whole-day fasting category includes 5:2 and other weekly schedules, and the review gives no rating for 5:2 alone; the one 5:2-specific row comes from a single 5:2 trial, and no rating is transferred from one category or schedule to another. The banner's outcome is body weight because weight loss is the main claim made for these patterns, and the rating describes how weight changed compared with another approach, not whether a client should aim to lose weight.

The 2026 Cochrane review pools the fasting patterns into one estimate, so its ratings cannot fill the pattern-specific rows of this table; they are reported here as that review published them, using GRADE. Against regular dietary advice: low for percentage weight change, very low for reaching a 5 percent weight loss, low for quality of life and very low for adverse events. Against no intervention or a waiting list: moderate for percentage weight change, and very low for quality of life and for adverse events (Garegnani et al., 2026).

Ratings marked as this page's own judgment were assigned from the evidence given in each row's reasons; ratings from a published review are reported exactly as that review published them.

On a narrow screen, scroll the table sideways to see every column.

Evidence ratings, their sources and reasons

Outcome	Certainty	Who rated it, and for which comparison	Why it is rated this way
Body weight, as measured: time-restricted eating, such as 16:8, against daily calorie restriction (in the banner)	Moderate that weight change was about the same	Semnani-Azad et al., 2025, using a GRADE-based method for network meta-analysis (CINeMA). Adults, mostly with overweight or obesity, in trials of 3 to 52 weeks; 14 trials compared the 2 directly, and the network adds indirect comparisons.	Randomized trials start high. The review found no concerns for risk of bias, reporting bias, indirectness or incoherence, and some concerns for imprecision and for heterogeneity, the variation between trials, and lowered the rating 1 level. Difference 0.39 kilograms favoring daily calorie restriction, from 1.02 favoring calorie restriction to 0.23 favoring time-restricted eating, well inside the 2-kilogram difference the review used to judge whether a change in weight matters; the review used a separate 1-kilogram bound when judging imprecision. Reported exactly as published in the review's supplementary table 5.
Body weight, as measured: whole-day fasting, a category including 5:2 and other weekly schedules, against daily calorie restriction (in the banner)	Moderate that weight change was about the same	Semnani-Azad et al., 2025, using CINeMA. Same population and trial lengths; 23 trials compared the 2 directly. The category includes 5:2 trials and trials with 1 or 3 fast days a week; the review gives no rating for 5:2 alone, and this page does not transfer this rating to 5:2.	Randomized trials start high. The review recorded major concerns for heterogeneity and no concerns in the other domains, and published the rating as moderate. Difference 0.25 kilograms favoring whole-day fasting, from 0.38 favoring calorie restriction to 0.88 favoring whole-day fasting. This page carries the published label exactly; a reader should know that the review's table records major concerns about heterogeneity for this comparison.
Body weight, as measured: alternate-day fasting against daily calorie restriction	Moderate for a small difference below the review's threshold of importance	Semnani-Azad et al., 2025, using CINeMA. Same population; 15 trials compared the 2 directly.	Randomized trials start high. Lowered 1 level for some concerns about heterogeneity; no concerns in the other domains. Alternate-day fasting reduced weight by 1.29 kilograms more, from 0.59 to 1.99 more, which the review classed as trivial because it is below 2 kilograms. In the 17 trials of at least 24 weeks, the difference was not statistically significant.
HbA1c, each of the review's 3 fasting categories against daily calorie restriction	Moderate for no difference detected, for each of the 3 comparisons separately	Semnani-Azad et al., 2025, using CINeMA. Each category was rated on its own: time-restricted eating, whole-day fasting (a category including 5:2 and other weekly schedules) and alternate-day fasting, each against daily calorie restriction.	Randomized trials start high. Each comparison was lowered 1 level: for time-restricted eating, some concerns for imprecision and heterogeneity; for whole-day fasting, major concerns for heterogeneity; for alternate-day fasting, major concerns for imprecision. The point estimates in the review's summary figure are close to zero. The 3 ratings are the review's, given separately, and are not combined here.
HbA1c in adults with type 2 diabetes: the 5:2 pattern against daily calorie restriction	Low that little or no difference was detected	This page's own judgment, using GRADE. One trial: 137 Australian adults with type 2 diabetes, mostly well controlled, over 12 months, with medication adjusted under an endocrinologist's protocol (Carter et al., 2018).	Starts high as a randomized trial. Lowered 1 level for risk of bias, mainly because 29 percent did not finish, so HbA1c was missing for many participants. Open assignment matters less for a laboratory HbA1c result, and the protocol's medication reductions are part of the strategy being tested, so neither is counted as bias on its own. Lowered 1 level for imprecision: a single trial of 137 people, and the trial reported a 90 percent interval for its equivalence test, from 0.2 lower to 0.5 higher, which reaches its 0.5-point margin; this is not a 95 percent interval, which the trial did not report and which would be wider. Inconsistency cannot be judged from one trial; indirectness not serious, because the trial tested 5:2 with clinician-managed medication changes, the strategy this row rates; publication bias uncertain and not rated down.
Heart attacks, strokes, cancer incidence, progression or survival, and deaths: the 3 schedules described here against any comparison	Not rated	This page's own judgment. No comparison could be rated.	The trials identified for this page measured risk factors, not these events, and a 2021 review found that none of its included trials reported them (Allaf et al., 2021). A cohort of people who reported routine fasting over decades (Bartholomew et al., 2022) and studies in people already living with cancer (Stringer et al., 2025) ask different questions and do not supply a treatment estimate for these schedules. A rating needs data on the outcome. This is a statement about what the searches found, not a finding of no effect.
Eating disorder symptoms in people who practice intermittent fasting, community samples	Very low for an association, provisional	This page's own judgment, using GRADE. Observational studies of adolescents, young adults and adults who chose to fast (Cuccolo et al., 2022; Ganson et al., 2022; He et al., 2025). He et al.'s main article was read in full and did not provide a certainty rating. This row remains provisional: Cuccolo et al. and Ganson et al. were available only as abstracts, and He et al.'s supplement, including Table S1 comparing retained and lost participants, was not obtained.	Observational evidence starts low. Lowered 1 level for risk of bias: self-selected and convenience samples, 2 of the 3 studies measured at one time, fasting and symptoms both self-reported, and confounding not ruled out. He et al. asked about fasting 8 months before the later scores, but kept 491 of the 800 adults who started (39 percent were lost), asked a single question about current or past fasting with no schedule or duration, and noted that its fasting and symptom questions may overlap; the link still cannot show cause, and it is not upgraded. Consistency is judged only in direction: all 3 studies report more symptoms with fasting, but they define fasting differently (fasting in the past year or month, about 16 hours a day, current or past fasting) and use different symptom measures. Precision is judged by outcome, using approximate intervals this page calculated from He et al.'s published standard errors. After accounting for earlier scores, gender, age and body mass index, past fasting was linked with a score 0.21 points higher 8 months later on the main eating disorder questionnaire, scored 0 to 6, with an interval reaching almost to no difference; current fasting's 0.17 was compatible with no difference. Both current and past fasting were linked with higher later scores for muscle-focused disordered eating, with intervals narrowly excluding no difference, and for impairment caused by eating problems. The other 2 studies' estimates and intervals were not available, and a large combined sample does not establish precision. Publication bias unknown. Screened trial participants did not show worsening on questionnaires (Vizthum et al., 2023), but those are different people and a different question. This rates an association, not whether fasting causes eating disorders.
Death from cardiovascular disease, usual daily eating window shorter than 8 hours against 12 to 14 hours	Very low for an association	This page's own judgment, using GRADE. One analysis of United States survey data, 19,831 adults, eating window from 2 days of recalled eating, median follow-up 8.1 years (Chen et al., 2025).	This row rates the association with a usual eating window, not intentional fasting. Observational evidence starts low. Lowered 1 level for risk of bias: the window was estimated from two 24-hour recalls, and residual confounding and reverse causation remain possible, as the authors note. Indirectness is not serious for the usual-window question; applying the finding to intentional 16:8 would add serious indirectness, and this page does not make that transfer. Imprecision is also serious: only 383 people were in the shortest-window group, with 33 cardiovascular deaths, the hazard ratio of 2.35 has an interval from 1.39 to 3.98, a range from a modest to a very large increase, and when the short window had to appear on both recalled days the interval, 0.85 to 3.74, included no association. Consistency cannot be judged: a second analysis of the same survey found a U-shaped pattern (Mao et al., 2025), but it is not an independent replication. Publication bias not assessed. No upgrade for the large hazard ratio, given the risk of bias.

How this archive talks about weight. This page reports what studies measured, including body weight, and does not treat weight change as a goal or as a measure of a diet's success. Weight a person loses without meaning to can be a sign of a health problem, and where it is, the page names it as a reason to see a clinician. People are described the way the study described them.

Status: reviewed and approved by James Peth, MS, MPH, PhD, on October 4, 2026, after a combined final review of 5 archive pages, a focused recheck and a final confirmation. Written October 3, 2026, and revised the same day after the combined final review.

What changed: Whole-day fasting is now named as the 2025 review's category, which includes 5:2 and other weekly schedules, wherever its numbers or ratings appear; a 52-week adolescent trial and a 12-month 4:3 trial were added; the 2026 Cochrane review was read in full; the cohort, warfarin, pregnancy, cancer and mechanism statements were narrowed to what the sources show; 3 rating rows have rewritten reasons; 4 references were added and 1 updated. After a focused recheck, the 2025 Chinese study was read in full and the eating disorder row updated; it stays provisional, as 2 of its papers were read only as abstracts. A final confirmation noted the study's supplement was not obtained.

Updated October 21, 2026: The pregnancy passage now reports 2 randomized trials of eating windows in or before pregnancy (Skarstad et al., 2024; Sujan et al., 2025), and the passage on earlier eating windows now reports a 2026 network meta-analysis (Y.-E. Chen et al., 2026), using wording from the outside review of the Time-Restricted Eating page. The 2018 Ramadan review is no longer cited and was removed from the references. No rating changed. Later the same day, 3 sentences about the 12-week app trial (Lowe et al., 2020) were corrected: that trial compared eating-window instructions with instructions for structured mealtimes, so the window was not the only difference between the groups.

Next scheduled review: August 2027. Pages built on annual guidance are rechecked each year. The diabetes guidance cited on this page is updated every year and should be rechecked first.

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Diet Reference Archive | One James

Prepared by James Peth, MS, MPH, PhD

© 2026–2027 One James. All rights reserved.

This page is for education and coaching practice. It is not medical or dietary advice.