The short version
All plant foods contain all 20 amino acids, including the 9 indispensable ones. The labels "complete" and "incomplete" are misleading (Mariotti & Gardner, 2019).
There's no evidence that plant proteins must be combined at the same meal. Variety across the day is enough.
Amounts matter more than labels: a cup of cooked lentils has about 18 g of protein; a cup of cooked broccoli has about 4 g (USDA FoodData Central).
Older adults, people who eat very little, and people whose diets skip legumes, nuts, and seeds deserve more attention, and a referral to a registered dietitian.
What the review found
Most coaches learned the "incomplete protein" idea somewhere along the way, and most clients have heard it too. Mariotti and Gardner (2019) reviewed the evidence on protein and amino acids in vegetarian and vegan diets, and their conclusions are worth knowing well.
Plant foods aren't missing amino acids. The authors state that "all plant foods contain all 20 amino acids, including the 9 indispensable amino acids," and call the claim that plant foods lack specific amino acids "demonstrably false." What differs is the proportion. Some plant proteins are lower in one amino acid than the body would ideally like. The best-known example is lysine in grains; wheat protein is low in lysine.
Lysine rarely becomes a real problem. In the EPIC-Oxford study, lysine intake averaged 58 mg per kg of body weight in vegetarians and 43 mg per kg in vegans, both well above the estimated average requirement of 30 mg per kg. The authors say lysine might be limiting only for vegans who get almost all their protein from grains.
No need to combine at each meal. The review found no evidence that plant proteins must be paired with complementary proteins at the same meal. A reasonable variety of protein sources over the day does the job. Combining within a meal can be a practical safeguard for someone whose total protein intake is low.
Vegetarian diets generally supply enough. The authors conclude that "classic vegetarian diets supply more than adequate protein and amino acids." A small share of vegans may be at modest risk, mostly because they eat too little food overall, not because of amino acid gaps. One disclosure: Mariotti holds a research contract with a French plant protein industry group.
The Dietary Guidelines for Americans 2025–2030 advise vegetarians and vegans to "diversify plant protein sources for amino acid balance," which fits the variety message. Clients may also hear that the new guidelines set a protein goal of 1.2–1.6 g per kg of body weight per day. What that means for a specific person is a question for a registered dietitian.
2 common corrections
Seitan is not a "complete" protein in the old sense. Seitan is made from wheat gluten, and wheat is low in lysine. Like every plant food, it contains all the amino acids. It simply works best as one protein among several.
Broccoli and spinach are not good protein sources. They contribute some protein, but not much. Per USDA FoodData Central, cooked broccoli has about 3.7 g per cup and cooked spinach about 5 g per cup.
Amounts, not labels
Here's how common plant foods compare, from USDA FoodData Central (cooked unless noted):
Edamame, 1 cup: 18.5 g
Lentils, 1 cup: 18 g
Black beans or kidney beans, 1 cup: about 15 g
Chickpeas, 1 cup: 14.5 g
Tofu made with calcium sulfate, ½ cup: 10 g
Quinoa, 1 cup: 8 g
Peanut butter, 2 tablespoons: 7 g
Almonds, 1 ounce: 6 g
Whole wheat bread, 1 slice: 4 g
Broccoli, 1 cup: about 3.7 g
Quinoa is often singled out as the "complete" grain. Like every plant food, it contains all the amino acids, but at 8 g per cup it carries less than half the protein of the same amount of lentils.
Using it with clients
For a client who eats mostly plants, the combining rule can make every meal feel like a puzzle. Correcting the myth, with permission, can come as a relief.
"What have you heard about getting enough protein from plants?"
"Would it be useful if I shared what a recent review found about combining proteins?"
"It sounds like matching up the right foods at every meal has felt like a lot of work."
"Which beans, soy foods, or grains already show up in your week?"
From there, build on what the client already enjoys: the bean dishes, soy foods, nut butters, and grains that are part of their usual meals and their culture. The client decides what, if anything, to change. You can share the FoodData Central amounts if they're curious, but setting protein targets or planning meals is outside the coach's role.
Pay closer attention when a client is an older adult, has a small appetite, eats very little overall, or follows a diet that leaves out legumes, nuts, and seeds. Mariotti and Gardner flag these as the situations where intake can run short.
Where coaching stops
Coaches don't set protein targets, assess whether a client's intake is adequate, or recommend protein powders or other supplements. Refer older adults with appetite or weight changes, people who eat very little, people with kidney disease, pregnant or breastfeeding clients, and families raising young children on vegan diets to a registered dietitian or their physician.
Go deeper
The Nutrition Science Deep Dive package from One James is in development and will cover macronutrients, including protein quality and what the newer evidence says about plant-based diets.
References
Mariotti, F., & Gardner, C. D. (2019). Dietary protein and amino acids in vegetarian diets: A review. Nutrients, 11(11), 2661. https://doi.org/10.3390/nu11112661
U.S. Department of Agriculture, Agricultural Research Service. (2018). FoodData Central: SR Legacy [Data set]. https://fdc.nal.usda.gov/
U.S. Department of Health and Human Services & U.S. Department of Agriculture. (2026). Dietary guidelines for Americans, 2025–2030. https://cdn.realfood.gov/DGA.pdf
