The short version
Eating seafood 1 to 2 times a week has the strongest support for heart health (Rimm et al., 2018).
Large prevention trials of fish oil pills mostly found no drop in major cardiovascular events. A Cochrane review of 86 trials found little or no effect on death, cardiovascular events, or stroke, though triglycerides fell about 15% (Abdelhamid et al., 2020).
Fish oil supplements are linked to a higher risk of atrial fibrillation, especially above 1 g a day (Gencer et al., 2021).
Mercury is a question about which fish, not about supplements. Any decision about a pill belongs with the client's prescriber, pharmacist, or dietitian.
A quick refresher on the 3 omega-3s
Alpha-linolenic acid (ALA) comes from plant foods such as walnuts, flaxseed, and canola and soybean oils. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) come mainly from seafood. Most of the heart and brain research is on EPA and DHA.
Your clients have probably heard that fish oil is "good for your heart." The large trials tell a different story.
What the heart trials show
The American Heart Association's science advisory (Rimm et al., 2018) recommends 1 to 2 seafood meals a week to lower the risk of heart failure, coronary heart disease, ischemic stroke, and sudden cardiac death, especially when seafood replaces less healthy foods. Harvard's Nutrition Source (2024) puts it plainly: eating fatty fish is strongly supported, and many large trials have not shown the same protection from supplements.
The NIH Office of Dietary Supplements (ODS) summarizes the big trials:
VITAL: 25,871 healthy older adults took 1 g a day for about 5 years. No significant drop in major cardiovascular events, though heart attacks alone fell 28% as a secondary finding.
ASCEND: 15,480 adults with diabetes took the same 1 g dose for about 7 years. No significant drop in serious vascular events.
STRENGTH: 13,078 high-risk adults took 4 g a day. The trial stopped early because benefit looked unlikely, and the supplement group had more atrial fibrillation.
REDUCE-IT: 8,179 high-risk adults took 4 g a day of icosapent ethyl, a purified EPA prescription drug. Cardiovascular events fell 25%. But the placebo was mineral oil, which ODS notes is not neutral: it affects lipids and inflammatory markers. That may explain part of the gap between REDUCE-IT and STRENGTH, which used a corn oil placebo.
The Cochrane review pooled 86 trials and 162,796 people (Abdelhamid et al., 2020). Long-chain omega-3s had little or no effect on all-cause death or cardiovascular events (high certainty) or stroke (moderate certainty). Triglycerides dropped about 15%.
The safety signal matters too. Gencer et al. (2021) pooled 7 trials with 81,210 people. Fish oil supplements were linked to a 25% higher risk of atrial fibrillation overall: 49% higher at doses above 1 g a day and 12% higher at 1 g or less.
Why would fish outperform pills? We don't fully know. The AHA notes that seafood helps most when it replaces less healthy foods, and Harvard points out that people in the later trials ate more fish and more often took statins, which leaves less room for a pill to add anything.
What about the brain?
The trials are disappointing here too. ODS reports that AREDS2 (3,501 older adults) found no effect on cognitive function, a UK trial of 748 healthy adults aged 70 to 79 found no difference after 2 years, and 2 g a day of DHA for 18 months did not slow decline in 295 people with Alzheimer's disease. Meta-analyses find no effect in healthy older adults or in Alzheimer's. A possible benefit in mild cognitive impairment still needs confirming.
Mercury: a question about fish, not pills
Clients sometimes worry that fish oil carries mercury. ODS says the opposite: omega-3 supplements have not been found to contain methylmercury, because processing removes it. Mercury varies by fish. The FDA and EPA advice for people who are pregnant or breastfeeding and for young children lists salmon, sardines, Atlantic mackerel, trout, anchovy, and canned light tuna among the "Best Choices," and advises avoiding king mackerel, shark, swordfish, bigeye tuna, marlin, orange roughy, and Gulf tilefish. The Dietary Guidelines for Americans 2025–2030 name low-mercury, omega-3-rich seafood such as salmon, sardines, and trout during pregnancy.
Affordable options carry plenty of EPA and DHA. Per ODS, 3 oz of canned sardines provides about 740 mg DHA and 450 mg EPA, and 3 oz of canned pink salmon about 630 mg DHA and 280 mg EPA.
Using it with clients
When a client brings up fish oil, start with curiosity about what they've heard and why it matters to them. Their heart health goal is the real topic; the pill is one idea they have about it.
"What have you heard about fish oil, and what got you interested?"
"Would it be helpful if I shared what the American Heart Association says about seafood and heart health?"
"You'd like to take care of your heart, and fish isn't something you enjoy. What else feels possible for you?"
Then explore what fits their life: which seafood they already like, how they like it prepared, and what they can afford. Canned and frozen fish are worth naming if the client is open to it. Clients who don't eat fish at all may want to know which plant foods contain ALA, which you can share from ODS.
If a client is already taking fish oil, take blood thinners seriously. ODS notes high-dose fish oil may raise INR in people taking warfarin, and package inserts for prescription omega-3s advise INR monitoring. That conversation goes to the prescriber or pharmacist: "That sounds like a great question for your pharmacist. What would you want to ask?"
Where coaching stops
Coaches don't recommend supplements, doses, or brands, and don't advise a client to start, stop, or change one. Prescription omega-3 products for high triglycerides or cardiovascular risk are a prescriber's decision. Refer a client who takes an anticoagulant, has atrial fibrillation or another heart rhythm problem, is pregnant, or wants a specific eating plan to their physician, pharmacist, or registered dietitian.
Go deeper
The Heart Health Conversations package from One James is in development and will cover the cardiovascular evidence coaches are asked about most, including supplements.
References
Abdelhamid, A. S., Brown, T. J., Brainard, J. S., Biswas, P., Thorpe, G. C., Moore, H. J., Deane, K. H., Summerbell, C. D., Worthington, H. V., Song, F., & Hooper, L. (2020). Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews, 3, CD003177. https://doi.org/10.1002/14651858.CD003177.pub5
Gencer, B., Djoussé, L., Al-Ramady, O. T., Cook, N. R., Manson, J. E., & Albert, C. M. (2021). Effect of long-term marine ω-3 fatty acids supplementation on the risk of atrial fibrillation in randomized controlled trials of cardiovascular outcomes: A systematic review and meta-analysis. Circulation, 144(25), 1981–1990. https://doi.org/10.1161/CIRCULATIONAHA.121.055654
Harvard T.H. Chan School of Public Health. (2024). Omega-3 fats. The Nutrition Source. https://nutritionsource.hsph.harvard.edu/omega-3-fats/
National Institutes of Health, Office of Dietary Supplements. (n.d.). Omega-3 fatty acids: Fact sheet for health professionals. Retrieved November 1, 2026, from https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/
Rimm, E. B., Appel, L. J., Chiuve, S. E., Djoussé, L., Engler, M. B., Kris-Etherton, P. M., Mozaffarian, D., Siscovick, D. S., & Lichtenstein, A. H. (2018). Seafood long-chain n-3 polyunsaturated fatty acids and cardiovascular disease: A science advisory from the American Heart Association. Circulation, 138(1), e35–e47. https://doi.org/10.1161/CIR.0000000000000574
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
U.S. Food and Drug Administration. (2024). Advice about eating fish. https://www.fda.gov/food/consumers/advice-about-eating-fish
