Nutrition

Omega-3 Fatty Acids: Not a Vitamin, and What Fish Oil Trials Show

Nutrition · Published 2026-10-24

Fish oil sits on the vitamin shelf, but omega-3s are not vitamins. They are essential fats: the body needs them and cannot make enough on its own. Whether capsules add anything to a diet that includes fish is a question large trials have tested, with mostly disappointing results for healthy people.

Why omega-3s are not vitamins

Vitamins are organic compounds needed in small amounts (milligrams or micrograms) for specific metabolic roles. Omega-3 fatty acids are fats, eaten in grams, that are built into cell membranes, especially in the brain and retina, and converted into signalling molecules involved in inflammation and blood clotting. They are classed as essential fatty acids. The distinction matters because the regulatory and scientific framework is different: there are no recommended dietary allowances for EPA and DHA in the United States, for example.

There are three main omega-3s in the diet:

• ALA (alpha-linolenic acid), from plant sources such as flaxseed (linseed), chia seeds, walnuts, rapeseed (canola) oil and soy. ALA is essential: the body cannot make it.
• EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), found mainly in oily fish and seafood, and in algae. The body can convert some ALA into EPA and, to a much smaller extent, DHA, but conversion is limited.

Reference intakes and dietary advice

EFSA also concluded that supplemental intakes of EPA and DHA combined up to about 5 g a day, and of DHA alone up to about 1 g a day, do not raise safety concerns for adults. That is a safety statement, not a recommendation to take such amounts.

The NHS sets limits on oily fish for some groups because of pollutants: people who are pregnant or might become pregnant, and girls, are advised to eat no more than two portions of oily fish a week, and to avoid shark, swordfish and marlin because of mercury. Other people are advised to have no more than four portions of oily fish a week.

What the trials of fish oil supplements found

Observational studies linked fish eating with lower rates of heart disease, which led to widespread use of fish oil capsules. Randomised trials then tested the capsules themselves:

• VITAL (United States, 25,871 adults without cardiovascular disease, 1 g a day of omega-3s, median 5.3 years): no significant reduction in major cardiovascular events or invasive cancer compared with placebo (Manson et al., NEJM 2019).
• ASCEND (United Kingdom, 15,480 adults with diabetes, 1 g a day): no significant reduction in serious vascular events over a mean of 7.4 years (2018).
• Cochrane review (Abdelhamid et al., 2020; 86 trials, more than 162,000 participants): increasing EPA and DHA has little or no effect on all-cause mortality and cardiovascular events. It probably reduces blood triglycerides, and may slightly reduce coronary heart disease mortality and events.
• REDUCE-IT (2018): 4 g a day of icosapent ethyl, a purified prescription form of EPA, reduced major cardiovascular events by 25% relative to placebo in people with raised triglycerides already taking statins. The result has been debated because the placebo was mineral oil, which may have raised cardiovascular risk in the control group. This is a prescription medicine for a defined group, not an over-the-counter supplement.
• STRENGTH (2020): a similar high-risk population given 4 g a day of a different EPA and DHA formulation, compared with corn oil, was stopped early because it showed no benefit.

Meta-analyses have also found that omega-3 supplements are associated with a higher risk of atrial fibrillation, an irregular heart rhythm, particularly at doses above 1 g a day (Gencer et al., Circulation 2021).

Pregnancy

A Cochrane review (Middleton et al., 2018; 70 trials) found that omega-3 supplementation during pregnancy reduced the risk of preterm birth before 37 weeks by about 11% and early preterm birth before 34 weeks by about 42%, with high-quality evidence for the first outcome. It did not find clear effects on child development outcomes. Pregnant people considering a supplement should discuss it with their midwife or doctor; fish liver oil products should be avoided in pregnancy because they contain vitamin A.

Other uses

Omega-3s have been studied for depression, dementia, dry eye, rheumatoid arthritis and asthma. Results are mixed or weak for most conditions. For example, a large trial of omega-3s for dry eye disease (DREAM, 2018) found no benefit over an olive oil placebo. Prescription omega-3 products are used to lower very high triglycerides under medical supervision.

Safety and interactions

Common side effects include a fishy aftertaste, belching, heartburn, nausea and loose stools. High doses may increase bleeding time, so people taking anticoagulant or antiplatelet medicines (such as warfarin) should discuss fish oil with their doctor or pharmacist. Cod liver oil differs from fish oil: it also contains vitamin A, which counts towards the vitamin A upper limit.

In short

• Omega-3s are essential fats, not vitamins.
• Dietary advice is to eat fish, including oily fish, about twice a week; EFSA's adequate intake is 250 mg a day of EPA plus DHA.
• Large trials found that standard fish oil supplements did not prevent heart attacks, strokes or cancer in the general population.
• A purified prescription EPA reduced events in one high-risk trial; that result does not apply to over-the-counter capsules.

This page is general information, not medical advice. If you take blood-thinning medicines, are pregnant or have a heart condition, ask a doctor or pharmacist before using fish oil or other omega-3 products.

Sources

• NIH Office of Dietary Supplements, Omega-3 fatty acids fact sheet for health professionals.
• NHS, Fish and shellfish.
• Manson JE et al. Marine n−3 fatty acids and prevention of cardiovascular disease and cancer. N Engl J Med 2019;380:23–32.
• Bhatt DL et al. Cardiovascular risk reduction with icosapent ethyl for hypertriglyceridemia. N Engl J Med 2019;380:11–22.
• Abdelhamid AS et al. Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease. Cochrane Database Syst Rev 2020.
• Middleton P et al. Omega-3 fatty acid addition during pregnancy. Cochrane Database Syst Rev 2018.
• EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on the tolerable upper intake level of EPA, DHA and DPA. EFSA Journal 2012;10(7):2815.

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