Omega-3 EPA and DHA: What the Science Actually Shows

Omega-3 supplements are a $4 billion global industry built largely on the strength of two long-chain fatty acids, EPA and DHA. The evidence behind them has had a turbulent decade. The REDUCE-IT trial in 2018 reported a 25 percent reduction in cardiovascular events with high-dose EPA, generating enormous excitement. Two years later, the STRENGTH trial reported no benefit from a combined EPA plus DHA preparation. Both were large, well-designed, and contradictory. This guide explains what EPA and DHA actually do, who benefits from supplementation, what the major trials showed, and how to choose a product.

EPA versus DHA: same family, different jobs

EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are the two long-chain omega-3 fatty acids that matter most for human health. They are structurally similar but functionally distinct. EPA is a 20-carbon fatty acid that gets incorporated into cell membranes and serves as a precursor for anti-inflammatory eicosanoids. DHA is a 22-carbon fatty acid especially concentrated in the brain and retina, where it makes up a large fraction of membrane phospholipids.

Blood levels of EPA and DHA are commonly summarized as the Omega-3 Index, which is the percentage of total fatty acids in red blood cell membranes that are EPA plus DHA. An Omega-3 Index of 8 percent or higher is associated with the lowest cardiovascular risk, while 4 percent or below is associated with the highest. The average American sits around 4 to 5 percent. You can raise it by eating oily fish regularly or by taking supplements.

A third omega-3, ALA (alpha-linolenic acid), is found in plant foods like flaxseeds, chia seeds, walnuts, and canola oil. The body can convert ALA to EPA and DHA, but the conversion is inefficient: roughly 5 to 10 percent for EPA and less than 1 percent for DHA. ALA has its own modest cardiovascular benefits, but for raising the Omega-3 Index, direct EPA and DHA intake from fish or supplements is far more effective.

Fish versus algae versus krill: which source?

Three supplement sources dominate the market. Fish oil is the oldest and best-studied. It is extracted from oily fish like anchovies, sardines, and mackerel and provides EPA and DHA in the triglyceride or ethyl ester form. Algae oil is derived from marine microalgae and provides EPA and DHA directly, without the fish middleman. Because algae is where fish get their omega-3 in the first place, algae oil is biologically equivalent and is the right choice for vegans and vegetarians. Krill oil provides EPA and DHA in phospholipid form, which some studies suggest is slightly better absorbed, but the differences are small and krill oil typically comes in lower doses per capsule.

Sustainability is a real consideration. Most fish oil is now sourced from small pelagic fish like Peruvian anchoveta, which are heavily fished. The Marine Stewardship Council and Friend of the Sea certifications indicate more sustainable sourcing. Algae oil avoids the fishery issue entirely and is increasingly cost-competitive.

A persistent worry about fish oil is mercury and PCB contamination. Large predatory fish (swordfish, shark, king mackerel, tilefish) accumulate mercury, but supplements are typically made from small fish low on the food chain. A 2008 analysis by ConsumerLab and several independent studies found that mainstream fish oil supplements have undetectable or very low levels of mercury and PCBs. The molecular distillation process used by reputable manufacturers removes the vast majority of contaminants.

The major trials: REDUCE-IT, STRENGTH, ASCEND

REDUCE-IT (2018)

REDUCE-IT randomized 8,179 patients with established cardiovascular disease or diabetes plus additional risk factors, all on statins with elevated triglycerides (135 to 499 mg/dL), to 4 grams per day of purified EPA (icosapent ethyl, brand name Vascepa) or placebo. After a median 4.9 years of follow-up, the EPA group had a 25 percent relative reduction in the composite primary endpoint of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, or unstable angina. Cardiovascular death alone fell 20 percent. The results were striking enough that the American Heart Association updated its advisory to support the use of icosapent ethyl in this specific high-risk population.

STRENGTH (2020)

STRENGTH randomized 13,078 patients with similar risk factors to 4 grams per day of a combined EPA plus DHA preparation (omega-3 carboxylic acids, brand name Epanova) or a corn oil placebo. The trial was stopped early for futility. There was no significant difference in the composite primary endpoint of cardiovascular death, myocardial infarction, stroke, revascularization, or unstable angina requiring hospitalization.

Reconciling the two

How do we explain the contradiction? Several hypotheses have been proposed. The most widely discussed is that EPA alone may have benefits that EPA plus DHA does not. Mechanistic studies suggest EPA has stronger anti-inflammatory and membrane-stabilizing effects. The choice of placebo also matters: REDUCE-IT used mineral oil, which may have worsened outcomes in the placebo group by increasing LDL and inflammation, while STRENGTH used corn oil, which is biologically more neutral. The truth is probably a combination of these factors.

ASCEND (2018)

ASCEND randomized 15,480 UK adults with diabetes but no prior cardiovascular disease to 1 gram per day of fish oil (EPA plus DHA) or placebo. After 7.4 years, there was no significant reduction in serious vascular events. There was a small reduction in vascular death that the authors described as a chance finding. ASCEND used a much lower dose than REDUCE-IT, which is one possible explanation for the null result.

The bottom line from these trials: low-dose fish oil (1 gram per day) does not meaningfully reduce cardiovascular events in primary prevention. High-dose purified EPA (4 grams per day of icosapent ethyl) does reduce events in a specific high-risk population with elevated triglycerides on statins. The picture for combined EPA plus DHA at high doses is less clear.

What about people with normal triglycerides?

Most of the cardiovascular benefit signal in the omega-3 literature comes from people with elevated triglycerides. For people with normal triglycerides and average cardiovascular risk, the evidence is much weaker. Observational studies consistently show that people who eat fish regularly have lower cardiovascular risk than those who do not, but it is hard to separate the omega-3 effect from the fact that fish eaters tend to have better diets and lifestyles overall.

A 2020 Cochrane review of 86 randomized trials concluded that increasing EPA and DHA has little or no effect on cardiovascular death or events, but slightly reduces coronary heart disease events and triglycerides. The review noted that the effect depends heavily on dose and baseline risk.

Pregnancy, breastfeeding, and infant development

DHA accumulates rapidly in the fetal brain and retina during the third trimester and the first 18 months of life. The World Association of Perinatal Medicine, the European Food Safety Authority, and the American Pregnancy Association all recommend that pregnant and breastfeeding women consume at least 200 to 300 mg of DHA per day. The FDA and EPA jointly advise pregnant women to eat 8 to 12 ounces per week of low-mercury fish (salmon, sardines, anchovies, cod, pollock) to meet this target. Fish to avoid during pregnancy include swordfish, shark, king mackerel, and tilefish due to mercury.

Meta-analyses of randomized trials of DHA supplementation in pregnancy show modest benefits: slightly longer gestation, slightly higher birth weight, and a small reduction in early preterm birth (before 34 weeks). Cognitive benefits in offspring are inconsistent and disappear by school age in most trials. The ISSFAL (International Society for the Study of Fatty Acids and Lipids) recommends 200 mg per day of DHA for pregnant and lactating women.

Brain health and cognitive decline

The brain is rich in DHA, and observational studies link higher fish intake and higher Omega-3 Index to slower cognitive decline and lower dementia risk. But randomized trials have been disappointing. A 2022 meta-analysis of 10 trials in JAMA Network Open found no significant effect of omega-3 supplementation on cognitive function in older adults without baseline cognitive impairment. In people with mild cognitive impairment or early Alzheimer's, some trials show modest benefits, others do not. The most honest summary is that omega-3 supplements are not a proven preventive therapy for cognitive decline, but eating fish regularly is reasonable.

Dosing: how much, for what, for whom

The American Heart Association recommends eating at least two servings of fish per week, preferably fatty fish, for cardiovascular health. This delivers roughly 250 to 500 mg per day of EPA plus DHA. For people with documented coronary heart disease, the AHA recommends about 1 gram per day of EPA plus DHA from fish or supplements, ideally after discussion with a clinician.

For people with elevated triglycerides (above 150 mg/dL), 2 to 4 grams per day of EPA plus DHA can lower triglycerides by 20 to 30 percent. The FDA approved icosapent ethyl (Vascepa) at 4 grams per day specifically for adults with triglycerides above 150 mg/dL and established cardiovascular disease or diabetes plus additional risk factors, based on REDUCE-IT.

The FDA has issued a qualified health claim stating that "supportive but not conclusive research shows that consumption of EPA and DHA omega-3 fatty acids may reduce the risk of coronary heart disease." Most over-the-counter fish oil supplements carry this claim on the label.

For general health in healthy adults, 250 to 500 mg per day of combined EPA plus DHA from fish or algae is a reasonable target. The Omega-3 Calculator can help you estimate your intake.

Choosing a supplement

Not all fish oil is equal. Look at the back of the bottle, not the front. The front may say "1,000 mg fish oil," but the back tells you how much of that is EPA and DHA. A typical cheap fish oil capsule contains 300 mg of combined EPA plus DHA per 1,000 mg capsule, meaning you would need 4 to 6 capsules per day to hit a meaningful dose. More concentrated products deliver 500 to 700 mg per capsule.

The triglyceride form is better absorbed than the ethyl ester form, though the difference is small and probably does not matter at clinical doses. Look for third-party testing from USP, NSF, or ConsumerLab to verify that what is on the label is in the bottle and that contaminants are below detection. Refrigeration is not strictly necessary but reduces oxidation. If a fish oil supplement smells rancid or fishy, throw it out. Oxidized fish oil may do more harm than good.

Vegans and vegetarians should look for algae oil, which provides DHA and often some EPA. Algae oil capsules typically deliver 200 to 400 mg of combined EPA plus DHA per capsule. They are biologically equivalent to fish oil and free of the fishy aftertaste that some people find objectionable.

Safety and drug interactions

Omega-3 supplements are generally safe. The most common side effects are fishy aftertaste, gastrointestinal upset, and easy bruising at very high doses. The FDA has stated that doses up to 3 grams per day of EPA plus DHA from supplements are generally recognized as safe. At doses of 4 grams per day, as used in REDUCE-IT, there is a small increase in atrial fibrillation, particularly in people with prior AF or elevated cardiovascular risk. This was noted in REDUCE-IT and confirmed in a 2021 meta-analysis in the European Heart Journal.

Omega-3 supplements have a mild antiplatelet effect. This is not usually clinically significant at standard doses, but high doses should be used with caution in people on anticoagulants or antiplatelet agents. Discuss with your clinician if you are on warfarin, clopidogrel, or direct oral anticoagulants.

The bottom line

For most healthy adults, eating oily fish twice a week is the best way to get EPA and DHA. If you do not eat fish, an algae or fish oil supplement providing 250 to 500 mg per day of combined EPA plus DHA is a reasonable substitute. People with established cardiovascular disease and elevated triglycerides on statins may benefit from high-dose purified EPA, an option now backed by REDUCE-IT and endorsed by the AHA. Pregnant and breastfeeding women should aim for 200 to 300 mg per day of DHA from low-mercury fish or supplements. Claims about cognitive benefits are weaker than the marketing suggests. The strongest evidence remains for cardiovascular risk reduction in specific high-risk groups, not for general prevention.