Omega-3: EPA and DHA Are Not the Same Thing

Two fats, one family name

Omega-3 is a category, not a substance. The parent molecule is alpha-linolenic acid, an 18-carbon fat found in flaxseed, walnuts, and canola oil. Humans can convert it into longer fats, but the conversion is poor. Estimates put the yield at somewhere under 10 percent for EPA and lower still for DHA, and the ratio worsens when linoleic acid from seed oils is high in the diet.

That's why the two long-chain fats get discussed directly. Eicosapentaenoic acid has 20 carbons and five double bonds. Docosahexaenoic acid has 22 carbons and six. The extra double bond and the extra two carbons change how each folds into a membrane and which enzymes act on it.

Both come from oily fish and from algae. Both are sold as capsules, and most capsules contain both.

What each one does

DHA is structural. It's the most abundant omega-3 in the membranes of photoreceptor cells in the retina and a major component of gray matter phospholipids. The body keeps a concentration of DHA in the brain that it defends across diet changes, which is one reason short-term supplementation studies in healthy adults often fail to move cognitive measures.

EPA is the precursor to a family of signaling molecules called resolvins and to series-3 prostaglandins. It competes with arachidonic acid for the same enzymes, which is the mechanistic story behind its anti-inflammatory and triglyceride-lowering effects. That competition is dose dependent. At 1 g a day it's a modest effect. At 4 g a day it's a pharmaceutical one.

The clinical split follows the biology. DHA matters most where the tissue is structural. EPA matters most where the outcome is a cardiovascular event.

The trials split by formulation

Three trials carry most of the weight here, and reading them side by side explains why the field looks confused.

REDUCE-IT, published in the New England Journal of Medicine in 2019. It enrolled 8,179 people with cardiovascular disease, or with diabetes plus another risk factor, whose triglycerides were still 135 to 499 mg/dL despite statin therapy. Participants took 4 g per day of icosapent ethyl, a purified EPA ester, or a mineral oil placebo, for a median of 4.9 years. Major cardiovascular events fell by 25 percent, an absolute reduction of 4.8 percentage points. The manufacturer funded the trial.

STRENGTH, published in JAMA in 2020. It enrolled 13,078 high-risk patients from 22 countries with similar triglyceride ranges. They took 4 g per day of an omega-3 carboxylic acid formulation containing both EPA and DHA, or corn oil. The trial was stopped after a median of about 3.5 years when benefit looked unlikely. There was no significant reduction in the composite endpoint, and the supplemented group had more atrial fibrillation.

VITAL, whose omega-3 arm reported in 2018. It gave 1 g per day, containing 840 mg of EPA and DHA combined, to 25,871 adults without existing cardiovascular disease. There was no reduction in major cardiovascular events or in cancer.

Two explanations circulate for the REDUCE-IT and STRENGTH gap. One is the formulation: purified EPA against a mixed EPA and DHA product, with some analyses suggesting DHA may blunt EPA's effect, a claim that hasn't been proven. The other is the placebo. REDUCE-IT used mineral oil, which appeared to raise LDL cholesterol and C-reactive protein in the control arm, possibly inflating the apparent benefit. STRENGTH used corn oil, which didn't.

The honest reading is that both explanations have support and neither has been settled by a trial designed to separate them. A 2021 editorial in the European Heart Journal of Cardiovascular Pharmacotherapy laid out the differences and concluded that more research was needed rather than picking a winner.

What the food record shows

Population studies point the same direction with less precision. Higher intake of oily fish is associated with lower cardiovascular risk in cohort work, and the American Heart Association advises two servings of fish per week for people without existing heart disease.

That advice is about food, not capsules. The dose in two servings of salmon is roughly 1 to 2 g of combined EPA and DHA per week, far below the 28 g per week used in REDUCE-IT.

Reading a label

The line that matters is the one under the total. A bottle that says 1,000 mg of fish oil and 300 mg of EPA plus DHA is 30 percent active material. The rest is other fatty acids and the capsule itself.

Check three things. The EPA and DHA numbers separately, since the split tells you which trial record applies. The serving size, because a capsule count of two or three changes the total. And the form, since ethyl ester and triglyceride forms differ in absorption when taken without a fat-containing meal. Taking capsules with food improves absorption of both.

Oxidation is the unlisted variable. Fish oil goes rancid, and independent testing has repeatedly found peroxide values above limits in consumer products. A capsule that smells sharply of fish when you cut it is a capsule to return. Algal oil avoids the fishy taste but not the oxidation problem.

Frequently asked questions

Is algal oil equivalent to fish oil?

For DHA, yes, and the source doesn't change the molecule. Most algal oils are DHA-dominant with little EPA, so a vegan looking for the EPA dose used in cardiovascular trials won't find it in a standard algal capsule. Read the split, not the total.

Does fish oil lower triglycerides?

Yes, and that's the best-established effect. Doses of 2 to 4 g per day lower triglycerides by roughly 20 to 30 percent in people with elevated levels. Whether that translates into fewer heart attacks depends on the formulation question above, which is still open outside the purified EPA trials.

Can I get the same effect from flaxseed?

Flaxseed supplies alpha-linolenic acid, not EPA or DHA. Conversion is limited and variable. Flaxseed has its own modest evidence for cardiovascular risk, but it isn't a substitute for the long-chain fats in the trials discussed here.

What about mercury?

Mercury accumulates in large predatory fish, not in the oil itself, and most purification steps remove it. Consumer testing of supplements has generally found low mercury levels. Eating fish twice a week is also low risk for most adults, with shark, swordfish, king mackerel, and tilefish the ones to limit.

Should I take omega-3 before surgery?

High doses can lengthen bleeding time. Several surgical guidelines ask patients to stop fish oil one to two weeks before an operation. That's a conversation with the surgeon, and it's worth raising if you take more than 3 g a day.

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This article is for general information only and does not constitute medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration and are not intended to diagnose, treat, cure, or prevent any disease. Talk with your health care provider before starting any supplement, especially if you take a blood thinner or have a bleeding disorder.

Sources

• NIH Office of Dietary Supplements, Omega-3 Fatty Acids Fact Sheet for Health Professionals
• Bhatt et al., Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia, NEJM 2019
• Nicholls et al., Effect of High-Dose Omega-3 Fatty Acids vs Corn Oil on Major Adverse Cardiovascular Events, JAMA 2020
• Manson et al., Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer, NEJM 2019
• Virani, Nambi, Ballantyne, Reconciling the results of REDUCE-IT and STRENGTH, EHJ Cardiovascular Pharmacotherapy 2021
• Yokoyama et al., Effects of eicosapentaenoic acid on major coronary events, JELIS, Lancet 200760290-1)