EPA and DHA are the two main omega-3 fatty acids in fish oil, and they’re the reason fish oil supplements exist. EPA (eicosapentaenoic acid) is a 20-carbon fatty acid with five double bonds, while DHA (docosahexaenoic acid) is a 22-carbon fatty acid with six double bonds. Both are classified as omega-3s because their first double bond sits on the third carbon from the end of the molecule. Though they’re closely related, they play distinct roles in your body.
Why Your Body Needs Them From Food
Your body can technically make EPA and DHA from a plant-based omega-3 called alpha-linolenic acid (ALA), found in foods like flaxseed and walnuts. The problem is that the conversion is remarkably inefficient. Healthy adults convert only about 5 to 10% of ALA into EPA and just 2 to 5% into DHA. Some estimates put the DHA conversion rate below 1% in adults. The International Society for the Study of Fatty Acids and Lipids concluded that ALA-to-DHA conversion is roughly 1% in infants and considerably lower in adults.
This poor conversion rate is why nutrition guidelines emphasize getting EPA and DHA directly, either through fatty fish or supplements. Relying on plant-based omega-3s alone leaves most people with very little of the forms their body actually uses.
What EPA Does
EPA’s primary strengths are in cardiovascular health and inflammation. It lowers triglycerides (a type of blood fat) without raising LDL cholesterol, which sets it apart from DHA. In the EVAPORATE trial, patients with existing heart disease who took 4 grams per day of EPA alongside statin therapy saw a 17% reduction in a type of arterial plaque strongly linked to heart attacks, compared to placebo.
EPA also acts as a direct competitor to arachidonic acid, an omega-6 fat that fuels inflammation. When EPA takes the place of arachidonic acid in cell membranes, the inflammatory signals your body produces are weaker. This translates to reduced production of pro-inflammatory molecules like prostaglandins and leukotrienes, along with less immune cell migration to sites of inflammation. Both EPA and DHA have anti-inflammatory properties, but EPA’s role in this competition with arachidonic acid is particularly well documented.
What DHA Does
DHA is the most abundant omega-3 fat in the brain and retina. It’s concentrated in the membranes of brain cells, particularly at synaptic terminals where neurons communicate with each other. By maintaining the fluidity of these membranes, DHA influences how well neurotransmitters are released, how receptors function, and how signals travel through the nervous system. It also plays roles in myelination (the insulation around nerve fibers), gene expression, and the growth of new neurons.
This structural role makes DHA especially critical during brain development. During the third trimester of pregnancy, the fetus has an obligatory need for DHA as the brain grows rapidly. In one study, pregnant women who supplemented with 300 mg of DHA per day during the third trimester had infants with measurably larger brain volumes on MRI. Another study found that 600 mg of DHA daily from before 20 weeks of gestation until delivery significantly reduced rates of preterm birth and low birth weight.
Where EPA and DHA Come From
Cold-water fatty fish like salmon, mackerel, sardines, and anchovies are the richest dietary sources. Fish accumulate EPA and DHA by eating algae and smaller fish throughout their lives, concentrating these fats in their tissues. The federal 2020 to 2025 Dietary Guidelines for Americans recommend meeting nutritional needs primarily through food, and two servings of fatty fish per week is a common benchmark.
For people who don’t eat fish, microalgae oil supplements provide both EPA and DHA. A direct comparison study found that the bioavailability of DHA and EPA from microalgal oil was statistically equivalent to fish oil. Plasma levels of combined DHA and EPA were 112% of the fish oil group’s levels after normalization, well within the range considered non-inferior. This makes algae oil a reliable plant-based alternative.
How Much You Need
There’s no single universal recommendation for EPA and DHA. The U.S. Institute of Medicine hasn’t set a specific daily intake target for these fats. For people with existing coronary heart disease, the American Heart Association recommends about 1 gram per day of combined EPA and DHA, preferably from oily fish. For managing high triglycerides, the AHA recognizes that 4 grams per day of prescription omega-3s can meaningfully lower levels. The AHA does not recommend omega-3 supplements for people without a high cardiovascular risk.
During pregnancy, studies showing benefits have used doses ranging from 200 mg to 600 mg of DHA per day, sometimes combined with EPA. One study using a higher dose of 2.2 grams of DHA plus 1.1 grams of EPA daily from the 20th week of pregnancy onward found improved visual and coordination abilities in the children.
Supplement Forms and Absorption
Not all fish oil supplements deliver EPA and DHA equally well. The two most common forms are triglycerides (the natural form found in fish) and ethyl esters (a concentrated, chemically modified form). Re-esterified triglycerides, where ethyl esters are converted back into a triglyceride structure, consistently show better absorption.
In a 16-week trial, supplements with a high triglyceride composition (over 96% re-esterified triglycerides) produced significantly greater increases in EPA and DHA levels in red blood cell membranes compared to a product with about 60% re-esterified triglycerides. The difference was apparent in blood serum as early as four weeks. This matters because the omega-3 index, which measures the percentage of EPA and DHA in red blood cell membranes, is the standard marker for whether supplementation is actually working.
Freshness and Quality
Fish oil is prone to oxidation, which degrades EPA and DHA and can produce harmful byproducts. Three international organizations, the Global Organization for EPA and DHA Omega-3s (GOED), the Council for Responsible Nutrition, and the International Fish Oil Standards program, have set voluntary safety thresholds. A quality fish oil should have a peroxide value below 5 mEq/kg (measuring early-stage oxidation), an anisidine value below 20 (measuring later-stage oxidation), and a total oxidation (TOTOX) value below 26.
These are voluntary standards, not regulatory requirements, so quality varies widely across the supplement market. If you’re choosing a fish oil supplement, look for brands that publish third-party testing results against these benchmarks. Store fish oil in a cool, dark place and discard any product that smells strongly fishy or rancid, which are signs of oxidation.