Omega-3 fish oil (EPA/DHA)

Omega-3 fatty acids are a family of polyunsaturated fatty acids characterized by a carbon-carbon double bond positioned three carbons from the methyl end of the molecule. The three omega-3s most relevant to human nutrition are alpha-linolenic acid (ALA), a short-chain omega-3 found in plant oils such as flaxseed and walnut; eicosapentaenoic acid (EPA), a long-chain omega-3 found primarily in fatty fish; and docosahexaenoic acid (DHA), also predominantly from fatty fish.¹ ALA is considered essential because the body cannot synthesize it. The body can convert small amounts of ALA into EPA and DHA, but the conversion is limited and variable, so direct dietary or supplemental sources of EPA and DHA are the practical way to increase tissue levels.¹

Standard fish oil supplements are derived from fatty fish (such as anchovy, sardine, mackerel, or salmon) and contain a mixture of EPA and DHA. Concentrations vary widely between products; a typical 1,000 mg fish oil softgel may contain between 200 mg and 600 mg of combined EPA and DHA. Algae-derived omega-3 supplements provide DHA (and in some products EPA) from algal sources and are a non-fish alternative.¹

Long-chain omega-3 fatty acids are incorporated into cell membranes throughout the body and play roles in cardiovascular function, vision, neurological development, and inflammatory pathways.¹

Cardiovascular outcomes have been the most extensively studied application. The body of evidence is mixed and has evolved over time. The large VITAL trial (n ≈ 26,000), which tested 1 g/day of a fish-oil-derived omega-3 supplement (containing 460 mg EPA and 380 mg DHA), did not show a significant reduction in the composite of major cardiovascular events or in total invasive cancer compared with placebo, although secondary analyses suggested possible benefit for myocardial infarction and certain subgroups.²

In contrast, the REDUCE-IT trial studied icosapent ethyl — a pharmaceutical-grade purified EPA ethyl ester at 4 g/day (2 g twice daily) — in 8,179 statin-treated patients with elevated triglycerides and either established cardiovascular disease or diabetes with additional risk factors. The trial found a 25% relative risk reduction in major adverse cardiovascular events with icosapent ethyl compared with placebo over a median follow-up of 4.9 years.³ On the basis of REDUCE-IT, icosapent ethyl received FDA approval for cardiovascular risk reduction in this defined population. It is important to note that this benefit was demonstrated for a specific high-dose, purified EPA pharmaceutical product in a defined clinical population, not for over-the-counter fish oil supplements at typical doses in the general population.

Omega-3 supplements are also commonly used to lower elevated triglyceride levels (where doses of 2–4 g/day of EPA + DHA have shown meaningful triglyceride reductions in clinical trials), to support neurological development during pregnancy and infancy (with DHA being the form most relevant to fetal and infant brain and retinal development), and for general dietary adequacy in individuals who do not consume fish.¹

The Institute of Medicine has not established an RDA for EPA or DHA specifically. The Adequate Intake (AI) for ALA in adults is 1.6 g/day for men and 1.1 g/day for women. The Dietary Guidelines for Americans recommend consuming at least 8 ounces of seafood per week (with at least some emphasis on fatty fish), which provides approximately 250 mg/day of combined EPA + DHA on average.¹

For supplemental use:¹

• General supplementation: Typical commercial fish oil products provide 200–600 mg combined EPA + DHA per softgel; 1–3 softgels per day is a common range.
• Triglyceride lowering: Clinical trials showing meaningful triglyceride reductions have generally used 2–4 g/day of combined EPA + DHA.
• REDUCE-IT prescription regimen (icosapent ethyl): 2 g twice daily of purified EPA ethyl ester, in the specific patient population for which it is FDA-approved.³

The FDA has stated that intakes up to 3 g/day of combined EPA + DHA from food and supplements are generally recognized as safe (GRAS), although intakes above 3 g/day should generally involve healthcare provider input.¹

Oral, typically as softgels, liquid, or chewable formulations.

Standard storage in a cool, dry place out of direct light; some manufacturers recommend refrigeration after opening, particularly for liquid fish oil. Oxidation produces rancidity, which can be detected by smell or taste — a strongly rancid product should not be consumed.

The most common side effects of fish oil supplementation are gastrointestinal: belching, fishy aftertaste, mild dyspepsia, or diarrhea.¹ Enteric-coated formulations may reduce these effects.

Omega-3 fatty acids have mild antiplatelet effects and may modestly prolong bleeding time at high doses. Clinical bleeding events with typical supplemental doses (1–3 g/day) are uncommon, but caution is reasonable in individuals taking anticoagulants or antiplatelet medications, and around surgical procedures — preoperative discontinuation policies vary and should be confirmed with the prescribing or surgical team.¹

Both VITAL and REDUCE-IT reported small absolute increases in atrial fibrillation in the omega-3 arms compared with placebo; the clinical implications of this finding are an area of active research.²³

Mercury exposure is a consideration with high consumption of certain large predatory fish but is generally not a concern with purified fish oil supplements, as the purification process removes most mercury and other contaminants.¹ Pregnant individuals, who require adequate DHA for fetal neurological development, are advised to consume lower-mercury fish (such as salmon, sardines, and trout) or to use supplements selected for low contaminant levels.