Fish oil / omega-3 · Evidence
Each point below summarizes specific sources, linked by number to the reference list at the bottom. Where the research is thin or mixed, we say so.
The NIH Health Professional fact sheet describes three main omega-3s: ALA (18 carbons), EPA (20 carbons) and DHA (22 carbons). The body can turn ALA into EPA and then DHA, but the fact sheet says this conversion is very limited (under 15%).[1]
The fact sheet says re-esterified triglycerides, natural triglycerides and free fatty acids are absorbed somewhat better than ethyl esters, but all forms significantly raise EPA and DHA in the blood. Results for krill oil’s phospholipid form are mixed. It also says methylmercury is removed when fish oil is processed.[1]
The NIH fact sheet says recommended amounts have been set only for ALA (1.6 g a day for men, 1.1 g for women), not for EPA or DHA. Supplements include fish oil, krill oil, cod liver oil and algal oil, in a wide range of doses and forms.[2]
Grouped by how much research backs each outcome. A stronger group means more consistent results across more studies, not a bigger effect.
The same review found a small drop in coronary heart disease events (risk ratio 0.93, 28 trials, 84,301 people), but it did not hold up in sensitivity analyses, and the authors say EPA and DHA probably make little or no difference to this risk.[3]
The review says EPA and DHA slightly lowered triglycerides and raised HDL cholesterol. The NIH fact sheet notes higher doses are sometimes used to lower triglycerides, under a healthcare provider’s care.[3][2]
A Cochrane review of randomized trials found that more EPA and DHA (mostly from supplements) made little or no difference to death from any cause (risk ratio 0.98, 39 trials, 92,653 people, high-quality evidence) or to cardiovascular events (0.99, 38 trials, 90,378 people, high-quality evidence). Cardiovascular deaths (0.95), stroke (1.06) and arrhythmia (0.97) also showed no clear change. The authors concluded that earlier suggestions of benefit came from trials with a higher risk of bias.[3]
The NIH Health Professional fact sheet describes the ORIGIN trial, in which 1 g a day of omega-3s for about 6 years lowered triglycerides but had no effect on heart attacks, strokes or cardiovascular deaths.[1]
The NIH fact sheet says the FDA recommends no more than 3 g a day of EPA and DHA combined, including up to 2 g a day from supplements.[2]
According to the NIH fact sheet, side effects are usually mild: an unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhea, headache and smelly sweat. The Cochrane review found no evidence that EPA and DHA changed serious adverse events.[2][3]
The fact sheet says high doses may cause bleeding problems when taken with warfarin or other anticoagulant medicines, and could possibly affect immune function.[2]