Omega-3: fish, fish oil and what it really does

This is general information, not medical advice. Always discuss your blood values with your doctor.

1. 1

   Heart and blood vessels: little to no effect from supplements

   A Cochrane review of 86 randomised trials (Abdelhamid et al., 2020) concluded, with moderate- to high-quality evidence, that increasing EPA and DHA has little or no effect on mortality or cardiovascular disease. The evidence comes mainly from supplement trials. A later meta-analysis of 40 studies (Bernasconi et al., 2021) did find small, dose-dependent reductions, for example 13% fewer heart attacks (RR 0.87). So the reviews differ in their conclusions, and the effects are small in any case.

2. 2

   Triglycerides: this does work, but mainly at high doses

   Omega-3 lowers triglycerides almost linearly with the dose, and non-HDL cholesterol too (dose-response meta-analysis of 90 RCTs with 72,598 participants, Wang et al., JAHA 2023). For LDL and HDL the relationship was J-shaped. In a randomised trial in people with moderately raised triglycerides, 3.4 g of EPA+DHA a day lowered them by 27%, while 0.85 g a day had no effect (Skulas-Ray et al., 2011). These are medical doses: with very high triglycerides, a doctor decides.

3. 3

   High doses: more risk of atrial fibrillation

   A meta-analysis of 7 large trials (81,210 patients, followed for 4.9 years on average, Gencer et al., Circulation 2021) found 25% more atrial fibrillation with fish oil supplements (HR 1.25). The risk was higher above 1 g a day (HR 1.49) than at 1 g or less (HR 1.12). Participants were 65 years old on average with higher cardiovascular risk, so this cannot be applied directly to young athletes. It does show that supplements are not harmless.

4. 4

   Muscles and training: at most a very small effect

   A meta-analysis in healthy young and older adults (Adv Nutr 2023) found no significant effect of omega-3 on muscle mass (SMD 0.07) or muscle function (SMD 0.03), and a very small effect on muscle strength (SMD 0.12). The certainty of the evidence was moderate and many studies had a high risk of bias. Dose, age and combining it with strength training did not change this.

5. 5

   Eating fish is the best-supported option

   For eating fish itself, the message is more consistent: about two servings of fatty fish a week, such as salmon, mackerel or herring (ACC overview, 2021: the benefit for fatal cardiovascular events appears from about that level). Over-the-counter fish oil is not routinely recommended for prevention. Plant-based ALA (flaxseed, walnuts) had little or no effect on mortality in the Cochrane review, with a small reduction in cardiovascular events and arrhythmias.

Updated in October 2026