There is insufficient evidence to say that intake of fish oil supplements (high in omega-3 fatty acids DHA and EPA) reduce the risk of heart or other disease.
1) Omega-3 intake for cardiovascular disease (Cochrane, 2018):
Moderate- and high-quality evidence suggests that increasing EPA and
DHA has little or no effect on mortality or cardiovascular health
(evidence mainly from supplement trials). Previous suggestions of
benefits from EPA and DHA supplements appear to spring from trials
with higher risk of bias.
2) Fish oil supplementation and insulin sensitivity: a systematic review and meta-analysis (Lipids in Health and Disease, 2017):
This systematic review and meta-analysis showed that fish oil
supplementation did not affect estimates of insulin sensitivity
overall. However, short-term fish oil supplementation could improve
insulin sensitivity among patients with metabolic disorders, which
could be a significant intervention as secondary prevention for the
T2DM and metabolic syndrome.
3) How does high DHA fish oil affect health? A systematic review of evidence (Tandofline, 2018):
Although there is sufficient evidence to strongly suggest that DHA
plays a series of important and fundamentally beneficial and health
promoting roles, the currently available scientific literature
describing the roles and potential metabolic effects of DHA, as an
individual nutrient and/or in comparison to EPA, is far from
conclusive. The exact mechanisms and the extent of these actions are
not yet fully elucidated. DHA appears to play important roles,
different to those of EPA, in heart, cardiovascular, brain and visual
4) Omega-3 Supplements: In Depth (National Center for Complementary and Integrative Health, 2018):
- Research indicates that omega-3 supplements don’t reduce the risk of heart disease. However, people who eat seafood one to four times a
week are less likely to die of heart disease.
- High doses of omega-3s can reduce levels of triglycerides.
- Omega-3 supplements may help relieve symptoms of rheumatoid arthritis.
- For most other conditions for which omega-3 supplements have been studied, the evidence is inconclusive.
5) FDA Announces New Qualified Health Claims for EPA and DHA Omega-3 Consumption and the Risk of Hypertension and Coronary Heart Disease (The US Food and Drug Administration, 2019):
The agency found that while there is some credible evidence suggesting
that combined intake of EPA and DHA from conventional foods and
dietary supplements may reduce the risk of hypertension by lowering
blood pressure, this evidence is inconclusive and highly
Omega-3 supplements and vegetarians/vegans
Most vegans refuse to take fish oil supplements (here: table 5), so this is why there are no related studies. In one small 1996 study, the intake of DHA from algae for 6 weeks increased DHA and EPA levels and slightly decreased LDL/HDL ratio in vegetarians.
Vegetarians and vegans, in general, consume much less saturated fats than omnivores (Nutrients, 2014).
According to one 2017 systematic review, vegan diets are associated with a decreased risk of ischemic heart disease and cancer, which triggers the question if fish and fish oil supplements are that beneficial at all.
In one 2016 review, they have found an association between vegetarian diets and lower risk of ischemic heart disease and diabetes.
Here are also two small studies: in one, vegan and vegetarian diets were associated with significantly lower levels of triglycerides and LDL cholesterol and in another one, vegan, but not vegetarian diets, were associated with lower LDL levels compared to omnivores.
This very limited, but quite conclusive, evidence suggests that vegans and vegetarians who tend to consume less saturated fat and have lower cholesterol and triglyceride levels than omnivores may not benefit significantly from omega-3 supplements.
It is also good to know that fish oil can reduce triglyceride, but not LDL cholesterol levels (Pharmacy and Therapeutics).