A meta-analysis of 35 small, randomized trials found that fish oil can slightly improve depression when compared to control. However, this improvement may be too small to be noticeable. Also, adding fish oil to antidepressant medication seems to be more beneficial than antidepressant treatment alone.^[8] Fish oil incorporates into the cell membrane phospholipid layer, increasing its fluidity and permeability, which in turn might reduce inflammation and attenuate depressive symptoms. Another meta-analysis suggested that doses of 1-2 grams daily containing ≥60% proportion of total EPA + DHA seem to be especially beneficial.^[9]

The short answer is “yes”, but it ultimately depends on your goal. For the purpose of covering the minimum amount of omega-3 fatty acids required for general health, it’s not necessary to consume fish or fish oil; you can just eat flaxseed.

Flaxseed is a rich source of alpha-linolenic acid|alpha-linolenic acid (ALA), the only essential omega-3 fatty acid (i.e., the only omega-3 that can’t be synthesized by the body). Following ingestion, ALA is converted into the longer chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the same fatty acids found in fish oil.

Animal-based foods are the primary sources of EPA and DHA in the diet. Consequently, vegetarians and vegans consume trivial amounts of these omega-3 fatty acids,^[18] and their blood levels of EPA and DHA tend to be lower than meat-eaters'.^[19]

Despite the near exclusion of EPA and DHA from plant-based diets, these diets are not associated with adverse health effects or impairments in cognitive development,^[20] which suggests that the average ALA intake of people following these diets allows for sufficient endogenous synthesis of EPA and DHA. Furthermore, the consumption of a healthy plant-based diet is associated with a variety of health benefits, such as a reduced risk of chronic diseases like cardiovascular disease and type 2 diabetes.^[21]

However, while it’s evident that the average person can cover their basic omega-3 fatty acid needs through ALA-rich foods (like flaxseed) alone, that doesn’t mean that ALA-only intake is optimal, at least with respect to certain health outcomes.

Higher intakes of EPA and DHA are associated with a number of health benefits,^[22] and higher blood levels are, too. A higher blood level of EPA and DHA is associated with a reduced risk of all-cause mortality and death from cardiovascular disease or cancer,^[23] as well as a reduced risk of developing type 2 diabetes.^[24] Higher blood levels of EPA and DHA might also be protective against cognitive decline and dementia,^[22] but more high-quality studies are needed to confirm whether this is the case.

Only consuming flaxseed may mean missing out on the potential benefits of higher-than-adequate levels of EPA and DHA, because the available evidence suggests that supplementing with ALA does not significantly increase the blood level of EPA and DHA.^[25][26] (To be more specific, supplementing with ALA seems to increase the blood level of EPA to a small degree, but it either doesn’t affect or even slightly decreases the blood level of DHA, typically resulting in no effect on the combined level of EPA and DHA.^[27])

As such, to maximize certain health outcomes, it might be best to consume direct sources of EPA and DHA — either in the form of fatty fish or fish oil, or, for vegetarians and vegans, in the form of algal oil — but further research is needed to confirm this.^[28]

There are some mixed views on whether fish oil can cause abnormal heart rhythms or not. Generally speaking, most healthy people don’t need to worry: A meta-analysis of 30 randomized studies and 77,990 patients shows that taking fish oil does not affect the risk of atrial fibrillation.^[13] However, in people who have (or are at risk for) heart disease, there’s a possibility that fish oil can increase the risk of new-onset atrial fibrillation. Several meta-analyses of randomized studies at least one year long in people with or at risk for cardiovascular disease show that fish oil can increase the risk of new-onset atrial fibrillation compared to control.^[14][15][16] It’s important to note that, while this increase in risk is statistically significant, it’s still small in absolute terms and there is no known mechanism to explain this risk.

For most people, regular fish oil supplements do not affect bleeding risk. One meta-analysis of 11 randomized studies with over 80,000 patients concluded (with low certainty) that fish oil does not increase bleeding risk,^[13] and another meta-analysis, which stratified by type of fish oil supplement, reported similar findings. Interestingly, the latter meta-analysis noted that EPA-only supplements did increase bleeding risk compared to control, but the absolute increase in risk was very small: about 0.5%, suggesting that for every 200 people taking an EPA-only supplement, one might experience bleeding.^[17]