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,^[43] and their blood levels of EPA and DHA tend to be lower than meat-eaters'.^[44]

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,^[45] 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.^[46]

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,^[47] 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,^[48] as well as a reduced risk of developing type 2 diabetes.^[49] Higher blood levels of EPA and DHA might also be protective against cognitive decline and dementia,^[47] 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.^[50][51] (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.^[52])

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.^[53]