Fish Oil Supplements and Your Health: The Good and the Bad

You may have heard that fish oil is good for you. You may even have heard of omega-3 fatty acids, the components of the oil credited with its health benefits.

Perhaps you’ve even taken a capsule of the stuff yourself, hoping it could boost your brain power or your cardiovascular health.

But is there any truth to these commonly held beliefs? Is it really the case that seafood lovers are healthier than those of us less well-oiled? Let’s take a look at the research.

The good

Fatty fish such as anchovies, herring, mackerel and sardines are particularly rich in two major omega-3 fatty acids: docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA).

Fish oil and the heart

This natural abundance is good news for anyone who regularly eats such fish, because decades of research has linked DHA and EPA to better cardiac health. Indeed, the American Heart Association recommends people eat two servings of fatty fish per week.

Much of the evidence for DHA and EPA’s health-boosting effects comes from population-correlation data.

For instance, in 2018, the largest prospective cohort study ever conducted (involving 421,309 participants) reported a significant inverse association between EPA and DHA intake (from eating fish) and various mortality outcomes. On average, both men and women who ate the most fish had 10% fewer cases of cardiovascular disease mortality than the participants who ate the least amount of fish.

So, how do DHA and EPA actually benefit cardiac health? It’s thought they reduce the blood’s level of triglycerides (the main constituent of body fat in humans), high levels of which are linked to a greater risk of heart attack and stroke.

A recent study (involving 441,000 participants) even found that those with a genetic predisposition to higher triglyceride levels had lower levels than expected when they regularly consumed omega-3 supplements.

DHA and EPA are also thought to improve circulation, prevent blood clots, lower blood pressure and maintain a healthy heart rhythm.

Fish oil and the brain

Higher up in the body, the fishy compounds also appear to benefit the brain.

According to a recent review of randomized controlled trials investigating the relationship between omega-3 acids and brain function, ingestion of DHA and EPA increases learning, memory, cognitive well-being and blood flow in the brain. The authors even concluded that “lonelier people, the elderly and those who eat fewer healthy foods containing omega-3 may benefit from an omega-3 supplement.”

Clinical trial data published after that review also concluded that a subset of older adults with a genetic predisposition to Alzheimer’s disease may benefit from fish oil supplements, although the study found no statistically significant benefit for all older adults in general.

Other studies have demonstrated that DHA in particular plays a key role in a child’s brain and eye development, as well as the fluidity and function of cellular membranes.

After scanning the brains of participants and measuring the omega-3 fatty acid levels in their blood, a study published in 2022 found that a higher omega-3 index was associated with larger hippocampal volumes. DHA and EPA consumption was also associated with better abstract reasoning.

“We saw the worst outcomes in the people who had the lowest consumption of omega-3s,” Claudia Satizabal, an assistant professor of population health sciences at University of Texas Health San Antonio and co-author of the 2022 paper, said at the time. “So, that is something interesting. Although the more omega-3, the more benefits for the brain, you just need to eat some to see benefits.”

The fatty acids have even demonstrated cognitive improvements when injected. In a recent study on newborn rodents, an injectable emulsion of DHA and EPA markedly reduced brain damage in newborn rats after a disruption in the flow of oxygen to the brain near birth.

The bad

So, judging by this pile of research, fish oil tablets seem like the ideal supplement, don’t they? Time to glug one down and enjoy the cardiac and cognitive benefits!

But other research into omega-3 acids hasn’t been quite as glowing. In fact, some studies have questioned the positive conclusions of others.

Fish oil and the heart, a second opinion

Published in the New England Journal of Medicine in 2018, one randomized, placebo-controlled trial (involving 25,871 participants) found that omega-3 supplements didn’t result in a lower incidence of major cardiovascular events or cancer when compared to a placebo.

More concerningly, some supplement studies have observed an apparent increased risk of cardiac events.

Published in BMJ Medicine this year, one study (involving 415,737 participants) found that those who regularly took fish oil tablets and had no initial cardiovascular disease went on to have a 13% higher risk of developing atrial fibrillation and a 5% higher risk of having a stroke.

For those with pre-existing cardiac conditions, however, the omega 3-rich fish oil did appear to provide its usual purported benefits; among these participants, the supplements were associated with a 15% lower risk of progressing from atrial fibrillation to a heart attack and a 9% lower risk of progressing from heart failure to death.

The researchers behind the study said their findings suggested that fish oil supplements may have different roles in the progression of cardiovascular disease depending on the health of a person.

Other researchers, who were not involved in the study, did caution such conclusions, though.

“It is quite likely that the association between taking cod liver oil and cardiovascular health outcomes could be a product of other factors,” Dr. Duane Mellor, a registered dietician and spokesperson for the British Dietetic Association, told Technology Networks at the time.

“On its own, cod liver oil supplements would have a tiny effect if any on cardiovascular disease risk, so the findings of this study are not that significant. It is the overall lifestyle in terms of dietary pattern (of which only meat and vegetables were considered in this study) and not other foods, known to decrease risk of heart disease,” they said.

Fish oil and the brain, a second opinion

Back up in the brain, other studies have hinted that DHA and EPA’s cognitive benefits may have their limits.

One paper published in 2018 found no evidence that omega-3 fish oil supplements aid or improve the reading ability of under-performing schoolchildren.

Published in 2019, a review of 31 clinical trials – which all investigated the effects fish oil could have on anxiety and depression – found the fatty acids had little or no effect on the conditions.

And while older adults with a predisposition towards Alzheimer's disease may benefit from omega-3 supplements, one study from 2020 claimed that extraordinary doses may be needed to have any effect.

The researchers noted dramatic increases in blood levels of omega-3s were accompanied by far smaller increases within the brain. At the end of the 6 month-long trial, participants who took omega-3 supplements had 200% more DHA in their blood compared to those who took placebos, however, the DHA found in cerebrospinal fluid of the omega-3 group was only 28% higher than the control group’s samples. 

Final thoughts

Fish oil is certainly a wholesome food component, and anyone who eats fish regularly can enjoy their seafood all the more knowing their triglyceride levels may just be a tad lower, and their thinking that bit sharper.

Fish oil supplements, however, may not be quite as enriching. Given that the much of the compelling evidence for the cardiac benefits of omega-3 acids comes from studies following seafood consumption – and several trials of actual fish oil supplements showed no cardiac effect – those oily capsules on the pharmacy shelves may only be beneficial to certain people, such as vegans and those with a genetic predisposition to higher triglyceride levels.

That said, anyone with such a condition or any other coronary disease should ideally only take omega-3 supplements in addition to treatments arranged by a medical professional.