For over a decade, fish oil has been touted by doctors, nutritionists, and armchair health enthusiasts alike as a near cure-all for health. Whether you have heart disease, depression, diabetes, or joint or skin problems, or you just want to stay healthy, somebody has probably told you to take a fish oil supplement.

The general notion was that it might help, and at the very least, it couldn’t hurt. Unfortunately, that isn’t necessarily the case. Fish oil supplements continue to gain in popularity, but the research supporting their efficacy is shaky.

Does fish oil really prevent heart disease?

It’s safe to say that the benefits of fish oil supplementation for heart health have been significantly overstated. But a majority of the evidence available now suggests that fish oil provides no benefits for preventing or improving heart disease.

For example, two randomized controlled trials (RCTs) published in 2010 found that in adults with preexisting heart disease, long-term supplementation (three-plus years) with fish oil had no significant impact on cardiovascular end points.

Some studies do still come up with positive results. For example, one meta-analysis published in 2013 found a protective effect of fish oil for preventing cardiac death, sudden death, and myocardial infarction. But there are also studies with negative results. Overall, the majority of studies show neither benefit nor harm from supplementing with fish oil for heart disease.

Does fish oil improve metabolic syndrome?

Metabolic syndrome is a collection of symptoms and biomarkers that often precedes heart disease or diabetes.

On the positive side, a recently published RCT found that in adults with metabolic syndrome, supplementation with 3g/d of fish oil along with 10 mL/d of olive oil for 90 days improved several blood markers. This includes a statistically significant lowering of triglycerides and LDL cholesterol, an improvement in LDL/HDL ratio, and improved markers of oxidative stress.

It’s interesting to note that the fish oil plus olive oil group had better results than either the fish oil or olive oil group alone. One possible reason for this is that olive oil is rich in antioxidants and may have protected against the potentially greater risk of oxidative damage from consuming more polyunsaturated fat.

On the negative side, a recent study in women with metabolic syndrome found that 3g/d of fish oil resulted in an increase in LDL cholesterol, blood glucose, and markers for insulin resistance after 90 days, although they did observe a decrease in blood pressure. And in overweight men, supplementation with 5g per day of krill and salmon oil resulted in increased insulin resistance after eight weeks, compared with a canola oil control.

As you can see, the evidence for fish oil supplementation for metabolic syndrome is mixed, with some studies showing a benefit, others showing harm, and still others showing no significant effect either way.

Recently, attention has been drawn to the quality of over-the-counter fish oil supplements. Long-chain polyunsaturated fatty acids, including DHA and EPA, are especially susceptible to oxidation due to double carbon bonds at multiple locations.  Light, oxygen exposure, and heat can all contribute to oxidation. Oxidized lipids have been linked to a number of health issues, including organ toxicity and accelerated atherosclerosis—the exact opposite outcomes usually desired by those who supplement with fish oil.

In 2016, the top three selling fish oil supplements in the United States were shown to have oxidation levels up to four times higher than recommended “safe” levels. As oxidation level measurements of omega-3 supplements have increased over the last several years, this has been the common finding. Studies examining fish oil supplements available around the world, including in Canada, New Zealand, and South Africa, consistently show that a vast majority (up to 80 percent!) exceed at least one of the measures of acceptable oxidation levels. Also noteworthy is that most of these supplements contain lower levels of DHA and EPA than the labels claim, probably partly due to oxidation.

Furthermore, the most recent study from 2016 demonstrated that over-the-counter omega-3 supplements had a decreased ability to inhibit small, dense LDL oxidation in a laboratory setting compared to pure omega-3 fatty acids. This means that whatever supposed benefits omega-3 supplements should have on blood lipids could likely be completely counteracted by the oxidized lipids also contained in the pills.

I think we still have a lot to learn about this subject. One of the challenges is that the effects of polyunsaturated fats on overall physiology are complex and probably depend on multiple factors that can vary individually, including uncontrolled oxidation, eicosanoid production, cell membrane effects, and signal transduction via specialized fatty acid receptors (i.e., PPAR receptors). This could explain why we see such a wide variation in study results. Is it possible that 3g/d of fish oil is beneficial for one person and harmful for another? Absolutely. Unfortunately, at this point, it’s difficult to predict that individual response with accuracy and certainty.

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