Note: This article was originally published in June 2015 and was updated in January 2017 to include the latest research. My original recommendations still stand, and the case for high-dose fish oil supplementation has become even weaker.
Fish oil supplements continue to gain in popularity, but the research supporting their efficacy is shaky.
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 and prevent nutrient deficiencies, 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.
Does Fish Oil Really Prevent Heart Disease?
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. (2, 3)
A meta-analysis of RCTs in 2012, focusing on cardiovascular end-points, found that fish oil did not reduce cardiovascular events or death and concluded that the evidence does not support using fish oil supplements for the secondary prevention of heart disease. (7)
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. (11)
Is it possible that fish oil is beneficial for one person and harmful for another? #fishoil
But there are also studies with negative results. Back in 2010, I wrote an article highlighting one study where long-term fish oil supplementation resulted in an increase in heart disease and sudden death and another that found increased LDL levels and insulin resistance in people who took 3g per day of fish oil. (12, 13)
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. (14)
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. (15)
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. (16)
Finally, an impressively large RCT involving over 12,500 patients with diabetes, elevated fasting glucose, or impaired glucose tolerance found that supplementation with 1g/d of omega-3s for six years did not reduce disease endpoints compared to placebo. Endpoints measured included incidence of cardiovascular events, death from cardiovascular events, and death from all causes. (17)
Can Fish Oil Prevent Cancer? Or Does Fish Oil Cause Cancer?
Many of you probably recall headlines from 2013 proclaiming that fish oil may increase the risk of prostate cancer (18). But despite the extensive media attention garnered by the study, it’s actually one of the weaker cases that have been brought against fish oil.
Believe it or not, the study in question actually had nothing to do with fish oil, or even omega-3 supplements. The researchers simply measured circulating levels of omega-3 fatty acids in men with and without prostate cancer and found that men with prostate cancer tended to have higher concentrations of omega-3s in their blood.
There are several reasons this could be the case; for instance, some evidence indicates that having prostate cancer might itself increase blood levels of omega-3s, or that certain genetic polymorphisms can increase both circulating omega-3s and cancer risk.
It didn’t take long for other researchers to publish a slew of comments pointing out these possibilities, but the media had already taken the “fish oil causes cancer” stance and run with it.
More recently, a meta-analysis found that in general, omega-3 consumption is associated with a decreased risk of prostate cancer, but that the correlation is too weak to be statistically significant. (19) In 2016, a massive meta-analysis looked at 44 studies and concluded overall that higher omega-3 supplementation had no effect on prostate cancer mortality (20).
A handful of reviews found that fish oil intake was associated with a lower risk of breast cancer, although no distinction was made between fish oil supplements and fish consumption. (21, 22) And one RCT published in 2012 found that supplementation with 600mg of omega-3s per day had no effect on cancer risk in men, but increased cancer risk in women. (23)
As with heart disease and metabolic syndrome, the research on omega-3 and fish oil supplementation on cancer is decidedly mixed.
High Levels of Oxidative Products Found in Fish Oil Supplements
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. (24)
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. (25) One caveat of this alarming study is that oxidation levels were normalized per 1g of omega-3s in the supplements, instead of the industry standard of normalizing per 1g of fish oil. Although this does inflate their three measures of oxidation, all three fish oil brands were still above acceptable levels of peroxidase and TOTOX levels, while one (instead of the study’s reported two) was above acceptable anisidine levels if instead normalized per 1g of fish oil.
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. (26, 24, 27) 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. (25) 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. Overall, I am quite wary of most over-the-counter products out there.
Should You Take Fish Oil?
To avoid making this article so long that nobody will read it, I haven’t included research on fish oil and other aspects of health, including mental health, skin health, pregnancy, and cognitive function. As you might imagine, the research on fish oil supplementation to prevent or improve these conditions is also somewhat mixed, with some studies showing significant benefit and others showing no change.
This is certainly an important topic, and I’m glad to see such a strong interest in it in the research community. I will continue to follow the literature and update my recommendations if and when new evidence comes to light, but for the time being this is what I would suggest:
If you are generally healthy, the best strategy is to consume about 12 to 16 ounces of cold-water fatty fish or shellfish each week. When possible, whole foods are always my first recommendation. Most studies show an inverse relationship between fish consumption and heart disease and mortality, so while fish oil may not protect you, eating fish does seem to. Perhaps this is because fish and shellfish contain many other beneficial nutrients that fish oil does not, including selenium, zinc, iron, and highly absorbable protein. (Fortunately, most cold-water fatty fish and shellfish are also low in mercury and other toxins, and mercury in fish may not be as big a problem as some have led us to believe.)
If you don’t eat fish (for whatever reason), I’d suggest supplementing with 1 teaspoon of high-vitamin cod liver oil. In addition to about 1.2 g of EPA + DHA, it is rich in the active forms of vitamin A and vitamin D, both of which are difficult to obtain elsewhere in the diet. There are very few studies suggesting the possibility of harm from supplementation with 1 gram or less of fish oil per day, and so I think one teaspoon of cod liver oil a day is likely to be safe even for those eating fish regularly—and beneficial for those not eating liver or other foods that contain active vitamin A. My current favorite cod liver oil is Rosita Extra Virgin Cod Liver Oil, as this company has consistently demonstrated very low levels of oxidative products from independent laboratory testing.
Based on the evidence I’ve reviewed in this article, I would not recommend consuming high doses of fish oil (i.e., more than 3g/day) over the long term. If you do choose to take a higher dose of fish oil, I would make sure to consume plenty of antioxidant-rich foods, like olive oil; blueberries; nuts; dark, leafy greens; and dark chocolate.
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, so I think the conservative approach I suggested above is probably the most sensible until we learn more.