Fish oil might not be the cure-all it’s often advertised to be, and in some cases, it may even cause problems.
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, joint or skin problems, or just want to stay healthy, somebody has probably told you to take a fish oil supplement.
The general notion was (and often still is) 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?
It’s safe to say that the benefits of fish oil supplementation for heart health have been significantly overstated. As I mentioned earlier, studies initially found that fish oil was beneficial for heart disease, particularly over the short term and for secondary prevention. (1)
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 (3+ years) with fish oil had no significant impact on cardiovascular end points. (2, 3)
A meta-analysis of RCTs 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) Two other meta-analyses published around the same time came to similar conclusions. (8, 9)
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. (10)
Is it possible that fish oil is beneficial for one person and harmful for another?
But there are also studies with negative results. Back in 2010, I wrote an articlehighlighting 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. (11, 12)
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. (13)
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. (14)
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. (15)
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 include incidence of cardiovascular events, death from cardiovascular events, and death from all causes. (16)
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.
Can fish oil prevent cancer? Or does it cause cancer?
Many of you probably recall headlines from 2013 proclaiming that fish oil may increase the risk of prostate cancer. 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. (17)
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. (18, 19) 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. (20)
As with heart disease and metabolic syndrome, the research on omega-3 and fish oil supplementation on cancer is decidedly mixed.
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 12–16 ounces of cold-water, fatty fish or shellfish each week. 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 form 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, 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 EVCLO.
Based on the evidence I’ve reviewed in this article, I would not recommend consuming high doses of fish oil (i.e. >3 g/d) 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 3 g/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.