I was boiling some pasta for dinner once when my daughter and I both noticed a fishy smell (something we both detest) permeating the kitchen. Since we never cook fish in our house, that seemed odd. It took a moment to locate the source – I had unknowingly purchased a box of pasta that had been fortified (or poisoned, depending on your perspective) with omega-3 fatty acids. This became known in my house as the “fish-pasta incident.”
Omega-3 fatty acids, also known as fish oil, is what give fish that fishy taste. That’s unfortunate for those like me who have sensitive taste and cannot tolerate even the slightest hint of fish oil, even in pill form.
Are omega-3 fatty acids, in pasta form or whatever, a net health benefit, however? A recent study which is getting a lot of press suggests that omega-3 may be associated with a higher risk of prostate cancer in men. The cardiac benefits of omega-3 have also recently been called into question. Let’s sort through the data to see what it says.
Omega-3 and Cancer
Prior research suggests that there may be an association between omega-3 and prostate cancer, but this kind of data can be tricky because it is mostly epidemiological. In other words, it is correlational only, and cannot prove causation. There are no double-blind placebo controlled trials in which omega-3 vs placebo is given to randomized subjects who are then followed to see who develops prostate cancer.
The new study is also epidemiological – it is an analysis of data from the SELECT trial (Selenium and Vitamin E Cancer Prevention Trial). They found:
This study confirms previous reports of increased prostate cancer risk among men with high blood concentrations of LCω-3PUFA. The consistency of these findings suggests that these fatty acids are involved in prostate tumorigenesis. Recommendations to increase LCω-3PUFA intake should consider its potential risks.
The study design seems solid, and supports a growing consensus of studies that there is some association. However, the study did not measure survival, so we don’t know the net effects of taking omega-3 on mortality.
Interestingly, omega-3 and polyunsaturated fats in general have been studied to see if they reduce cancer risk. A 2006 systematic review concluded:
A large body of literature spanning numerous cohorts from many countries and with different demographic characteristics does not provide evidence to suggest a significant association between omega-3 fatty acids and cancer incidence. Dietary supplementation with omega-3 fatty acids is unlikely to prevent cancer.
So omega-3 does not prevent cancer, and may increase prostate cancer risk, at least for high-grade prostate cancer.
Omega-3 and Heart disease
Most people who take omega-3, however, take it for the prevention of heart disease. Omega-3 promotes the “good” cholesterol – the kind that promotes removal of fatty acids from the inside of arteries. The “bad” cholesterol (increased by saturated fats) promotes the deposition of fatty acids on the lining of arteries.
That’s a nice story, but the history of biology and medicine is such that neat, simple stories like this rarely reflect the complex and ugly reality. Supplements have multiple possible effects in the body requiring careful study from multiple angles to sort out. Perhaps the best type of evidence is from those studies that look at net outcomes – do people who take omega-3 live longer, for whatever reason?
A 2009 systematic review of omega-3 and cardiovascular events found:
The mortality benefit was largely due to the studies which enrolled high risk patients, while the reduction in nonfatal cardiovascular events was noted in the moderate risk patients (secondary prevention only). Meta-regression failed to demonstrate a relationship between the daily dose of omega-3 fatty acid and clinical outcome.
These results are a bit encouraging, but mixed. There was no clear difference in net clinical outcome. However, there also seems to be a protective effect of omega-3 for those at high risk of cardiovascular disease. This review, however, is from 2009, which is ancient history in the world of medicine.
A 2012 meta-analysis concluded:
Our meta-analysis showed insufficient evidence of a secondary preventive effect of omega-3 fatty acid supplements against overall cardiovascular events among patients with a history of cardiovascular disease.
A separate 2012 systematic review and meta-analysis concluded:
Overall, omega-3 PUFA supplementation was not associated with a lower risk of all-cause mortality, cardiac death, sudden death, myocardial infarction, or stroke based on relative and absolute measures of association.
Ooh – that’s a one-two punch for omega-3. It’s not looking good, which is too bad. It would be nice to have a relatively low-cost supplement without significant side effects that could reduce the risk of such a common cause of death.
On the bright side, however, the study found no increased risk of all-cause mortality from omega-3 supplementation. If there were an increased risk from prostate cancer or anything else it might show up in this data.
Conclusion
At this point it looks like there may be an increased risk of high-grade prostate cancer from omega-3. Further, there may be a small benefit for those at high risk of cardiovascular disease. However, the best evidence we have to date does not show any statistically significant net health harm or benefits.
One can always argue that the studies were not sufficient to detect the benefit, but since we now have systematic reviews of many large studies, any net benefit (or harm) must be very small in order to be missed by these studies.
What this data also shows is the complexity of medical research in general, and evaluating the net health effects of any intervention. Omega-3 is a dietary supplement, something you can get in food. This makes it seem safe. However, even supplements can have multifarious and complex effects in the body, and sorting out all these effects can take time and many studies.
The meta-lesson here is to be skeptical of the latest supplement being touted with simplistic stories of their health effects and benefits. Also, preliminary studies are unlikely to give confident answers or tell the whole story. It takes years, and many rigorous studies, to sort out the potential harms and benefits of any intervention, even something that seems as benign as fish oil.
Steven Novella, M.D. is the JREF's Senior Fellow and Director of the JREF’s Science-Based Medicine project.