The medical community is in the midst of a fierce debate over whether fish oil supplements are more likely to decrease or increase the risk and severity of prostate cancer. Studies in 2011 and 2013 found a higher incidence of prostate cancer among men who consumed omega-3 fatty acids, but a 2016 experiment found that fish oil hindered the growth of prostate tumors in mice. These conflicting results have raised new questions about the safety of fish oil supplements.

The 2013 study reported a 71 percent higher risk of aggressive prostate cancer among men with high levels of omega-3 fatty acids in their blood. Alan Kristal, senior author of the study, said, “We’ve shown once again that use of nutritional supplements may be harmful.” Although scientists are still puzzled as to why omega-3 fatty acids would increase the risk of prostate cancer, the findings suggest that these acids are somehow involved in the formation of tumors. Researchers concluded that “the consistency of these findings suggests that these fatty acids are involved in prostate tumorigenesis and recommendations to increase long-chain omega-3 fatty acid intake, in particular through supplementation, should consider its potential risks.”

Vocal critics of the study, however, have pointed out reasons to be skeptical. First, no fish oil supplements were given to the subjects, and no crossover studies were conducted. Researchers merely looked at blood levels of long-chain fatty acids, such as EPA and DHA, which are found in fish. Second, the study was based only on accumulated data from participants in the Prostate Cancer Prevention Trial, which was conducted from 1993 through 2003. And third, the study reported only an observation that levels of omega-3 fatty acids and incidents of high-grade prostate cancer were found together in this particular population.

The critics further emphasize that “correlation is not causation.” Observational studies like this one are not randomized or controlled; they simply point out associations. Because the study contains a slew of undocumented variables and contradictory findings, the possible confounding risk factors should be mentioned:

  • Fifty-three percent of the subjects with prostate cancer were smokers.
  • Sixty-four percent of the cancer subjects regularly consumed alcohol.
  • Thirty percent of the cancer subjects had at least one first-degree relative with prostate cancer.
  • Eighty percent of the cancer subjects were overweight or obese.

These statistics were compiled by Robert Rountree, MD, chief medical officer at Thorne Research.

On the other side of the debate, proponents of fish oil supplements point out that prostate cancer is much less frequent among Japanese and Inuit men, who traditionally consume at least eight times the amount of fish that Western men do. In addition, a 2016 study indicated that omega-3 fatty acids may fight the progression of prostate cancer. This experiment compared prostate tumor growth between mice that were fed fish oil and mice that were fed corn oil and found that the tumors remained significantly smaller in the mice on the fish oil diet.

Of course, these findings must also be interpreted with caution. The adage “correlation does not equal causation” bears repeating: the lower rates of prostate cancer in Japanese and Inuit populations may not be caused by high omega-3 fatty acid intake but by another factor, such as lower consumption of saturated fats or high-glycemic foods. As for the mouse study, the difference in tumor volume between the two groups of mice might not indicate the protective benefits of fish oil but the well-known fattening and inflammatory effects of corn oil. And of course, mice may not process fish oil the same way humans do.

I have not yet seen a good peer-reviewed randomized controlled trial testing the effects, negative or positive, of omega-3 fatty acids. Until I do, I must abide by the age-old medical adage “Do No Harm.” Doctors generally do not recommend fish oil supplements because the true benefits are unclear. Therefore, under no circumstances would I take these supplements, nor would I recommend them to my patients unless I had a compelling reason to do so.


Photo by exoimperator, CC BY-SA 2.0