Something Fishy about Fish Oil

Dudley S. Danoff, MD, FACS

The research reported a 71 percent higher risk of dangerous high-grade prostate cancer among men who ate fatty fish or took fish oil supplements. These findings were widely reported in the media and generated telephone calls from many of my patients.

Alan Kristal, researcher at the Fred Hutchinson Cancer Research Center and 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 appear to be linked to a greater risk of prostate cancer, the findings suggest that these acids are somehow involved in the formation of tumors. A large European study also found the same link between omega-3 fatty acids and prostate cancer.

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 cautious. 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 the 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 to associations. Because the study contains a slew of undocumented variables and contradictory findings, it might be worthwhile pointing out the participants and their possible confounding risk factors:

• 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 Roundtree, MD, chief medical officer at Thorne Research.

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 go back to the age-old medical adage Do No Harm. Doctors generally do not recommend fish oil supplements because the true benefits are unclear.

Even though this published study did not evaluate the supplements themselves but rather the blood levels of omega-3 fatty acids, the data strongly supports the connection between omega-3 fatty acids and the incidents of high-grade prostate cancer. 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. To date, I have not found that compelling reason.