This study was published the American Journal of Epidemiology this month. It involves a study of the effects of omega-3 and omega-6 fatty acids on the risk of prostate cancer. Omega-3 fatty acids, like those found in fish oil, are known to decrease inflammation, heart disease and cancer risk while omega-6 fatty acids and trans-fatty acids are pro-inflammatory and increase the risk of heart disease and cancers. This was a subset of 3000 patients taken from the prostate cancer prevention trial. In this subset of patients, 1/2 of them developed prostate cancer and 1/2 did not. The researchers compared blood levels of omega-3 DHA as well as trans-fatty acids. The main source for the omega-3 fatty acids in this trial was fatty fish, not fish oil capsules.
What they found was perplexing. The risk of aggressive prostate cancer was highest in the group of men who had the highest DHA levels. Conversely, those with higher transfat levels, had a 50% decrease in the risk of aggressive prostate cancer. Furthermore, neither fat affected the risk of low-risk prostate cancer.
These results are difficult to explain because of the known health benefits of omega-3 fatty acids. They by no means prevent enough data to make recommendations to stop eating fatty fish or fish oil capsules. Perhaps, since fish was the main source, mercury or some other toxin played a role in the findings of this study.
We do know that another source of omega-3 fatty acids, flaxseed, has direct anti-prostate cancer effects. So, I would recommend people continue taking fish oil capsules or eating fatty fish, as well as eating flaxseed every day.