Blog | Monday, August 26, 2013

Prostate cancer--a fish tale


Have you heard that fish oil supplements cause prostate cancer? The news items come from an article published online July 11, 2013 by Brasky et al. in the Journal of the National Cancer Institute, "Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial."

This study looked at 834 men diagnosed with prostate cancer, and compared them with 1,393 men, matched for age and race, that did not have prostate cancer. They then looked at the amount of omega-3 in their blood and compared the groups.

They found that those with the highest omega-3 levels had the highest risk of prostate cancer, 44% higher overall. This study followed up on one published earlier by the same group that suggested increased risk from fish consumption. The levels of omega-3 in the highest group were fairly modest, equivalent to eating an oily fish, such as salmon, twice a week.

Before jumping to the conclusion that men should not take fish oil or consume much fish, there are a number of things to consider. First of all, association does not imply causation. What does that mean? Just because two things occur together, it does not mean that one caused the other. If you look outside on a rainy day, you will see many people carrying umbrellas. But you would be wrong to conclude that carrying umbrellas caused it to rain.

It could be that it wasn't the omega-3 in fish, or fish oil supplements, that caused prostate cancer, but rather something else in the products, such as mercury or other toxins in the fish. If you ate fish raised in places low in pollution, or consumed ultra-filtered fish oil, then perhaps it would not be a problem. This study does not answer that question.

Even if eating fish or taking supplements increases the risk of prostate cancer, studies have shown it decreases the risk of cardiovascular (heart) disease, which is far more common.

This study was not the preferred double-blind, placebo controlled study, and the conclusion may just be wrong. After all, other studies have shown that fish consumption decreases prostate cancer. For example, one in the Lancet showed decreased risk of prostate cancer in those who ate moderate or high amounts of fish. Also, consider that Japanese men consume much more fish then American men, yet have far less prostate cancer.

So until I see more convincing evidence, I'll continue to take my fish oil capsules, and enjoy eating salmon.

Daniel Ginsberg, MD, FACP, is an internal medicine physician who has avidly applied computers to medicine since 1986, when he first wrote medically oriented computer programs. He is in practice in Tacoma, Washington. This post originally appeared on his blog, World's Best Site.