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Fish Oil Linked to Prostate Cancer?

Fish Oil Linked to Prostate Cancer?

Do you ever get the feeling that you are damned if you do and damned if you don’t? That could easily be the conclusion of a new study about omega-3 fatty acids and prostate cancer.

For the last 30 years we have been told to eat more fish to protect our hearts and our brains and improve our overall health. And if we can’t stand fish, we are told to take fish oil capsules.

Now here comes a study published in the prestigious Journal of the National Cancer Institute telling us that men who have higher levels of omega-3 fats circulating in their blood stream are at higher risk (44%) for prostate cancer compared to men with lower levels of such fats. The men with the highest levels of omega-3s were also at greater risk (71%) for aggressive prostate cancer compared to men who had lower levels.

And it’s not just supplements. Eating lots of fatty fish (salmon, tuna, trout, etc) may also pose a problem. The study involved 834 men diagnosed with prostate cancer. They were matched with roughly 1,400 men who did not develop cancer. Levels of omega-3 fats (DHA, EPA, DPA) were measured. Most of the men did not take fish oil supplements, so presumably their levels of omega-3 fats were correlated in large measure with the amount of fish they consumed.

This is not the first time omega-3 fats have been linked to prostate cancer. The same research group picked up a similar signal in 2011. They reported in the American Journal of Epidemiology that men with higher levels of DHA in their blood had significantly more high-grade prostate cancer than men with lower levels of this omega-3 fat.

The lead author, Thomas Brasky, PhD, was quoted:

“What’s important is that we have been able to replicate our findings from 2011 and we have confirmed that marine omega-3 fatty acids play a role in prostate cancer occurrence. It’s important to note, however, that these results do not address the question of whether omega-3s play a detrimental role in prostate cancer prognosis.”

In other words, even though fish oil is associated with a greater risk of being diagnosed with prostate cancer, the researchers don’t yet know whether this means men with high levels of omega-3 fats in their blood will die from prostate cancer faster than other men.


Ok, so that was the official story and what the media picked up on. Let’s dig a bit deeper.

This was NOT a placebo controlled trial that randomly assigned men to fish oil or placebo and tracked them for years. That would be the highest qualilty study and would more reliably answer the question whether fish oil actually caused prostate cancer. This epidemiological study could only detect an association.

The study took one snapshot of omega-3 fats in blood at the enrollment stage and did not determine  blood levels over time. The investigators did not know whether the levels of omega-3 fats in blood were because men were taking fish oil supplements or eating fish. There has been the suggestion that most of the men did not take supplements, so presumably their omega-3 blood levels were due to diet rather than pills. Dietary data was not included in the report, however. It seems counterintuitive that eating fish two or three times a week would increase the risk of prostate cancer. This issue clearly requires much greater investigation before we tell men to stop eating fish.


So, what are we to make of this latest research? First, we can only say that there may be an association between levels of omega-3 fats and prostate cancer but we cannot say eating fish or taking fish oil causes prostate cancer.

We can say that the cardiovascular benefits of fish oil supplements have been disappointing. A large meta-analysis of well-conducted studies in the Journal of the American Medical Association revealed that fish oil supplements did not seem to protect against death from heart attacks. And a large Italian study published in the New England Journal of Medicine on May 9, 2013 also produced disappointing results. Over 12,000 patietns were recruited for this research. They all were at high risk for heart attacks because of previous events or diagnosed heart disease. They were randomized to receive either fish oil supplements (1 gram daily) or placebo. These patients were followed for an average of five years. The fish oil supplements did not reduce death or disease from cardiovascular complications.

One begins to think that a lot of conventional health wisdom is falling by the wayside.

We are not ready to give up on fish, however. We still think that fish is a healthy food choice, but we are beginning to think that vegetarians may be on to something. And if a man is at high risk for developing prostate cancer because of family history or other factors, he might want to cut back on fish oil supplements until we have much better data on this controversial topic.

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About the Author
Joe Graedon is a pharmacologist who has dedicated his career to making drug information understandable to consumers. His best-selling book, The People’s Pharmacy, was published in 1976 and led to a syndicated newspaper column, syndicated public radio show and web site. In 2006, Long Island University awarded him an honorary doctorate as “one of the country's leading drug experts for the consumer.” .
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