BLOG: Does fish oil cause prostate cancer?
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I received a comment on my last blog regarding the study linking omega-3 fish oil and prostate cancer. There have been several responses to this published study, and I think this warrants yet another review.
The original Selenium and Vitamin E Cancer Prevention Trial (SELECT) was intended to determine the effects of vitamin E, alone or with selenium, on prostate cancer. In the original study, more than 35,000 subjects were enrolled, and the results showed a 17% increased risk of prostate cancer among those taking 400 IU of vitamin E supplementation. This most recent meta-analysis of that study looked at the high plasma phospholipid concentration of long-chain omega-3 polyunsaturated fatty acids and showed an association with an increased risk of prostate cancer. However there are several limitations to this latest review. Among those are:
- The original study was designed to evaluate the relationship between prostate cancer and vitamin E, not omega-3 fatty acids.
- This was an observational study, which means that it can only show an association and not a cause-and-effect relationship between omega-3s and prostate cancer risk.
- Specific variables with regard to the omega-3 fatty acids were not addressed. This included, but was not limited to, the type of omega-3 fatty acid supplemented, quantity of omega-3 fatty acid intake, distinction between dietary intake and supplementation, and/or quality of omega-3 fatty acid intake.
- Red blood cell analysis of DHA and EPA is the conventional and accepted marker for measuring omega-3 fatty acid intake. Blood plasma evaluation, as performed, in this study, may not yield an adequate assessment.
- Singular measurements of fatty acid intake were taken at the beginning of the study only.
- The difference in mean plasma omega-3 levels (EPA plus GPA plus DHA) in the combined prostate cancer groups vs. the control group of men without prostate cancer was tiny: 4.66% vs. 4.48%, respectively, or about a 0.2% difference. This difference could have occurred by consuming a single serving of fish prior to the blood draw for data collection in the original study.
- The authors acknowledge that no mechanism is known that would explain how omega-3 increases prostate cancer risk. In fact, inflammation is believed to play a causal role in many cancers, and the omega-3s possess anti-inflammatory actions.
When speaking on how to evaluate nutrition research, I am always cautious when I review the process of reading a study that is actually a meta-analysis. This is not a true study, but a study of studies, taking data that has already been accumulated and sorting it out by other statistical methods. Thus, any meta-analysis should be reviewed with caution and an open mind.
I am glad to see that doctors are reading research studies and hope they will continue to do so. Unfortunately, many studies have critical flaws, and we should use caution when applying data from one study to make broad generalizations.