Fatty acids and prostate cancer: lessons learned from the SELECT study

Headlines were made when the Selenium and Vitamin E Cancer Prevention Trial (SELECT) announced that plasma phospholipid fatty acids can increase the risk of prostate cancer.

This surprising finding was published in the Journal of the National Cancer Institute. Several different experts agree that there are limitations in the trial that make the link between omega-3s and prostate cancer questionable.

As a result, the study 'should not change clinicians’ dietary recommendations or prescribing patterns' for omega-3s, said Douglas 'Duffy' MacKay, ND, vice president of scientific and regulatory affairs for the Council for Responsible Nutrition.

SELECT is a randomised, placebo-controlled trial of 35,533 men over age 50. For the case-cohort on fatty acids, the researchers identified 834 participants with primary prostate cancer, along with 1364 case-matched controls without prostate cancer.

Researchers analysed the men’s venous blood samples at baseline for levels of alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), docosahexaenoic acid (DHA), linoleic acid (LA), arachidonic acid (AA) and trans-fatty acids (TFA). The subjects were placed into quartiles based on each fatty acid’s weight percentage of total plasma phospholipids.

The researchers found that compared with men in the lowest quartile, the subjects in the highest quartile of long-chain omega-3 fatty acids (EPA, DPA and DHA) had a 44% higher risk of low-grade prostate cancer, a 71% higher risk of high-grade prostate cancer and a 43% higher risk of all grades of prostate cancer.

ALA and AA weren’t associated with an increased risk of any type of prostate cancer, and LA actually reduced the risk of low-grade and total prostate cancer by a non-significant 25%. The data on TFA was inconclusive.

MacKay explained that there were numerous 'red flags' in the trial. First of all, SELECT was designed to assess the effects of selenium and vitamin E supplementation, so subjects weren’t given fish or fish oil supplements and there was no documentation of whether they took them on their own.

In addition, plasma phospholipid omega-3 levels are heavily influenced by recent intake and thus aren’t a good measure of medium or long-term intakes and their correlations to chronic diseases like cancer.

Finally, these findings directly contradict results from other studies, as well as dietary recommendations from the American Heart Association, the World Health Organisation, the US Institute of Medicine’s Food Nutrition Board and the 2010 Dietary Guidelines.