US scientists said Wednesday they have confirmed a surprising 2011 study that found a higher risk of prostate cancer among men who consume omega-3 fatty acids, raising new questions about the safety of supplements.
The research in the Journal of the National Cancer Institute reported a 71% higher risk for dangerous high-grade prostate cancer among men who ate fatty fish or took fish-oil supplements, which are often touted for their anti-inflammatory properties.
"We've shown once again that use of nutritional supplements may be harmful," said Alan Kristal, researcher at the Fred Hutchinson Cancer Research Center and senior author of the paper.
Scientists are still puzzled as to why omega-3s appear linked to a greater risk of prostate cancer, but they said the findings suggest they are somehow involved in the formation of tumours.
The same team of researchers published similar findings in 2011, linking high blood concentrations of DHA to a more than double risk of high-grade prostate cancer, which is more likely to be fatal than other types.
A large European study also found the same omega-3 and prostate cancer link.
"The consistency of these findings suggests that these fatty acids are involved in prostate tumorigenesis and recommendations to increase long-chain omega-3 fatty acid intake, in particular through supplementation, should consider its potential risks," the US study said.
The difference in blood concentrations of omega-3 fatty acids between the highest and lowest risk groups was about 2.5 percentage points (3.2% vs. 5.7%), or just higher than the effect of eating salmon twice a week, Kristal said.
The latest study was based on an analysis of specimens and data from a large randomized, controlled trial that tested whether selenium and vitamin E would reduce prostate cancer risk.
The trial, known as SELECT (the Selenium and Vitamin E Cancer Prevention Trial), found that vitamin E raised the prostate cancer risk and selenium showed no impact either way.
For the July 11 study in the Journal of the National Cancer Institute, researchers analyzed the data on 834 men diagnosed with prostate cancer and compared them to a random sample of 1 393 taken from the SELECT trial.
Those who had high blood concentrations of the fatty acids EPA, DPA and DHA were shown to have a 71% increased risk of high-grade prostate cancer.
Supplements may do harm
The increased risk of low grade prostate cancer was 44% higher in those with elevated fatty acid levels, and the combined risk was 43% for all prostate cancers.
Due to the nature of the study, it was not possible to tell for certain whether the elevated blood levels were due to men taking supplements or eating fish rich in omega-3s.
However, Franklin Lowe, associate director of the department of urology at St Luke's-Roosevelt Hospital in New York, said the findings should remind consumers that supplements may not help, and may even do harm.
"In general, there is nothing that has been proven to actually limit the risk of prostate cancer," said Lowe, who was not involved in the study.
"For the most part, doctors do not recommend this stuff because it is unclear what the true benefits are for most of the supplements that people take."