The large-scale prospective study, published in the Journal of the National Cancer Institute, follows up on findings from 2011 which suggested that a high omega-3 status may be linked to prostate cancer.
Led by senior author Dr Alan Kristal from Fred Hutchinson Cancer Research Center, USA, the new follow up study confirms the findings of the earlier research by analysing data from a in a large European population study - finding that high blood concentrations of long chain omega-3 polyunsaturated fatty acids (PUFAs) were linked to a 71% higher risk of developing high grade prostate cancer.
The study also found a 44% increase in the risk of low-grade prostate cancer and an overall 43% increase in risk for all prostate cancers.
"We've shown once again that use of nutritional supplements may be harmful," said Kristal, who added that the findings of both the 2011 study and the recent research are surprising because omega-3 fatty acids are generally believed to have a host of positive health effects based on their anti-inflammatory properties.
"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," said the team.
"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," said corresponding author Dr Theodore Brasky, of The Ohio State University Comprehensive Cancer Center - and lead author of the 2011 study.
"It's important to note, however, that these results do not address the question of whether omega-3's play a detrimental role in prostate cancer prognosis," he said.
The new prospective study analysed data and specimens collected from men who participated in the Selenium and Vitamin E Cancer Prevention Trial (SELECT), a large randomised, placebo-controlled trial in Europe which aimed to test whether selenium and vitamin E, either alone or combined, reduced prostate cancer risk.
Using data from the SELECT study, the team analysed blood plasma omega-3 status for 834 men who had been diagnosed with prostate cancer - of which 156 were high-grade cancer - along with a comparison group of 1,393 men selected randomly from the 35,500 participants.
Compared with men with the lowest blood plasma levels of long chain omega-3 polyunsaturated fatty acids (PUFA) those with the highest levels had increased risks for low-grade (44% increased risk), high-grade (71% increased risk), and total prostate cancer (43% higher risk).
The team added that these associations were 'similar' for individual long-chain omega-3 fatty acids - noting that higher linoleic acid (omega-6) was associated with reduced risks of low-grade and total prostate cancer; but noting that there was no dose response.
The authors noted that it remains unclear from their results exactly why high levels of omega-3 fatty acids could increase prostate cancer risk, however, they suggested that replication of their finding in two large studies indicates the need for further research into possible mechanisms.
Sources: Journal of the National Cancer Institute
Published online ahead of print, doi: 10.1093/jnci/djt174
"Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial"
Authors: Theodore M. Brasky, Amy K. Darke, Xiaoling Song, et al