Omega-3 fatty acids from fish (such as mackerel, trout, and salmon) or from fish oil supplements have been previously described to decrease heart disease and cancer risk. A recent study from multiple centers in the US, however, showed that men with the most intake of omega-3 fatty acids had a 43% increased risk of prostate cancer (both low grade (44%) and high-grade prostate cancer (71%)) versus men with the least intake. This is similar to other studies in which men with the highest serum DHA (docosaheaenoic) acids had a 2.5 times increased risk of high-grade prostate cancer (the type that often requires treatment and can be dangerous). It is possible that omega-3 fatty acids are converted into DNA damaging compounds. However, higher levels of omega-3 fatty acids have been associated with favorable sperm characteristics (morphology).
The >2000 men in this study had participated in the SELECT (Selenium and Vitamin E Cancer Prevention Trial) study which demonstrated that vitamin E supplementation was associated with elevated prostate cancer risk (17%) with more than five years of follow-up. Moderation, even of nutrients such as omega-3 and vitamin E, is essential for optimal health.