As you know, omega-3 fatty acids are very unsaturated (polyunsaturated). As such, they are prone to oxidation, they go rancid easily, both before we consume them, and inside our bodies. That oxidation is thought to be one way in which they raise the risk for prostate cancer:
The Relevance Of Serum Levels Of Long Chain Omega-3 Polyunsaturated Fatty Acids And Prostate Cancer Risk: A Meta-Analysis, Canadian Urological Association Journal, 2013
Reports have also shown that marine fish contaminated with environmental toxins, such as polychlorinated biphenyls or methylmercury compounds, can disrupt androgen and estrogen balance and could be linked to high-grade prostate cancer.
Furthermore, the presence of long chain n-3 PUFA (DHA and EPA) in the prostate cell’s beta-oxidative metabolic process leads to the formation of lipid hydroperoxides [products of oxidation] in the microenvironment of the cell; this can generate reactive species. With chronic exposure to these reactive molecules, the prostate cell can become dysplastic and develop into an aggressive cell.
Plasma Phospholipid Fatty Acids And Prostate Cancer Risk In The SELECT Trial, Journal of the National Cancer Institute, 2013
In conclusion, in this large, prospective study of plasma phospholipid fatty acids and prostate cancer risk, contrary to our expectations, we found that long-chain ω-3 PUFA overall, and DPA and DHA in particular, were associated with strong, linear increases in prostate cancer risk. We note that this is not a novel finding because it has been reported previously in two other prospective blood biomarker studies that have examined the associations between long-chain ω-3 PUFA and prostate cancer risk.
This study confirms previous reports of increased prostate cancer risk among men with high blood concentrations of LCω-3PUFA [EPA and DHA, what you find in fish oil]. The consistency of these findings suggests that these fatty acids are involved in prostate tumorigenesis. Recommendations to increase LCω-3PUFA intake should consider its potential risks.
Why on Earth would anyone, especially men, take omega-3 supplements? The algae-based ones are a marketing ploy designed to appeal to vegans. They contain the same DHA as fish oil, at several times the price.