>>> we are back with the story we mentioned from tonight's medical news. it will reverberate in a lot of american households. the chances you or someone you know is consuming more fish oil these days to protect the heart, whether from fish oil supplements or eating more of the right types of fish. today, we learned of a big potential downside for men getting high amounts of fish oil in their system and that's an increased risk of prostate cancer . story tonight from chief medical editor dr. nancy snyderman .

>> reporter: researchers followed more than 2,000 men over the age of 50 who did not have prostate cancer when the study began. blood from each man was stored analysis. men who started with the highest blood levels of omega 3 fatty acids , those found in fish oil supplements and fatty fish , had a higher risk of getting a deadly form of prostate cancer . they did not expect this link.

>> we know that omega three fatty acids decrease inflammation, but this was an association, but in the opposite direction what we expected.

>> reporter: a confusing message for men like 54-year-old bill ross who takes fish oil supplements to protect his heart. confusion, because bill also has a family history of prostate cancer .

>> my father is a prostate cancer survivor. battling prostate cancer now for about 20 years. and so, you know, i'm at risk.

>> reporter: conventual wisdom for years, fish oil protects against cancer and heart disease , but cardiologists say you don't need fish oil supplements and a good diet with oily fish two or three times a week is enough.

>> i am never a proponent of supplements. more is not necessarily better. studies look at the intake of foods high in omega 3s, antioxidants, follate, we see consistent protection for cancer and heart disease .

>> the leading trade council representing supplement calls the study overblown. the study demonstrates no cause and effect , it can only purport to show an association between higher mass wra levels of omega 3 fatty as iz and those whom the researcher advise had a higher rate of prostate cancer . this has people talking, but the american heart association is urging people to get their omega three acids from their food, and that means from the right kind of fish. salmon, mackerel, herring, three times a week. and too much of a good thing can be too much for the human body . people will keep talking about this one.