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Omega-3 fatty acids may have a link to aggressive prostate cancers, while the long-maligned trans fats may protect against prostate tumors.

In a new study, researchers at the Fred Hutchinson Cancer Research Center in Seattle examined levels of omega fatty acids and trans-fatty acids in the blood from a group of 3,461 men over age 55. The men were part of the Prostate Cancer Prevention Trial, which ran from 1994 to 2003. About half of these men had prostate cancer.

The researchers expected that omega-3s would reduce the risks of prostate tumors while trans fatty acids would up the risk, but they were surprised to learn that the opposite appeared to be true. High levels of omega-3 fatty acids (specifically DHA) were associated with a 2.5-times increased risk of high-grade tumors. These are the cancers that grow faster and have a greater likelihood of spreading than lower-grade tumors.

Men in the study who had the highest levels of two kinds of trans fatty acids - which has been widely regarded as the "bad" fat - had about half the risk of prostate cancer of those with lower levels.

The researchers published their findings this month in the American Journal of Epidemiology.

"We were stunned to see these results and we spent a lot of time making sure the analyses were correct," lead researcher Theodore Brasky said in a news release.

But the researchers caution that the apparent association could possibly be the result of other diet or lifestyle factors, and nobody should avoid omega-3 fatty acids and increase their consumption of trans fats based on the results of this study.

"Overall, the beneficial effects of eating fish to prevent heart disease outweigh any harm related to prostate cancer risk," Brasky said.

While prostate cancer is a concern for many men, heart disease is the leading cause of death in males.