US researchers were stunned to discover that higher blood levels of omega-3 fatty acids, normally promoted as good for the heart, were linked to higher risk of aggressive prostate cancer, and conversely, raised levels of trans-fats, considered bad for the heart, were linked to a lower risk.

However, neither of the fats was found to be linked to risk of low-grade prostate cancer.

These were the findings of the largest study ever to look at links between dietary fats and prostate cancer risk.

Lead and corresponding author Theodore M. Brasky, of the Fred Hutchinson Cancer Research Center in Seattle, Washington, told the press:

"We were stunned to see these results and we spent a lot of time making sure the analyses were correct."

He and colleagues in the Hutchinson Center's Public Health Sciences Division, and also from the University of Texas Health Science Center at San Antonio and the National Cancer Institute, analyzed a subset of data on more than 3,4000 men taking part in the Prostate Cancer Prevention Trial, a large nationwide randomized clinical trial, involving more than 19,000 men aged 55 and over, that tested the performance of the drug finasteride in preventing prostate cancer.

Their paper was published this week in the American Journal of Epidemiology. Brasky, a postdoctoral research fellow in the Hutchinson Center's Cancer Prevention Program, said:

"Our findings turn what we know - or rather what we think we know - about diet, inflammation and the development of prostate cancer on its head and shine a light on the complexity of studying the association between nutrition and the risk of various chronic diseases."

Brasky and colleagues found that men with the highest blood levels of docosahexaenoic acid, or DHA, an omega-3 fatty acid that lowers inflammation and is common in fatty fish, had 2.5 times the risk of developing aggressive, high-grade prostate cancer compared to men with the lowest blood leves of DHA.

They also found that men with the highest blood percentages of trans-fatty acids, which have been linked to inflammation and heart disease, and are common in processed foods made with partially hydrogenated vegetable oils, had a 50% reduced risk of developing high-grade prostate cancer.

In contrast, they found neither of the two types of fatty acids was linked to raised risk of low-grade prostate cancer.

They also found that omega-6 fatty acid, common in most vegetable oils, and which has been linked to inflammation and heart disease, was not linked to raised risk of either high-grade or low-grade prostate cancer.

The results stunned the researchers, whose prime motive was to test the idea that concentrations of these various types of fatty acids in the blood would show some relationship with prostate cancer risk, in a similar pattern to their various links with inflammation and heart disease risk.

"Specifically, we thought that omega-3 fatty acids would reduce and omega-6 and trans-fatty acids would increase prostate cancer risk," said Brasky.

Why omega-3s should be linked to raised risk of high-grade prostate cancer is a mystery to the researchers.

Brasky said they need to do more research on it, but speculated that perhaps omega-3s affect biological mechanisms other than inflammation, and these have a bigger effect on the development of some prostate cancers.

Although many nutritionists and doctors recommend 450 mg daily dose of omega-3 DHA as part of a healthy diet, there are currently no official guidelines in the US on recommended daily allowance.

Half of the participants in the nationwide trial developed prostate cancer during the course of the study, and half did not. The trial is considered unique because all participants underwent biopsy to confirm the presence or absence of prostate cancer.

Few of the participants took fish oil supplements, the most common non-dietary source of omega-3 fatty acids, which are known to prevent heart disease and other diseases caused by inflammation. Most of the participants got their omega-3s from consuming fish.

Brasky and colleagues don't think that these findings should discourage men from continuing to get their omega-3s from oily fish or food supplements.

"Overall, the beneficial effects of eating fish to prevent heart disease outweigh any harm related to prostate cancer risk," said Brasky, stressing that the study highlights the complexity of nutrition and its links with disease risk, and we need to study them much more rigorously before jumping to conclusions.

Funds from the National Cancer Institute helped pay for the study.

"Serum Phospholipid Fatty Acids and Prostate Cancer Risk: Results From the Prostate Cancer Prevention Trial."
Theodore M. Brasky, Cathee Till, Emily White, Marian L. Neuhouser, Xiaoling Song, Phyllis Goodman, Ian M. Thompson, Irena B. King, Demetrius Albanes, and Alan R. Kristal
Am. J. Epidemiol. first published online 24 April 2011
DOI:10.1093/aje/kwr027

Additional source: Fred Hutchinson Cancer Research Center.

Written by: Catharine Paddock, PhD