Though the research, which covered more than 5,000 people and appeared in The American Journal of Clinical Nutrition, didn't prove that seafood protects against polyps, earlier experiments in animals have showed that the omega-3 fatty acids in fish can reduce the risk of colorectal cancer.

About 140,000 new cases of colon and rectal cancer are diagnosed each year in the United States, and the lifetime risk of developing it is about 20 per cent, according to the American Cancer Society.

"(The research) does increase our confidence that something real is going on," said Edward Giovannucci, a professor at the Harvard School of Public Health, who didn't work on the study.

The idea that researchers have been pursuing is that the omega-3 fats in fish might have an anti-inflammatory effect, similar to aspirin, that could prevent the growth of polyps.

For the latest study, researchers led by Harvey Murff, a professor at Vanderbilt University in Nashville, Tennessee, surveyed more than 5,300 people about their eating habits. All had come in for a routine colonoscopy.

The team then compared more than 1,400 women without polyps to 456 who had polyps, also called an adenoma, detected during the procedure.

Among women with polyps, 23 per cent were in the bottom fifth among fish eaters, while 15 percent were in the top fifth. That means that people who eat lots of seafood are somehow protected, because otherwise the percentages should have been the about the same.

After accounting for differences such as age, smoking and aspirin use, women who ate the most fish, three servings a week, were 33 per cent less likely to have a polyp detected than those who ate the least, or less than a serving a week.

But other factors explaining the findings can't be ruled out. For instance, fish lovers may have other healthy behaviors that decrease their risk of polyps.

The study also didn't follow the women to see whether either group was more likely to go on to develop cancer, but Murff said that polyps are a reliable predictor of cancer risk.

"You would think most things that would reduce adenoma risk would subsequently reduce cancer risk," he said.

The men in Murff's study who ate a lot of fish did not have the same reductions in polyp risk as women, however.

Murff said he didn't have a good explanation for that, but that perhaps men are less sensitive to the omega-3s in fish and need to eat more to get the benefit.

It could also be that men eat more omega-6 fats, found in a broad range of foods including chicken and nuts, which counteract the impact of the omega-3s. Omega-6 fatty acids are related to the production of a hormone called prostaglandin E2, which is associated with inflammation.

Eating Omega-3 fatty acids keeps down the level of Omega-6 acids, which in turn cuts levels of the hormone, Murff said. He noted that the women in the study who ate more fish -- and presumably, more Omega-3s -- had lower levels of the hormone.

"We know people who have higher levels of this (hormone) are more likely to develop colorectal cancer. So in essence, by eating more Omega-3 fatty acids, it's almost like taking an anti-inflammatory medication," he told Reuters Health. SOURCE: http://bit.ly/zWMCyQ

-Reuters