Women who eat about three servings of fish per week have a somewhat lower chance of having polyps found during a routine colonoscopy than women who eat just one serving every two weeks, according to a new study.
The research doesn't prove that seafood protects against polyps, but it "does increase our confidence that something real is going on", said Dr Edward Giovannucci of the Harvard School of Public Health in Boston, who was not involved in this study.
The idea 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 development of polyps.
Dr Giovannucci said that earlier experiments in animals have showed that omega-3 fats can reduce the risk of this cancer, but that studies of humans have had mixed results.
Three servings a week
In the latest study, reported online in the American Journal of Clinical Nutrition, the researchers surveyed more than 5 300 people about their eating habits before they underwent colonoscopy.
The team then compared more than 1 400 women without polyps to 456 who had adenomas detected during the procedure.
Among women with adenomas, 23% were in the bottom fifth among fish eaters, while 15% were in the top fifth.
After accounting for differences like age, smoking and aspirin use, women who ate the most fish – three servings a week – were 33% less likely to have a polyp detected than those who ate the least – less than a serving a week.
Of course, it's never possible to rule out the possibility that other factors could explain the findings. For instance, it's possible that fish lovers have other healthy behaviours that decrease their risk of polyps.
What's more, the study didn't follow the women to see whether either group was more likely to go on to develop cancer.
In the study, which was led by Dr Harvey Murff at Vanderbilt University in Nashville, Tennessee, men who ate a lot of fish did not see the same reductions in polyp risk as women.
Dr Murff told Reuters Health he doesn't have a good explanation for that, but perhaps men are less sensitive to the omega-3s in fish and need to eat more to get any benefit. It could also be that men might eat more omega-6 fats, counteracting the effects of the omega-3s.
Dr Murff said eating omega-3 fatty acids tamps down the body's levels of omega-6 fatty acids. In turn, the body then has reduced levels of prostaglandin E2.
He and his colleagues demonstrated this by showing that the women in the study who ate more fish – and presumably, more omega-3s – had lower levels of prostaglandin E2.
"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," Dr Murff told Reuters Health.
There is still more work to be done to prove that the omega-3 fatty acids are actually the reason for the reduced colorectal cancer risk.
But, Dr Giovannucci said, "I think this association has a pretty strong biologic rationale."
(Kerry Grens, Reuters Health, February 2012)
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