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, a professor at the Harvard School of Public Health in Boston, who was not involved in this study.
A polyp, also called an adenoma, is a mushroom-shaped tag of tissue that grows in the colon and can develop into colorectal cancer.
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.
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.
In the latest study, the researchers surveyed more than 5,300 people about their eating habits. All of the participants had come in to the researchers' practices for a 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 percent were in the bottom fifth among fish eaters, while 15 percent were in the top fifth. That means people who eat lots of seafood are somehow protected against polyps, because otherwise the percentages should have been the same.
After accounting for differences like age, smoking and aspirin use, women who ate the most fish -- three servings a week -- were 33 percent less likely to have a polyp detected than those who ate the least -- less than a serving a week.