Studies in animals and a couple of small trials in people suggest that fish oil supplementation can fight inflammation and may have cancer-fighting properties, Dr. Sangmi Kim of the National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina, and colleagues note. But so far studies looking at the diets of larger groups of people have had equivocal results.
To investigate further, Kim and colleagues examined the relationship between polyunsaturated fatty acid intake and bowel cancer risk in 1,503 whites (including 716 colon cancer patients and 787 healthy controls) and 369 African Americans (213 with colon cancer, 156 controls).
Among whites, the researchers found, those in the top fourth based on their omega-3 consumption had half the risk of colon cancer compared to those in the bottom fourth.
When the researchers looked separately at the two main fatty acids contained in fish oil --eicosapentaenoic acids and docosahexaenoic acids -- they found risk also fell with increasing intake.
When the researchers looked at whites and blacks together, they also found a reduced risk of colon cancer with increasing omega 3 intake; separate analysis of the black study participants didn't find this relationship.
They also found that people who consumed more omega-6 fatty acids in relation to omega-3s were more likely to have colon cancer, although omega-6 intake in and of itself didn't affect risk.
In addition to fish oils, omega-3 fatty acid sources include seed oils, such as walnut oil and flax-seed oils, and leafy green vegetables. People in the US typically eat more omega-6 fatty acids than omega-3s; top sources include palm oil, soybean oil, and sunflower oil.
The researchers also found an "unexpected" association between higher omega-3 intake and colon cancer in African-Americans, but urged caution in interpreting this finding, which they say "may have been due to chance." Nevertheless, they conclude, "whether the possible benefit from this dietary modification varies by race warrants further evaluation."