Eating Fish and Risk of Colorectal Cancer in Men
Men who ate fish five times a week or more had a 40 percent lower risk of developing colorectal cancer compared to men who ate fish less than once a week, according to a new analysis of data from 22,071 participants in the Physicians' Health Study (PHS).
The researchers say the reduction in colorectal cancer risk is substantial in comparison to other dietary components, and while they don't suggest that everyone starts eating fish daily simply because of these results, they say the health benefits of fish consumption have already been proven.
"We already know that eating fish can reduce the risk of sudden cardiac death, and this might provide another reason to add fish to your diet," said Megan Phillips, a doctoral student at the Harvard School of Public Health and the lead author of this study.
The researchers believe the health effects of fish consumption in relation to colorectal cancer may lie in their content of the n-3 fatty acids that can inhibit the cyclooxygenase-2 (COX-2) enzyme. This enzyme acts as a mediator of inflammatory responses thought to be associated with cancer development.
The PHS was designed as a randomized, double blind, placebo-controlled clinical trial to examine the effect of aspirin and beta-carotene supplements on development of cancer and cardiovascular disease, and the participants filled out a one-time food questionnaire 12 months after starting the study. In this analysis, investigators were also trying to determine if fish consumption had a different effect on men who received aspirin for five years compared to men who weren't randomized to use aspirin, which is also a COX-2 inhibitor. "We thought that maybe for men who received aspirin, it wouldn't matter whether they ate fish or not," Phillips said.
The researchers looked at four different categories of fish consumed - tuna fish, dark meat fish (salmon, sardines, bluefish, etc.), a general fish category, and shellfish including shrimp, lobster and scallops - and asked how many times the participants ate them on average during the previous year. They found almost 10 percent ate fish less than once a week, 31 percent ate it less than two times a week, 48 percent ate fish less than five times a week, and about 11 percent ate it five times or more a week. They then compared these figures with incidence of colorectal cancer that later developed in the men. (The average follow-up was 19.4 years).
They found that compared to men who ate the least amount of fish, the risk of developing colorectal cancer was 40 percent lower in men who ate the most fish, was 20 percent lower in men who ate fish 2-5 times a week, and 13 percent lower among participants who ate fish less than two times a week.
The relationship between fish consumption and colorectal cancer was similar for men randomized to aspirin and those who weren't, possibly because the researchers only had information on aspirin use during the first five years in the trial, and "it may take more years of aspirin use to see an effect," Phillips said.
While she said the results are promising, Phillips also noted that they are based on the assumptions that the pattern of fish consumption observed in the sole food questionnaire represented a diet that the men subsequently followed for many years.
In addition, men who ate more fish may also have a healthier lifestyle perhaps including better cancer screening. Although this study controlled for some of these factors such as cigarette smoking, vigorous exercise, and multivitamin use, the investigators do not have information on colorectal endoscopies. Thus, these findings need additional confirmation through other prospective studies with more complete information and a definitive answer might require a randomized trial, said senior author Jing Ma, M.D., Ph.D., a researcher at the Brigham and Women's Hospital-based Channing Laboratory and associate professor of medicine at Harvard Medical School.