CHICAGO -- Two studies shed new light on the link between diet and cancer, bolstering evidence that red meat may raise colorectal cancer risks but casting doubt on whether fruits and vegetables can help prevent breast cancer.
The new research doesn't settle the questions, partly because both studies asked about eating habits only in adulthood. Some researchers think that may have less impact on cancer risk than lifelong eating habits.
Breast cancer risk, especially, may be more dependent on a woman's diet during adolescence, when breast cells are rapidly dividing and are more vulnerable.
Still, both studies, published in today's Journal of the American Medical Association, are consistent with evolving thinking about specific foods and their influence on cancer risks.
In numerous previous studies examining diet and cancer, the relationship between meat consumption and colorectal cancer is the among the strongest, with most finding that eating lots of red meat and processed meats increases the risk.
The new study, led by American Cancer Society researchers and involving 148,610 men and women whose average age was 63, is among the biggest. Participants recorded their meat intake in 1982 and again in 1992 to 1993. Those with a high meat intake were about 30 to 40 percent more likely to develop lower colon or rectal cancer than those with a low intake. High meat intake for men was at least 3 ounces daily -- about the size of a large fast-food hamburger -- and 2 ounces daily for women. Low intake was about 2 ounces or less of red meat no more than twice weekly for men and less than an ounce that often for women.
Study coauthor Dr. Michael Thun, the cancer society's epidemiology chief, said the results should be put into perspective: Smoking, obesity, and inactivity are still thought to be more strongly linked with colon cancer than eating lots of red meat.
The breast cancer study, involving 285,526 European women, found no protective effect from fruits and vegetables in women followed for an average of about five years. Studies on whether diets rich in fruits and vegetables might protect against various cancers including breast, colon, and stomach cancer have had mixed results, though no effect was seen in the more recent research on breast cancer.