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 are consistent with evolving thinking about specific foods and their influence on cancer risks. The studies are published in Wednesday's Journal of the American Medical Association.
In numerous previous studies examining diet and cancer, the relationship between meat consumption and colorectal cancer is 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 aged 63 on average, is among the biggest. Participants recorded their meat intake in 1982 and again in 1992-93. 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.
Slightly higher risks were found for a high consumption of processed meats including bacon and bologna.
Study co-author 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.
Still, Thun said, the results support cancer society dietary guidelines recommending against heavy meat consumption and favoring a variety of healthful foods.
The breast cancer study, involving 285,526 European women, found no protective effect from fruits and vegetables in women questioned about diet and 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 some of the more recent research on breast cancer.
The results don't rule out that a diet rich in fruits and vegetables might reduce breast cancer risks for certain subgroups of women, including those with a family history of breast cancer, said lead author Dr. Petra Peeters of University Medical Center Utrecht in The Netherlands.
But even if they don't help prevent breast cancer, fruits and vegetables, as well as limiting red meat intake, are good for the heart, said Dr. Walter Willett, a Harvard University nutrition expert and author of a book promoting those habits.
"Fortunately, substituting pistachio-encrusted salmon and gingered brown basmati pilaf for roast beef with mashed potatoes and gravy is not a culinary sacrifice," Willett said in a JAMA editorial accompanying the studies.
By Lindsey Tanner