Diets high in fiber do not reduce the likelihood of colon and rectal cancer, Harvard researchers have concluded.

The researchers followed 88,757 women for 16 years without finding evidence that fiber in whole-grain bread, cereals, fruits, and vegetables protect against colorectal cancers, as commonly believed. This was the largest study of this kind to date and the results were unexpected.

Another Harvard study of more than 47,000 men, done in 1994, also failed to find that high-fiber diets reduce the risk of these cancers.

"After adjustment for age, established risk factors, and total energy intake, we found no association between the intake of dietary fiber and the risk of colorectal cancer," says Charles Fuchs, assistant professor of medicine. Fuchs and his colleagues at the Harvard Medical School and Harvard School of Public Health report their findings in today's issue of The New England Journal of Medicine.

But don't give up on eating the recommended 25 to 35 grams of fiber a day, the scientists add. They found "strong associations" between dietary fiber and reductions in coronary heart disease, high blood pressure, non-insulin dependent diabetes, and diverticular disease.

In the early 1970s, the rarity of colorectal cancer in Africa led some scientists to propose that the high-fiber diets of Africans protected them against this malady. As appealing as the idea sounds, however, a conclusive link never was established. Most studies were not thorough enough to make a clear distinction between the effects of fiber and other ingredients of fruits, vegetables, and cereals, such as vitamins and minerals.

The Harvard team selected 88,757 women who participated in an ongoing study of nurses and followed them by questionnaires from 1980 through 1996. During this time, 787 cases of colorectal cancer occurred among the women, but the investigators found no evidence that a diet low in fiber offered any significant protection against that cancer or benign tumors in the colorectal area.

Despite the unexpected lack of protection, however, the researchers note that "there are cogent reasons for increasing fiber intake," particularly the association between fiber intake diets and a reduction in heart disease seen in many other studies.