Colorectal cancer is the third most common cancer in the U. S. According to the American Cancer Society, an estimated 105,500 new cases of colon and 42,000 new cases of rectal cancer will be diagnosed in 2003. Colon cancer is a slow-growing form of cancer that is often preceded by identifiable changes in the lining of the colon or rectum. One type of change is the development of polyps —growths that project from the lining of the colon or rectum. Some types of polyps are benign, while others—such as adenomatous polyps—may be precancerous.

As many as 30 years ago, researchers began observing what they believed to be an association between high fiber diets and lowered risk for colorectal cancer. Since then, a number of large case-control studies have supported these initial observations, finding an inverse relationship between the amount of fiber in the diet and the development of precancerous adenomas. However, over this same period of time, a number of clinical trials have failed to support these findings.

Recently, a group of researchers from the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial set out to assess, once again, the relationship between high fiber diets and the risk for colorectal cancer. The results of this analysis were published in the May 3, 2003 issue of The Lancet. The researchers found that high fiber diets, particularly those emphasizing fiber from grains, cereals, and fruits, were associated with a significantly decreased risk of colorectal cancer.

About the study

Researchers compared the fiber intake of 33,971 participants determined to be free of colorectal adenomas (polyps) by sigmoidoscopy with 3,591 participants who had at least one verified adenoma in the lower half of their large bowel (descending colon, sigmoid colon, or rectum).

Using a 137-item food frequency questionnaire, the researchers assessed each participant’s usual dietary intake for the year prior to their enrollment in the PLCO study. The questionnaires were designed to control for 11 other potential cancer risk factors including past smoking behavior, history of cancer and other diseases, red meat intake, level of physical activity, weight, alcohol use, calcium intake, and other factors known to affect an individual’s overall risk for cancer.

The findings

The study found that, after adjusting for other potential risk factors, participants with the highest level of dietary fiber intake had as much as a 27% lower risk for developing precancerous colon polyps that those with the lowest levels of dietary fiber intake. Dietary fiber from grain, cereals, and fruit was the type most strongly associated with this reduction in risk.

This same reduction in risk did not apply to rectal adenomas.

How does this affect you?

While not claiming to be the decisive vote in the matter, and acknowledging that the study was not a randomized controlled trial (which generally provide the most reliable results), the researchers emphasize the importance of their findings based on the fact that this was a large study with a large number of participants who were consuming diets over a wide range of fiber intake levels, criticisms which have been levied against other studies on this topic.

Also coming down on the yes side of the argument, is a second study published in the same issue of The Lancet , which supports the findings of the PLCO study. This study, using similar research techniques, found not only that there was an inverse relationship between high fiber diets and the risk for colorectal cancer, it also concluded that doubling the amount of fiber in those with the lowest fiber intake, could potentially reduce their risk of colon cancer by as much as 40%.

No doubt the debate will continue. Nevertheless, since there are no serious risks associated with diets high in fiber, and there may be other benefits, it seems more than reasonable to recommend such a diet for reducing the risk of colorectal cancer, especially given the results of these two studies.