Cancer prevention: not all fiber helps.

Cancer prevention: Not all fiber helps

In the past, studies have linked diets high in fiber with a reduced risk of colon cancer -- a cancer whose incidence in the United States trails only that of the lung and breast. But dietary fiber comes in many forms. While many breakfast-cereal makers have extolled the potential anti-cancer benefits their products' fiber may offer, a new study now suggests that it's fruits and vegetables -- not grains -- that offer the most beneficial fiber.

Researchers asked 231 colon-cancer patients and 391 other randomly selected individuals to recall the foods, portions and preparation methods typifying their diets. Both groups -- comprising mostly Mormons -- contained only white, Utah residents. Based on their responses, researchers used three different processes to measure fiber in foods mentioned by study participants -- and thereby gauge their consumption of fiber.

The crude-fiber process yields mostly the insoluble cellulose and lignin. A detergent-processed fraction yields roughly the same cellulose and lignin, together with much of the soluble fiber. Fort the best estimate of "total fiber," an "imputed-fiber" measurement relies on computer analysis of sugars and starch in a sample.

In general, the study found a strong dose-response relationship between crude fiber and protection against colon cancer. In contrast, there was only a "weak, inconsistent" suggestion of a link between the detergent fraction and decreased cancer risk -- and no link between risk and imputed dietary fiber.

While this trend held for both sexes, there was a difference in the apparent sites being protected. For women, crude fiber had a greater protective effect against cancer of the ascending (right) colon. That quarter of the study population consuming the lowest proportion of this fiber fraction were twice as likely as the top quarter of crude-fiber eaters to develop cancer of the descending (left) colon and 3.3 times as likely to develop right-colon cancers. In men, however, crude fiber was more protective of the left colon. And the detergent-separated fraction, strongly associated with protection against right-colon cancers in men, had no affect on women.

In terms of actual food, decreased colon-cancer risk was most strongly associated with men eating diets high in fruit, and with women eating many vegetables, according to a report of the study in the NOV. 16 JOURNAL OF THE NATIONAL CANCER INSTITUTE. Somewhat unexpectedly, notes epidemiologist Martha L. Slattery, who headed this study at the University of Utah School of Medicine in Salt Lake City, neither group experienced any apparent protective effect from eating grains.

Due to this study's small size, its findings must be viewed more as trends than as strong, quantitative measures of a fiber fraction's protective role, says Elaine Lanza of the National Cancer Institute (NCI) in Bethesda, Md. And, she notes, its trends are very interesting. She points, for example, to the finding that while crude fiber, an indicator of insoluble fiber, was most associated with colon-cancer risk reduction, foods whose fiber was almost exclusively insoluble -- grains -- offer no similar risk reduction.

To Padmanabhan P. Nair, studying diets and cancer prevention at the Agriculture Department's Human Nutrition Research Center in Beltsville, Md., what really makes this a "very valuable paper," is the new direction it suggests for future researchers to follow. Lanza agrees. In fact, concern that some fiber fractions may be selectively protective led NCI this year to launch a three-year analysis of 400 foods for total fiber and soluble, insoluble, hemicellulose, cellulose and lignin fractions. Once these data are available, Lanza says, "We'll reanalyze a lot of these studies" in hopes of identifying more specific-fiber trends.