For more than 30 years, the association between a lower risk of colorectal cancer and increased amounts of dietary fiber has been a matter of controversy. Studies using different methodologies and outcome measures have given contrasting results. Two studies published in the same issue of Lancet conclude that dietary fiber intake is inversely related to the risk of large bowel cancer.

In the first study, Peters and colleagues compared the fiber intake of about 34,000 participants in a large cancer screening trial who had no polyps on sigmoidoscopy with about 3,600 participants who were found to have at least one adenoma in the distal bowel. Questionnaires were used to gather sociodemographic information and detailed information about food intake. The researchers calculated fiber intake and nutrient values for each participant using standardized techniques. The total dietary fiber intake ranged from 12.6 to 36.4 g per day. Participants with the highest fiber intake tended to be older, male, and better educated. They also had better health habits (e.g., exercised more, smoked less, drank less alcohol, and were more likely to take aspirin, folate, and calcium).

The risk of adenoma was found to be inversely related to fiber intake. After multivariate analysis to adjust for other risk factors, participants in the highest quintile of fiber intake had a 27 percent decreased risk compared with participants in the lowest quintile. The inverse relationship was strongest for fiber derived from grains, cereals, and fruit.

In the second report, Bingham and colleagues described the relationship between dietary fiber and the incidence of colorectal cancer in more than 500,000 participants in the European Prospective Investigation into Cancer and Nutrition (EPIC) study. This study enrolled healthy adults 25 to 70 years of age living in 22 communities in 10 countries. The extensive data collected included comprehensive dietary information. In the nearly 2 million person-years of accumulated follow-up since 1992, more than 1,000 cases of colorectal cancer have been confirmed. The hazard ratio for the highest quintile of fiber intake (0.75) showed a significant reduction in risk. The protective effect was dose-related and appeared to be operative for both sides of the colon and for men and women. The authors calculate that a mean intake of 35 g of fiber per day corresponded to a relative risk of 0.58 compared with the lowest quintile intake of 15 g per day. The protective effect appeared to be related to the total fiber intake regardless of the source of the fiber.

Both studies recommend increased fiber intake as a preventive measure against colon cancer. The EPIC study stresses the need for change in population eating habits because a doubling of fiber intake could lead to a reduction of 40 percent in colorectal cancer risk.