To examine the association of dietary fiber with the risk of colorectal cancer in a population with a high incidence of cancer and a low fiber intake, we analyzed the data from the Japan Collaborative Cohort Study. From 1988 to 1990, 43,115 men and women aged 40 to 79 years completed a questionnaire on dietary and other factors. Intake of dietary fiber was estimated using a food frequency questionnaire. Rate ratios (RR) were computed by fitting proportional hazards models. During the mean follow-up of 7.6 years, 443 colorectal cancer cases were recorded. In all participants, we found a decreasing trend in risk of colorectal cancer with increasing intake of total dietary fiber; the multivariate-adjusted RRs across quartiles were 1.00, 0.96 [95% confidence interval (95% CI), 0.72-1.27], 0.72 (0.53-0.99), and 0.73 (0.51-1.03; P trend = 0.028). This trend was exclusively detected for colon cancer: the corresponding RRs were 1.00,0.90 (95% CI, 0.64-1.26), 0.56 (0.38-0.83), and 0.58 (0.38-0.88; Ptrend = 0.002). The decrease in RRs with increasing intake of dietary fiber was larger in men than in women. No material differences appeared in the strength of associations with the risk between water-soluble and insoluble dietary fiber. For food sources of fiber, bean fiber intake was somewhat inversely correlated with colorectal cancer risk. This prospective study supported potential protective effects of dietary fiber against colorectal cancer, mainly against colon cancer. The role of dietary fiber in the prevention of colorectal cancer seems to remain inconsistent, and further investigations in various populations are warranted.
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