Background: Fiber-based prebiotic supplements are marketed for maintaining bowel health and promoting beneficial gut bacteria. However, the association between prebiotic supplement use and colorectal cancer risk and mortality is unknown. Methods: The association between prebiotic use and colorectal cancer risk and mortality was evaluated in postmenopausal women in the Women's Health Initiative study. Self-reported prebiotic use was documented at study enrollment. Adjudicated colorectal cancer cases and mortality were captured using medical and death records. Cox proportional hazards models were used to estimate the HR related to prebiotic use and colorectal cancer risk and mortality. Results: In total, 3,032 colorectal cancer cases were diagnosed during an average 15.4 years of follow-up. Overall, 3.7% of women used a prebiotic with psyllium, the major fiber type. Use of any prebiotic supplement was not associated with colorectal cancer risk or mortality. The type of prebiotic supplement (none vs. insoluble or soluble) was not associated with colorectal cancer risk; however, use of insoluble fiber prebiotics compared with none was associated with higher colorectal cancer mortality [HR, 2.79; 95% confidence interval (CI), 1.32–5.90; P ¼ 0.007]. Likelihood ratio tests indicated no significant interactions between prebiotic use and other colorectal cancer risk factors, including metabolic syndrome. Conclusions: Prebiotic fiber supplement use was not associated with colorectal cancer risk. Insoluble, but not soluble, prebiotic fiber use was associated with higher colorectal cancer mortality. These findings do not support the promotion of prebiotic fiber supplements to reduce colorectal cancer risk or colorectal cancer mortality. Impact: Further investigation is warranted for findings regarding insoluble prebiotic fiber and higher colorectal cancer mortality in postmenopausal women.
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