Inverse associations of dietary fiber and menopausal hormone therapy with colorectal cancer risk in the Multiethnic Cohort Study

In the Multiethnic Cohort Study, we previously reported that dietary fiber intake was inversely associated with colorectal cancer risk in men only. In women, the inverse relationship was weaker and appeared to be confounded by menopausal hormone therapy (MHT). We re-examined this observation with a greatly increased power. Using Cox proportional hazards models, we analyzed data from 187,674 participants with 4,692 cases identified during a mean follow-up period of 16 years. In multivariable-adjusted models, dietary fiber intake was inversely associated with colorectal cancer risk in both sexes: HR50.73, 95% CI: 0.61-0.89 for highest vs. lowest quintile, p(trend)=0.0020 in men and HR=0.76, 95% CI: 0.62-0.91, p(trend)=0.0067 in women. Postmenopausal women who ever used MHT had a 19% lower risk of colorectal cancer (95% CI: 0.74-0.89) compared with MHT never users. In a joint analysis of dietary fiber and MHT, dietary fiber intake was associated with a lower colorectal cancer risk in MHT never users (HR=0.75, 95% CI: 0.59-0.95, p(trend)=0.045), but did not appear to further decrease the colorectal cancer risk of MHT ever users (p(trend)=0.11). Our results support the overall protective roles of dietary fiber and MHT against colorectal cancer and suggest that dietary fiber may not lower risk further among women who ever used MHT. If confirmed, these results would suggest that MHT and dietary fiber may share overlapping mechanisms in protecting against colorectal cancer.