Pre-diagnostic intake of vitamin D and incidence of colorectal cancer by anatomical subsites: the Norwegian Women and Cancer Cohort Study (NOWAC).

According to the World Cancer Research Fund International, vitamin D might decrease the risk of colorectal cancer (CRC). However, less is known about the association with cancers in different subsites of the colon and in the rectum. The aim of this study was to examine associations between pre-diagnostic intake of vitamin D and risk of CRC by anatomical subsites. Data from 95,416 participants in the Norwegian Women and Cancer Cohort Study was included, and vitamin D intake was estimated from two repeated food frequency questionnaires. Associations between vitamin D intake and incidence of CRC were assessed using multivariable Cox regression. During follow-up, there were 1774 incident cases of CRC. A small but borderline significant inverse association was found for a 5 µg increase in vitamin D intake and risk of CRC (HR=0.97. 95% CI,0.93-1.01) and colon cancer (HR=0.96. 95% CI,0.91-1.01). High (≥20 µg) compared to low (<10 µg) vitamin D intake was associated with 17% borderline significant reduced risk of CRC (HR=0.83. 95% CI, 0.68-1.02). Medium (10-19 µg) versus low intake (<10 µg) was associated with 27% reduced risk of proximal colon cancer (HR=0.73. 95% CI, 0.57-0.94). No significant associations were observed between vitamin D intake and risk of distal colon or rectal cancer. Our study indicates that vitamin D may be differently associated with subsites of the colon. The association between vitamin D intake and proximal colon cancer is novel.