Serum levels of vitamin D metabolites and the subsequent risk of colon and rectal cancer in Finnish men.

Experimental and human epidemiologic data suggest a protective role for vitamin D in large bowel cancer. To investigate this association, we conducted a nested case-control study within a Finnish clinical trial cohort. Cases (n = 146) were participants diagnosed with primary adenocarcinoma of the large bowel. Controls were matched (2:1) to cases on age, date of baseline blood draw, and study clinic. Prediagnostic serum levels of the vitamin D metabolites, 25-hydroxyvitamin D (25-OH D), and 1,25-dihydroxyvitamin D (1,25-DIOH D) were used as primary exposure measures. The baseline geometric-mean serum level of 25-OH D was 11.6 percent lower in cases than in controls (12.2 cf 13.8 ug/l, P = 0.01) while serum levels of 1,25-DIOH D did not differ by case-control status. No association was seen between serum levels of 1,25-DIOH D and large bowel cancer risk. However, the estimated relative risk (RR) of large bowel cancer decreased with increasing level of serum 25-OH D and the association was more pronounced for rectal cancer (55 cases; RR by quartile = 1.00, 0.93, 0.77, 0.37; trend P = 0.06). Neither exclusion of early cases nor multivariate adjustment for potential confounders materially altered these estimates. There was no evidence of effect modification by level of 1,25-dihydroxyvitamin D or with other known risk-factors for large bowel cancer.