In this nested case-referent study, we related plasma concentrations of vitamin B12 to the risk of colorectal cancer, taking into consideration prediagnostic plasma folate and total homocysteine concentrations. Subjects were 226 cases and double matched referents from the population-based Northern Sweden Health and Disease Study. Follow-up times from recruitment to diagnosis ranged from 0.1 to 12.7 years, with a median of 4.2 years. Plasma vitamin B12 concentrations were inversely associated with the risk of rectal cancer: univariate odds ratio for the highest versus lowest quintile 0.34 (95% confidence interval (95% CI) 0.13-0.83), p(trend) = 0.004. Risk estimates were attenuated slightly but remained statistically significant after adjustment for body mass index, current smoking, recreational and occupational physical activity, alcohol intake and prediagnostic plasma folate and total homocysteine concentrations: OR 0.30 (95% CI 0.08-0.99), p(trend) = 0.025. The corresponding univariate and fully adjusted odds ratios for colon cancer were 1.25 (CI 0.66-2.36), p(trend) = 0.185 and 1.42 (CI 0.67-3.05), p(trend) = 0.113, respectively. The observed over-risk was attributable to left-sided colon cancer. Interaction analyses including vitamin B12, folate and homocysteine were in line with the results for vitamin B12 alone. In conclusion, these results suggest that increasing levels of plasma vitamin B12, alone or together with other factors involved in one-carbon metabolism, may reduce the risk of rectal cancer, whereas for colon cancer, the association appears to be less clear.
(c) 2007 Wiley-Liss, Inc.