Epidemiologic studies of folate and colorectal neoplasia: a review.

Dietary folate influences DNA methylation, synthesis and repair. Aberrations in these DNA processes may enhance carcinogenesis, particularly in rapidly proliferative tissues such as the colorectal mucosa. DNA methylation abnormalities may influence the expression of cancer-related genes, and inadequate levels of folate may lead to uracil misincorporation into DNA and to chromosomal breaks. Folate deficiency enhances intestinal carcinogenesis in several animal models. An increasing number of epidemiologic studies indicate that higher intakes of folate either from dietary sources or from supplements may lower the risk of colorectal adenoma and cancer. More limited data also suggest that dietary methionine, which might also influence methylation, may have a similar protective role. High alcohol consumption, which has a strong antifolate effect, also has been related to higher risk of colorectal neoplasia. The deleterious effects of alcohol are accentuated when folate or methionine intake is low. Some evidence also suggests that the risk of colorectal neoplasia may vary according to genetic polymorphisms in methylenetetrahydrofolate reductase, an enzyme that is involved in folate metabolism. The cumulative data indicate that maintaining adequate folate levels may be important in lowering risk of colorectal cancer.