Interaction among folate, riboflavin, genotype, and cancer, with reference to colorectal and cervical cancer.

Epidemiological studies have linked low folate intake with an increased risk of epithelial cancers, including colorectal cancer and cervical cancer. Riboflavin has received much less attention, but there is increasing interest in the well-established role that flavins play in folate metabolism and the possible synergy of a protective effect between these 2 vitamins. Folate plays a key role in DNA synthesis, repair, and methylation, and this forms the basis of mechanistic explanations for a putative role for folate in cancer prevention. The role of folate in these processes may be modulated by genotype for the common C677T thermolabile variant of methylene tetrahydrofolate reductase (MTHFR), homozygosity for which is associated with lower enzyme activity, lower plasma and red blood cell folate, and elevated plasma homocysteine. Riboflavin, as FAD, is a cofactor for MTHFR and there is evidently some interaction among riboflavin status, folate status, and genotype in determining plasma homocysteine, a functional marker of folate status. The MTHFR C677T polymorphism appears to interact with folate and riboflavin in modulating cancer risk in a manner that varies according to cancer site. Most evidence points to a protective effect of this polymorphism for risk of colorectal cancer, but the effect on cervical cancer risk is not clear. The effect of this polymorphism on cancer risk seems to be further modulated by other factors, including alcohol and, in the case of cervical cancer, infection with the human papilloma virus. An additional factor determining the effect of diet and genotype interactions on cancer risk may be the stage of cancer development.