Figure 1.

Folate metabolism and interaction with other nutrients such as vitamin B12 and choline. Folate and betaine are methyl donors. They provide the methyl group required for methylation of homocysteine to methionine and production of S-adenosylmethionine. Folate metabolism depends on the availability of vitamin B12 (the warning sign: a rate-limiting step in the re-methylation pathway) and is slowed in subjects with MTHFR polymorphisms/or riboflavin deficiency (the warning sign: a rate-limiting step in the folate cycle). DMG, dimethylglycine; DHFR, dihydrofolate reductase; MS, methionine synthase; MTHFR, methylenetetrahydrofolate reductase; 5MTHF, 5-methyltetrahydrofolate; SHMT, serine hydroxymethyltransferase; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine; THF, 10fTHF, 5,10-CH=THF, and 5,10-Methylen-THF are derivatives of tetrahydrofolate.