Overview of the methionine-homocysteine-folate-B12 cycle, which provides methyl donors for methyltransferases. Nutritional regulations of the cycle by phytochemicals and metals are shown in red. BHMT, betaine homocysteine; CS, cystathionine synthase; DNMT, DNA methyltransferase; EGCG, epigallocatechin-3-gallate; MAT, methionine adenosyl transferase; MS, methionine synthase; SAAH, S-adenosyl-L-homocysteine hydrolase; SAH, S-adenosyl-L-homocysteine; SAM, S-adenosyl-L-methionine; THF, tetrahydrofolate.