Betaine is a biochemical component of intracellular methionine metabolism. Exogenous betaine supplementation can elevate intracellular SAM, a universal methyl donor, and glutathione levels through modulating intracellular methionine metabolism. SAM, S-adenosyl methionine; SAH, S-adenosyl homocysteine; MAT, methionine adenosyltransferase; BHMT, betaine-homocysteine S-methyltransferase; SAHH, SAH hydrolase; MS, methionine synthase; CBS, cystathionine beta synthase.