SAM (S-adenosylmethionine)

SAM (S-adenosylmethionine) is your body's main methyl-group donor. It is the molecule that hands out the chemical tags used to mark DNA, histones, neurotransmitters, and fats. Your cells build it by joining the amino acid methionine to ATP (energy). An enzyme called methionine adenosyltransferase runs that step. After SAM gives away its methyl group, it becomes SAH (S-adenosylhomocysteine). SAH is then broken down to homocysteine. Homocysteine sits at a fork. It can be recycled back to methionine, or sent down the transsulfuration path toward cysteine and the antioxidant glutathione. The ratio of SAM to SAH inside a cell is a gauge of its methylation capacity. As SAM falls with age or with methionine restriction, it is thought to reshape epigenetic programming. In several lab organisms it shifts longevity, though the net effect in mammals depends on context.

Last reviewed: June 22, 2026

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