The SAM to homocysteine loop
Methylation is the transfer of a single carbon (methyl) group, and the cycle that supplies it turns on four molecules. Methionine is activated to S-adenosylmethionine (SAM), the universal methyl donor. Once SAM hands its methyl group to a target, it becomes S-adenosylhomocysteine (SAH), which is hydrolysed to homocysteine. Homocysteine is then either remethylated back to methionine (using folate and B12) or diverted into the transsulfuration pathway to make cysteine and glutathione.
The diagram shows this as a loop, SAM to SAH to homocysteine to methionine, because the same carbon traffic recurs continuously rather than running once.
What the methyl groups actually build
These methyl groups are not abstract: SAM-driven methylation tags DNA (influencing which genes are expressed), helps synthesise and break down neurotransmitters, and builds phospholipids such as phosphatidylcholine for cell membranes. That breadth is why the cycle is invoked in discussions of mood, cardiovascular risk, and epigenetics.
Popular accounts often overstate how directly supplements tune this cycle; the system is buffered and the clinical payoff of forcing it is uncertain. This is general educational information, not medical advice; consult a qualified professional before supplementing to alter methylation.