Methionine is an essential sulfur-containing amino acid: the body does not synthesize it itself and must obtain it from food (eggs, meat, fish, sesame, Brazil nuts). The assembly of almost every protein begins with methionine — its AUG codon serves as the start signal in synthesis.
Even more important is its role as a donor of methyl groups: methionine forms S-adenosylmethionine (SAM) — the universal "supplier" of methyl marks for hundreds of reactions, including DNA methylation.
The methionine–homocysteine cycle
Having donated its methyl group as part of SAM, methionine turns into homocysteine. There are then two paths. The first is "remethylation" back into methionine: this needs active folate (activated by the enzyme MTHFR) and vitamin B12. The second is "transsulfuration" into the amino acid cysteine. If these vitamins are lacking, homocysteine accumulates in the blood.
Methionine and methylation
Through SAM, methionine supplies methyl marks to DNA, proteins and many small molecules. The balance of these reactions determines which genes are switched on (DNA methylation) — that is, the workings of epigenetics. So an adequate supply of methionine and its cofactor vitamins (folate, B12) is important not only for proteins but also for the fine-tuning of the genome.
Methionine restriction and longevity
In animal experiments, restricting methionine in the diet extended lifespan — presumably through reduced growth signalling (including the mTOR pathway) and a mild stress response. This has not yet been proven in humans, but plant-based diets contain less methionine on average. It is important to understand: methionine is essential, and this is about balance, not complete exclusion.
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Frequently asked questions
What is the danger of too much methionine?
A constant excess of methionine increases the load on the cycle and can raise homocysteine if folate and B12 are lacking. In a number of studies, a very high intake is linked to increased growth signalling.
Is it true that methionine restriction extends lifespan?
This has been shown in animals but not yet proven in humans. Methionine is essential, so this is about a sensible balance of the diet, not its exclusion.