Restricting cysteine extends life, not methionine

Methionine restriction has attracted attention for years. Diets very low in this amino acid appear to extend lifespan in laboratory animals. But according to a new study, that conclusion is not quite right. The authors argue it is actually cysteine restriction that makes the difference.

The reasoning goes like this. Methionine and cysteine are the only two sulphur-containing amino acids in proteins. A diet low in methionine almost always contains little cysteine too. The body also uses methionine to produce cysteine. This makes it nearly impossible in animal experiments to separate the effects of one from the other. The researchers solved this by using genetically engineered animals in which the conversion of methionine to cysteine was blocked.

Cysteine as the key player

In those animals, the protective metabolic effect occurred only when cysteine was low, not simply when methionine was low. Cysteine has a unique chemical property: it is the only sulphur-containing amino acid with a thiol group, making it central to redox signalling (the balance between oxidation and antioxidant activity in cells), the production of glutathione (a major antioxidant), and the transport of coenzyme A, a key molecule in energy metabolism.

Low cysteine also activated a stress response in the central nervous system. That triggered the sympathetic nervous system and promoted fat burning through brown adipose tissue (fat cells that generate heat rather than store energy). These responses are similar to those seen with overall calorie restriction.

Implications for longevity research

If cysteine is the actual lever, that changes how diet research should be interpreted. Plant-based diets tend to be lower in sulphur-containing amino acids, which may partly explain why some studies report favourable effects. Whether deliberate cysteine restriction is practically feasible and safe in humans has not yet been studied.

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