A mouse gets food that is missing a single amino acid, and within five days it loses a fifth of its weight. Its white fat turns darker, as if tiny stoves had lit up inside it. This is not hunger and not exercise: cysteine was simply removed from the diet.
Cysteine is a building block of proteins that our body can make on its own from another amino acid, methionine. It used to be thought that restricting methionine was good for health and longevity. But such diets were usually also short on cysteine, so it was unclear which of the two was doing the work. The authors of the study used mice with the CTH gene switched off, which cannot convert methionine into cysteine. That way they could restrict each amino acid separately.
A cysteine deficiency made the liver release the hormone FGF21 into the blood, which switches metabolism over to burning fat. In old mice, five days of cysteine deprivation improved the state of fat tissue and the liver. And long-term 70% restriction over ten months prevented age-related weight gain, improved glucose tolerance, and reduced inflammation. In nematode worms, cysteine restriction extended life, while methionine restriction did not.
Cysteine turned out to be the gas pedal of metabolic aging: let it go, and the body starts spending its reserves and wears out more slowly. Now scientists want to check whether the same can be achieved in people, for example with drugs that block cysteine synthesis. For now this is work done in mice, but it shows that even a single amino acid on your plate can decide how fast we age.
In C. elegans nematodes, cysteine restriction extended lifespan more than methionine restriction, and restoring cysteine in the diet completely abolished this effect.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.