A mouse eats food missing one tiny detail, the amino acid cysteine, and in five days loses a fifth of its weight. Its white fat darkens, as if a furnace had been switched on inside. Cysteine is a building block for proteins. Our body can make it on its own from another amino acid, methionine. Researchers used to think that eating less methionine was good for health, but it turned out that cysteine is what really matters.
Scientists took mice in which the gene needed to produce cysteine from methionine had been switched off. That way they could remove cysteine on its own. When only methionine was removed, nothing changed. But when cysteine was removed, the mice lost weight, their fat began burning energy, and the liver released a hormone that helps metabolism. In old mice, long-term cysteine restriction prevented weight gain, improved blood sugar, and reduced inflammation.
Cysteine works like a gas pedal in a car: let it go, and the body starts burning its reserves and wears out more slowly. Now scientists are wondering whether the same can be done in people, for instance with food containing less cysteine or with drugs that block its production. But so far these are only the first steps.
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.