Scientists have long known that people on calorie-restricted diets can live longer than people who eat a normal number of calories, but the actual mechanism behind such increased longevity may have just been discovered.

Our cells build two kinds of recycling factories. One kind, known as the proteasome, is a tiny cluster of proteins. It slurps up individual proteins like a child sucking a piece of spaghetti. Once inside the proteasome, the protein is chopped up into its building blocks.

For bigger demolition jobs, our cells rely on a bigger factory: a giant bubble packed with toxic enzymes, known as a lysosome. Lysosomes can destroy big structures, like mitochondria, the sausage-shaped sacs in cells that generate fuel. To devour a mitochondrion, a cell first swaddles it in a shroudlike membrane, which is then transported to a lysosome. The shroud merges seamlessly into the lysosome, which then rips the mitochondrion apart. Its remains are spit back out through channels on the lysosome’s surface. ...

Unfortunately, as we get older, our cells lose their cannibalistic prowess. The decline of autophagy may be an important factor in the rise of cancer, Alzheimer’s disease and other disorders that become common in old age. Unable to clear away the cellular garbage, our bodies start to fail.

If this hypothesis turns out to be right, then it may be possible to slow the aging process by raising autophagy. It has long been known, for example, that animals that are put on a strict low-calorie diet can live much longer than animals that eat all they can. Recent research has shown that caloric restriction raises autophagy in animals and keeps it high. The animals seem to be responding to their low-calorie diet by feeding on their own cells, as they do during famines. In the process, their cells may also be clearing away more defective molecules, so that the animals age more slowly.

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