Monkeys that ate less for many years burn fewer calories while sleeping, even when you account for how much muscle they have, as if their bodies learned to use less energy just to stay alive.

Even though monkeys on a low-calorie diet burn less energy while sleeping and eat less, their total daily energy use stays the same because they move around more and spend less energy moving, so it all balances out.

Monkeys that eat less over a long time use less energy when they move around, which means their muscles work more efficiently — like a car that gets better gas mileage.

Monkeys that eat fewer calories for a long time tend to move around more and stay active longer than monkeys that eat normally, even though they're taking in less energy.

When healthy, normal-weight people eat 25% fewer calories for two years, they lose weight—but their bodies also burn even fewer calories at rest than you’d expect just from losing weight, as if their metabolism slows down on its own.