
Mitochondrial dysfunction
The power stations falter
Mitochondria are the small engines that turn food and oxygen into the energy every other process on this page runs on. With age there are fewer of them, they leak, and the broken ones are cleared away more slowly. Tiredness that sleep does not fix is often this. So is a good deal of what people mean when they say they feel old.
Exercise, and it is not close. It is the only intervention known to build new mitochondria in a living human being: endurance work grows them, interval work does it fastest. Nothing on any shelf comes near it, whatever the label promises.
The science, in one sentence
Mitochondria become fewer, leakier and less efficient, and the damaged ones are cleared away more slowly.