MTOR

In one line: The master "build" switch — turn it on to grow muscle, off to trigger cellular cleanup.

mTOR is a foreman that watches the cell's food supply. When protein and energy are plentiful (after a meal, especially with the amino acid leucine), the foreman shouts "build!" and the cell grows and lays down muscle. When food is scarce, the foreman goes quiet and the crew switches to cleaning and recycling instead.

This is the central tension between fitness and longevity. To build muscle, you want to spike mTOR after training — protein does exactly that. But for long-term health, you periodically want it low (through fasting, or the drug rapamycin) so cells can clean house — a process linked to longer life in animals.

You can't have it maximally on and off at once. The smart play: spike it around training, and let it fall the rest of the day.