mTORMechanistic target of rapamycin

mTOR (mechanistic target of rapamycin) is a central signalling pathway that senses nutrients, energy and mechanical load and, when activated, switches cells toward growth and building — including muscle protein synthesis.

It matters because resistance training, adequate protein and especially leucine activate mTOR, linking the pathway directly to muscle growth. It also sits at the centre of a longevity debate: constant growth signalling may trade off against cellular clean-up (autophagy), so the ideal is thought to be cycling between the two rather than maximising either.

In practice, you don't manage mTOR directly; training hard and eating enough protein activates it for muscle building, while periods without constant feeding allow the opposing maintenance processes their turn.