mTOR

mTOR, short for mechanistic target of rapamycin, is a nutrient-sensing enzyme that acts as one of the cell's main growth switches. When nutrients are abundant, mTOR is active and drives the cell toward building and storing, switching on protein synthesis and switching off recycling. When nutrients are scarce, mTOR quiets down and the balance tips the other way.

This makes mTOR central to what happens during a fast. Protein and carbohydrate are its strongest activators, so when you stop eating, mTOR activity falls. That drop is what releases the brakes on autophagy, the cell's recycling process, and shifts cells away from growth and toward maintenance. mTOR and the energy-sensing enzyme AMPK work as a pair here, with mTOR falling and AMPK rising as fuel runs low.

mTOR is not something to suppress as hard as possible. The body needs periods of mTOR activity to build and repair tissue, including muscle, which is why fasting and eating are best understood as a cycle rather than a contest to keep mTOR permanently low.

Source: Saxton & Sabatini, Cell (2017)