Autophagy

Autophagy is the cell's built-in recycling process, in which damaged proteins, worn-out organelles, and other cellular debris are broken down so their building blocks can be reused. The name comes from the Greek for "self-eating," and it is one of the main ways a cell maintains itself by clearing out what is no longer working.

Autophagy is governed by nutrient availability, which is what links it to fasting. When food is plentiful, the nutrient-sensing pathway mTOR is active and holds autophagy in check. During a fast, mTOR activity falls and the energy-sensing enzyme AMPK rises, and together these shifts ease the brakes on autophagy so it can run more freely. The response is driven by these nutrient signals, not by reaching a fixed number of hours.

Within a fast, autophagy is part of the same broad shift that moves cells away from growth and storage and toward maintenance and breakdown, the nutrient-sensing change that also underlies the metabolic switch from burning glucose to burning fat.

Source: Intermittent fasting and autophagy: narrative review (2025)