The Build-and-Repair Cycle
mTOR and AMPK — why your body needs both
Your body cycles between two states: a build phase, active after eating and during resistance training, and a repair phase, active during fasting windows and rest. Both matter for long-term health — living permanently in one, at the expense of the other, works against you.
Why Neither Extreme Works
At the cellular level, this cycle is governed by two signalling pathways: mTOR drives growth when nutrients and mechanical loading (like resistance training) are present, while AMPK drives clean-up and repair when energy is scarce, and the two broadly suppress each other. The muscle-building side of this — how resistance training and protein timing activate the build phase — is covered in the Exercise & Recovery guide; the fasting and nutrient-timing side — how meal timing and caloric intake activate the repair phase — is covered in the Nutrition guide. This guide's interest is narrower than either: both phases need to happen on some kind of regular cycle, rather than living permanently in one or the other.
Section takeaway
Long-term health depends on cycling between build phases (feeding, resistance training) and repair phases (fasting, rest), not maximising either one permanently. For the underlying biology of each phase — muscle protein synthesis, nutrient-sensing pathways — see the Exercise & Recovery and Nutrition guides.