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Protein & muscle turnover (build vs break down)

Muscle is in constant flux between building (muscle protein synthesis, MPS) and breakdown (MPB). Net muscle gain happens when synthesis outpaces breakdown — the target of every muscle-building protocol and the reason protein timing and training matter.

BUILD — synthesis (MPS)BREAK DOWN — (MPB)Resistance trainingProtein + leucine → mTOR ↑Testosterone · growth hormoneInsulin — blocks breakdownFasting / low energy → AMPK ↑CortisolNet muscle GAIN when build > break down
Muscle grows when building outpaces breakdown.

How it works

  1. Training (especially resistance training) and dietary protein — particularly leucine — switch on the mTOR pathway, driving muscle protein synthesis.
  2. In the fasted/energy-low state, AMPK rises and breakdown (autophagy, proteolysis) dominates; amino acids can be used for fuel or gluconeogenesis.
  3. Adequate protein (~1.6–2.2 g/kg for trainees) and the training stimulus tip the balance toward net synthesis.
  4. Insulin is permissive (reduces breakdown); testosterone and growth hormone support the anabolic side.

Why it matters

Under-eating protein or skipping the training stimulus means breakdown wins and muscle is lost — critical in fat-loss (protect muscle in a deficit) and healthy-ageing (fight sarcopenia) protocols.

Key hormones: Insulin (anti-breakdown), Testosterone & growth hormone (anabolic), Cortisol (catabolic), IGF-1

Compounds that act on this