Reference
Net Muscle Protein Balance
**Net Muscle Protein Balance** is the difference between the rate of muscle protein synthesis (MPS) and the rate of muscle protein breakdown (MPB) over a given period. When synthesis exceeds breakdown the balance is positive and muscle tissue is gained; when breakdown exceeds synthesis the balance is negative and tissue is lost.
How the balance works
Skeletal muscle is in constant turnover. Existing proteins are degraded (MPB) while new ones are assembled from amino acids (MPS), and the two rates fluctuate across the day. Net Muscle Protein Balance is simply MPS minus MPB. Muscle growth, or hypertrophy, requires that synthesis exceed breakdown over time (Phillips et al., 2014, Sports Medicine, PMID:24791918).
In the fasted state the balance is typically negative: with no incoming amino acids, breakdown outpaces synthesis and the body draws on muscle as an amino acid reservoir. This is the same catabolic-versus-anabolic distinction captured at the whole-body level by nitrogen balance, where a negative reading indicates a catabolic state and a positive reading an anabolic one.
How protein feeding shifts it positive
Eating protein moves the balance toward positive through two mechanisms acting at once. Dietary amino acids stimulate muscle protein synthesis, and the insulin released with a meal suppresses muscle protein breakdown. Protein both raises MPS and inhibits MPB, which together facilitate repair of damaged tissue. Resistance exercise amplifies the synthesis side of the equation, so the combination of training and protein intake produces a larger and longer positive swing than either does alone.
The size of that swing depends partly on protein quality and leucine content, which is why protein source matters for athletes managing recovery and adaptation. For a fuller treatment of intake targets and timing, see the guide to protein for athletes. The relevant point for balance is that several distinct feedings across the day keep the net figure positive for longer than a single large dose.
Why it matters with aging
The synthetic response to protein feeding is blunted with age, a condition termed anabolic resistance (PMID:23558692). Because the synthesis side of the balance responds less strongly to the same amount of protein, older adults reach a positive net balance less easily and may require more protein, more leucine, or the addition of resistance exercise to do so. Decreased physical activity can produce a similar blunting even in younger people, and that reduced response cannot be overcome by dietary protein alone.
Over weeks and months, the cumulative direction of Net Muscle Protein Balance — not any single meal — determines whether muscle mass is maintained, gained, or lost.
Related terms