Net protein balance

Net protein balance is muscle protein synthesis minus muscle protein breakdown. Positive balance means tissue is being added; negative means it is being lost; and the sum of those balances across days and weeks is what determines whether you gain or lose muscle.

Across a normal day the balance swings both ways. It is negative between meals and overnight, when breakdown exceeds synthesis, and positive after eating protein, particularly when resistance training has raised the responsiveness of the muscle.

That is the actual reason both training and protein matter, and why either alone underperforms. Training raises the height of the synthesis response; protein provides the substrate and the leucine signal that triggers it; and the two together produce a larger and longer positive swing than either alone.

It also explains why distribution matters a little. Several protein feedings across the day produce several positive swings, whereas one large feeding produces one, since the synthesis response to a single meal saturates. Total intake still dominates.

Energy status shifts the whole picture. In a deficit the balance is harder to keep positive, which is why protein requirements rise during dieting and why resistance training becomes protective rather than optional.

In older adults the synthesis response to a given protein dose is blunted, called anabolic resistance, which is why recommendations for older people start higher and why loading matters as much as intake.

In practice, the balance is not something to measure, only to influence: train, eat enough protein spread across meals, and expect the accumulated result over months rather than the state on any afternoon.