Net protein balance

Net protein balance is the difference between the rate at which muscle protein is built and the rate at which it is broken down. Sustained positive balance means muscle mass increases; sustained negative balance means it declines.

It matters because it is the actual mechanism underneath every muscle-building recommendation. Resistance training raises synthesis for a day or more afterwards; a protein feed raises it acutely and suppresses breakdown a little. The two together tip the balance positive, and repeating that across weeks is how muscle accrues.

In practice, a single measurement tells you very little — balance swings positive after meals and negative during fasting throughout a normal day. What matters is the integrated total across days and weeks, which is why total daily protein and consistent training beat any acute intervention.