N equilibrium is shorthand for nitrogen equilibrium, the state in which the body’s nitrogen intake from dietary protein exactly matches nitrogen losses in urine, faeces, sweat, and skin. It represents maintenance of the body’s protein pool: no net gain or loss of lean tissue. Healthy adults eating adequate protein sit at equilibrium, and nutrition researchers use it as the reference point for estimating protein requirements, defining the minimum intake that keeps a person neither building nor losing protein. Because the body cannot store protein the way it stores fat or carbohydrate, daily intake matters, and equilibrium shifts with age, illness, and activity. Athletes in heavy training and patients recovering from surgery need intakes above equilibrium to reach a positive balance and rebuild tissue.