Most organisms have little capacity to store nitrogen. Thus, animals must continually replenish nitrogen supplies through the diet to replace nitrogen lost through catabolism. When dietary protein is insufficient, proteins manufactured for other purposes, such as most muscle proteins, are broken down and not replaced.

A well-nourished adult is said to be in nitrogen equilibrium or normal nitrogen balance if

Dietary intake of nitrogen = Nitrogen loss (through excretion and other processes, such as perspiration)

Positive nitrogen balance occurs when:

Dietary intake of nitrogen > Nitrogen loss

Negative nitrogen balance occurs when:

Dietary intake of nitrogen < Nitrogen loss