Nitrogen balance
In human physiology, nitrogen balance is the net difference between bodily nitrogen intake (ingestion) and loss (excretion). It can be represented as the following:
Nitrogen is a fundamental chemical component of amino acids, the molecular building blocks of protein. As such, nitrogen balance may be used as an index of protein metabolism. When more nitrogen is gained than lost by an individual, they are considered to have a positive nitrogen balance and be in a state of overall protein anabolism. In contrast, a negative nitrogen balance, in which more nitrogen is lost than gained, indicates a state of overall protein catabolism.
The body obtains nitrogen from dietary protein, sources of which include meat, fish, eggs, dairy products, nuts, legumes, cereals, and grains. Nitrogen loss occurs largely through urine in the form of urea, as well as through faeces, sweat, and growth of hair and skin.
Blood urea nitrogen and urine urea nitrogen tests can be used to estimate nitrogen balance.