Nitrogen balance
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In human physiology, nitrogen balance is the net difference between bodily nitrogen intake (ingestion) and loss (excretion):
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Nitrogen is a fundamental chemical component of amino acids, the molecular building blocks of protein. Nitrogen balance may therefore be used as an index of protein metabolism.[1] When more nitrogen is gained than is lost by an individual, they are considered "in positive nitrogen balance" and a state of overall protein anabolism. In contrast, a "negative nitrogen balance", in which more nitrogen is lost than is gained, indicates a state of overall protein catabolism.[2]
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 feces, sweat, and hair and skin growth.
Blood urea nitrogen and urine urea nitrogen tests can be used to estimate nitrogen balance.