In human physiology, nitrogen balance is the net difference between bodily nitrogen intake (ingestion) and loss (excretion):

${\displaystyle {\mbox{nitrogen balance))={\mbox{nitrogen intake))-{\mbox{nitrogen loss))}$

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.

Physiological and Clinical Implications

Positive nitrogen balance is associated with periods of growth, hypothyroidism, tissue repair, and pregnancy. Because of this, the intake of nitrogen into the body is greater than the loss of nitrogen from the body. Thus, there is an increase in the total body pool of protein.

Negative nitrogen balance is associated with burns, serious tissue injuries, fever, hyperthyroidism, wasting diseases, and periods of fasting. This means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested.^[3] A negative nitrogen balance can be used as part of a clinical evaluation of malnutrition.^[4]

Nitrogen balance is a method traditionally used to measure dietary protein requirements.^[5] This approach necessitates the meticulous collection of all nitrogen inputs and outputs to ensure comprehensive accounting of nitrogen exchanges.^[6] Nitrogen balance studies typically involve controlled dietary conditions, requiring participants to consume specific diets to determine total nitrogen intake precisely. Furthermore, participants often must remain at the study location for the duration of the study to facilitate the collection of all nitrogen losses. Physical exercise is also known to influence nitrogen excretion, adding another variable that requires control during these studies.^[7] Due to the stringent conditions required for accurate results, the nitrogen balance method may pose challenges when studying dietary protein requirements across different demographics, such as children.^[8]

Dietary nitrogen, acquired from metabolizing proteins and other nitrogen-containing compounds, has been linked to changes in genomic evolution. Species that primarily obtain energy from metabolizing nitrogen-rich compounds use more nitrogen in their DNA than species that primarily break down carbohydrates for their energy^{[citation needed]}. Dietary nitrogen alters codon bias and genome composition in parasitic microorganisms.^[9]