| The concept of nitrogen balance
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Because there is no significant storage form of nitrogen or amino compounds in humans, nitrogen metabolism is quite dynamic. A careful balance is maintained between nitrogen ingestion and secretion. In an average, healthy diet, the protein content exceeds the amount required to supply essential and nonessential amino acids for protein synthesis, and the amount of nitrogen excreted is approximately equal to that taken in. Such a healthy adult would be said to be 'in nitrogen balance'. When there is a need to increase protein synthesis, such as in recovering from trauma or in a rapidly growing child, the amount of nitrogen excreted is less than that consumed in the diet, and the individual would be in 'positive nitrogen balance'. The converse is true in protein malnutrition: because of the need to synthesize essential body proteins, other proteins, such as muscle protein or hemoglobin, are degraded and more nitrogen is lost than is consumed in the diet. Such an individual would be said to be in 'negative nitrogen balance'. Fasting, starvation and poorly controlled diabetes are also characterized by negative nitrogen balance, as body protein is degraded to amino acids and their carbon skeletons are used for gluconeogenesis. The concept of nitrogen balance reminds us of the continuous turnover in the normal human body of amino acids, proteins, and some nucleic acids.
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