Nitrogen Balance

Nitrogen balance, such a state of the body's metabolism in which the amount of nitrogen taken in with food per day equals the amount of N in the excretions (urine and feces; sweat is taken into account only in cases of significant perspiration) for the same period. Although the N in food, and consequently in excretions, is not always exclusively protein N, however, one often speaks, neglecting some inaccuracy, of protein balance instead of nitrogen balance. N. b. is achieved, as first shown (in 1857) by C. Voit on dogs, when adult animals are given for a series of (4-7) days the same amount of N in meat. Later research has established that other adult animals and humans also come into a state of N. b. within a few days with a constant protein content in the daily diet. The upper limit of protein content in food, at which N. b. is possible, is determined by the ability of the digestive apparatus to digest and assimilate proteins: this limit is higher in carnivores, lower in herbivores, omnivores, and humans. The lower limit is generally more or less close to the amount of protein that is broken down during established fasting of a given organism, and varies depending on its nutritional status and the composition of food. Voit's doctrine of N. b. is of great importance because it established the pathways of N excretion from the body, refuted the doctrine of N excretion by respiration (the doctrine of so-called N deficit), and showed that nitrogen balance, being the normal state of the adult organism, provides a very convenient starting point for the study of protein metabolism (see also Protein metabolism and Metabolism).