discriminate between body equilibrium, meaning thereby the maintenance from day to day of a constant body-weight, and nitrogen equilibrium, or carbon equilibrium. In the latter cases, what is meant is that the intake of nitrogen, or of carbon, exactly equals the output of these two elements. It is quite possible, however, to have a condition of nitrogen equilibrium without the body being in a state of balance, as when the outgo of carbon exceeds the intake of carbon, or when there is an increased output of water. As a rule, it may be stated that when a man puts out less carbon and less nitrogen than he takes in he must be gaining in weight; the only exception being the possible case of an increased excretion of water, which might more than counterbalance the gain. On the other hand, if he gives off more carbon and more nitrogen than he takes in, the body must lose in weight. Where the output of carbon is beyond the amount of carbon ingested, the lost carbon represents a drain upon body fat. In a reversal of this condition, _i. e._, where the carbon taken in is in excess of the outgo, the body is gaining in fat. Theoretically, gain or loss of carbon may mean gain or loss of either carbohydrate or fat, but practically stored-up carbon generally stands for accumulated fat; and, correspondingly, loss of carbon represents a withdrawal from the store of adipose tissue, since glycogen and sugar from a quantitative standpoint figure only slightly in these metabolic processes. When the body excretes more nitrogen than is taken in during a given period, there is only one interpretation possible, viz., that the body is losing proteid or flesh. If, on the other hand, the nitrogen import exceeds the outgo, then the body must be gaining flesh. Here, again, there is the theoretical possibility that gain or loss of nitrogen might represent increase or decrease of proteid in some glandular organ, or even in the blood; but practically it is the relatively bulky muscle tissue, with its high content of proteid matter, that is most subject to change in metabolism. Finally, it is easy to see how, knowing the percentage of nitrogen in proteid and the percentage of carbon in fat, one can calculate from the nitrogen and carbon lost or gained the amounts of proteid or fat added to the capital stock, or withdrawn from the store of nutritive material. When there is no income, as in fasting, the body loses rapidly, living during the hunger period upon its store of energy-containing material. Many careful observations have been made upon people who have fasted for long periods, some as long as thirty days, the income consisting solely of water. The following figures[22] show the daily excretion of nitrogen in several notable cases: [22] Taken from Landergren: Untersuchungen über die Eiweissumsetzung des Menschen. Skandinavisches Archiv für Physiologie, Band 14, p. 112; and from A. Magnus-Levy: v. Noorden’s Handbuch der Pathologie des Stoffwechsels, 1906, p. 312.