Metabolism is anabolic and katabolic. _Anabolism_ is constructive; it includes growth and the act of the tissues in selecting, appropriating, and making substances absorbed from the alimentary canal a part of themselves. The body is never stable; while growth and nutrition progress, destruction or demolition takes place, and this is called _katabolism_. To ascertain the exact amount of matter and energy used daily by the body a balance sheet of the exchange of material is necessary. The _income_ consists of (_a_) matter—food, drink, and the oxygen of the air; (_b_) _energy_, the potential energy of the food and drink. The _outgo_ consists of (_a_) matter in the feces, urine, perspiration, and breath; (_b_) energy—the potential energy of feces, urine, products of respiration, and perspiration. A complete account would show the amount of C, N, H, O, P, S, Cl, Na, Ca, K, Mg, and Fe in the income and outgo, it would also show the compounds in the excreta, including proteins, carbohydrates, water, and carbonic acid. Experiments show that the body requires a given quantity of given energy producing substances for sustenance, and something more to meet the demands for muscular work. The metabolism of nitrogen is usually measured by the amount of urea in the urine, taking into account the small proportion of uric acid and other nitrogenous bodies. In fasting the organism lives on its own flesh and fat. _The Effects of a Nitrogenous Diet on Metabolism._—The most striking effect of a purely nitrogenous diet is a large increase in the nitrogenous metabolism, but it also increases the metabolism of the non-nitrogenous elements of the body. With an ordinary mixed diet the normal excretion of urea varies from 33 to 37 grams a day, while with a meat diet the urea may rise to 50 or even 80 grams daily. _Carbohydrates as a Protein Protector._—It has long been observed that when there was a deficiency of protein in the food the metabolism of nitrogen will be spared and the tissues protected if the food contains plenty of carbohydrates and fat. The subject was fully investigated by Lusk. When the diet contains an abundance of protein, carbohydrates, and fat the organism gains a little nitrogen; when the diet contains the same amount of protein but no carbohydrates, the body loses considerable nitrogen. Again, when the food was the ordinary mixed kind, and contained a sufficiency of energy, but was of a low protein character, the excretion of nitrogen was normal. These results led to the conclusion that the carbohydrates were a protein protector. _Fat as a Protector of Proteins._—The metabolism of nitrogenous tissue and elimination of nitrogen is not prevented by the consumption of fat, but the consumption of fat reduces the metabolism of protein so much that one-quarter or one-third as much meat will suffice to maintain the nitrogen equilibrium as would have to be consumed if only lean meat was used.