In body building and repairing, the nitrogen of protein food is indispensable. In the other functions, nitrogen is not utilized. Carbohydrates and fats are the natural and convenient sources of energy, although the protein contains in itself non-nitrogenous material capable of oxidation. As already stated, however, to depend largely upon protein for oxidizable food means the ingestion of an enormous quantity of unnecessary nitrogen, the getting rid of which involves waste of energy and is thought to necessitate a strain upon the excretory system. Furthermore, when the diet consists chiefly of protein, much of the latter may not be digested and absorbed as such, but may remain in the intestine, there to undergo putrefactive changes. In the course of putrefaction certain soluble poisonous substances form and are absorbed, requiring elimination by the kidneys. The excess of nitrogen which cannot be utilized in tissue repair must also be eliminated by the kidneys. Thus while people may thrive on a diet rich in protein, there is considerable evidence that vitality, endurance and resistance to disease are better attained with a diet in which the fuel foods (carbohydrates and fats) predominate.

Nitrogen cannot be stored to any extent and is constantly eliminated in the urine as urea and other nitrogenous substances. When the diet-nitrogen corresponds in amount with the eliminated nitrogen, the metabolism of nitrogen is balanced, or in a state of equilibrium. If too small a quantity of nitrogenous food is taken it does not compensate for the waste of body nitrogen which is inseparable from life. The same thing occurs when for any reason the food nitrogen is not absorbed, or is eliminated without being utilized. This condition of nitrogen starvation is characteristic of underfeeding, wasting diseases, defective assimilation, etc. Conversely equilibrium between food nitrogen and excreted nitrogen represents good nutrition, health and energy; for while nitrogen cannot be permanently stored, it is possible for a balance to be maintained over long periods. It was once taught that increased elimination of nitrogen was due chiefly to the waste of tissue incidental to prolonged and severe muscular or nervous effort; at a later period, however, it became apparent that this waste was comparatively little, and constant for the individual regardless of the amount of muscular exercise, so that the urea nitrogen depends chiefly on the amount of diet nitrogen. The loss of nitrogen in wasting disease, starvation, etc., is, however, extensive; so that in convalescence the body is able to store some nitrogen.

Nitrogen retention is greatly facilitated in all cases by a liberal supply of carbohydrates and fats.

Nitrogen metabolism can hardly be studied in the ordinary patient, on account of the number and variety of tests which must be accurately made, so that our only guide is the results of scientific investigation of the amounts of nitrogen required under given conditions, and the general well-being of the subject. If weight is maintained, along with a healthy state of functions, the ration in question is suitable for the subject tested. If the individual be in the growing period, the ration should be such that the growth rate is normal. If he be a convalescent, the recovery of the original weight should be constantly progressive. Of late years, however, the emphasis on nitrogen values alone has been largly superseded by greater attention to total fuel values as being of greater practical worth. It is known that a relatively small and easily obtained quantity of nitrogen is sufficient to repair waste in the average individual, but the food required for producing energy is very significant in amount. With a liberal supply of carbohydrates and fats, the ordinary daily waste of nitrogen in one adult may be reduced as low as 3 to 4 grams daily (corresponding to 20 to 25 grams of protein).

To insure a full supply of nitrogen, and to maintain nitrogen equilibrium at a higher level than the bare maintenance requirement, considerably more than this amount is commonly taken. Prom 80 to 100 grams of protein per day is regarded as a liberal supply for an average man (weighing 70 kilograms or 154 lbs. ) on an ordinary mixed diet of suitable energy value.