The estimation of the nitrogen balance has already been referred to as one factor in the determination of the total food requirement by means of metabolism experiments; and it has been shown that the balance may be found either by comparing the total intake with the total output, or by comparing the amount absorbed with the amount catabolized and eliminated through the kidneys.* When intake exceeds output, there is a plus balance which indicates a storage of nitrogen and therefore of protein in the body; a minus balance (greater output than intake) indicates a loss of body protein. When the balance is o, or so near o as to be within the limits of experimental error, the body is said to be in nitrogen (or protein) equilibrium.

The healthy full-grown body tends to establish nitrogen equilibrium by adjusting its rate of protein metabolism to its food supply within wide limits. The time required by the body for this adjustment depends mainly upon the extent to which the diet is changed.

The following observations by Von Noorden illustrate the establishment of equilibrium after only moderate changes in the diet:

A young woman weighing 58 kilograms (128 pounds) at rest in bed was given food furnishing protein, 106 grams; fat, 71.6grams; carbohydrate, 200 grams; fuel value, 1860 Calories per day.

* Theoretically the elimination through the skin should also be determined and included in the calculation; practically this is usually neglected unless on account of warm weather or vigorous exercise the subject has perspired profusely. For data on nitrogen in perspiration see Benedict, Journal of Biological Chemistry, Vol. 1, page 263 (1906) and A Study of Prolonged Fasting, Publication No. 203 of the Carnegie Institution of Washington, pages 233-235.

Example Of Adjustment To Diminished Intake

Here there was practical equilibrium after the second day. The small amount of nitrogen represented as stored on the third, fourth, and fifth days was very likely lost through the skin. This was a case of adjustment to a lowered protein intake, for the food previously taken, although not accurately observed, was known to have been rich in protein.

Another experiment was made by Von Noorden with the same patient to show the time required to reach equilibrium after increasing the intake of protein. In this case the food furnished 2030 Calories per day and the nitrogen balance was as follows:

Here where the amount of protein fed was increased from 90 to 130 grams without change in the total fuel value of the diet, the body reached equilibrium on the fourth day after the increase.

It is apparent therefore:

(1) That the body tends to adjust its protein metabolism to its protein supply.

(2) That when the body is accustomed to a certain rate of protein metabolism, it requires an appreciable length of time to adjust itself to a materially higher or lower rate.

Hence the rate of protein metabolism on any given day will depend in part upon the rate of metabolism to which the body has been accustomed and in part upon the protein intake for the day. When the protein supply varies from day to day, the metabolism for each day is influenced by both the factors, with the net result that the elimination equals the intake when averaged for a sufficiently long period, although the data for any particular day might show a distinct gain or loss. When the protein supply is constant for a few days, the effect of previous habit usually disappears and equilibrium is established as in the above cases.

A transitory loss of nitrogen from the body is apt to be due simply to the taking of less than the usual amount of protein food, but a persistent loss indicates that the diet is insufficient, either in total food (calories) or in protein, to enable the usual adjustment to take place.

A transitory storage of nitrogen in the body may occur as the result of an increase either of the protein or of the total fuel value of the food; but a persistent storage occurs, as Von Noor-den has pointed out, only under the following conditions:

(1) In the growing body (or in pregnancy) where new tissue is being constructed.

(2) In cases where increased muscular exercise calls for enlargement of the muscles.

(3) In cases where, owing to previous insufficient feeding 01 to wasting disease, the protein content of the body has been more or less diminished and consequently any surplus available is utilized to make good the loss.