Since the diet has such a great influence upon the amount of protein metabolized, it might be expected that the basal protein metabolism could be observed best in fasting. But in fasting the energy metabolism of the body is only a little lower than with food; the amount of combustion continues nearly the same although only body material is available; and since the body must consume so much of its own substance to obtain the energy needed, there is always a chance that in fasting some protein may be burned simply as fuel. Accordingly the protein metabolism in fasting may be greater than that which represents the needs of the body when properly fed, while on the other hand it may be abnormally low through the effort of the body to adjust itself to the abnormal condition.

The amount of protein broken down in fasting is much influenced (1) by the previous habit as regards protein consumption, and (2) by the metabolism of stored glycogen and stored fat.

The direct effect of the level of protein metabolism on the days preceding the fast is shown in the following data obtained by Voit in experiments upon a dog weighing 35 kilograms:

Influence Of Previous Diet On Nitrogen Elimination In Fasting (Voit)

The influence of the metabolism of the previously stored glycogen upon the amount of protein metabolized in fasting is well illustrated by the following three experiments with one individual:l

1 Benedict, Influence of Inanition on Metabolism. Carnegie Institution of Washington (1907).

It will be seen that the nitrogen output was less when there was available for metabolism a considerable supply of previously stored glycogen. Since most of the stored glycogen is used up on the first day of fasting, its influence upon the protein metabolism is short-lived as compared with that of the stored fat.

The influence of the available supply of body fat upon the protein metabolism of fasting is shown by the following observations of Falck, on the protein metabolism of two fasting dogs - the one lean, the other fat:

A rise in protein metabolism of the lean dog after the 8th day showed that from this time he used protein largely as fuel - so largely that the results were fatal in 25 days of fasting. The fat dog, having plenty of other fuel in the form of fat, used protein to a much smaller extent, so that he was able gradually to accommodate himself to a lower level of protein metabolism and to endure a fast of 60 days' duration.

The professional faster, Succi, starting with a good store of body fat, fasted 30 days * with the following results:

Since Succi's health remained good throughout his fast, it might be thought that the true protein requirement of his body was not greater than the smallest figure found for any period - in this case about 30 grams per day. On the other hand, it may well be supposed that, since the body increases its protein metabolism to an abnormally high rate under influence of excessive protein feeding, so under the influence of fasting the body may be able to adjust itself to an abnormally low rate of protein metabolism; and the fact that the protein metabolism continues to diminish for such a long time in fasting gives weight to the supposition that the body is here gradually adapting itself to an abnormal condition. One might assume that in some particular period of Succi's fast, the effect of previous feeding might no longer be apparent and the conditions had not yet become abnormal as the result of the fasting, in which case the expenditure of protein during one of these periods would represent his normal requirement. Any such assumption must, however, be more or less arbitrary. A much more definite idea of the normal dietary need is obtained by determining experimentally how much protein must be contained in the daily food in order to keep the body in protein (or nitrogen) equilibrium.

* The output of nitrogen and of several other elements during a 31-day fast recently described by Benedict may be found in Chapter IX (Inorganic Foodstuffs And The Mineral Metabolism. The Elementary Composition Of The Body).