Apart from the question of the total food requirements of the human body, there is much discussion over the necessary protein supply. Investigation has shown there is a necessary daily minimum protein use. Even with persons in starvation, a certain protein destruction goes on, which, after a time, draws on the tissues. When insufficient protein is taken in the food, the necessary balance will be supplied from the body. On the other hand, where there is no growth of tissue or temporary storage of protein, any excess of protein above this minimum requirement is also broken down. In other words, the body maintains a nitrogen balance, the excretion of nitrogen compounds in the urine fluctuating with the intake of proteid.

197. Fixed And Circulatory Protein

Bio-chemists have come to regard the protein of the animal organism as existing in two general conditions, viz., what may be called the "fixed," " stable," or "tissue " protein, and the "circulatory" or "labile" protein. It is the latter that does not resist disintegration, and it probably consists in part of that surplus protein which is immediately derived from the food and which has not become deposited in the tissue form. This is the type of protein that fluctuates according to the food supply, and it is by the disintegration of this that the body keeps in protein balance when sufficiently long periods of time are considered. A sudden increase of food protein may cause a temporary storage, for there appears to be a "lag" in the adjustment of the supply to the expenditure, but the adjustment comes more or less gradually.

198. Protein Standards

The food standards which are based upon observations of practice call for not less than 100 grams of protein daily for professional men, and 175 grams for men at severe labor. Voit gave 118 grams as the standard for a strong man doing moderate work. Notwithstanding these estimates, men in various occupations have been found to maintain a protein balance, that is, no loss of protein occurred from the body, when the intake was much less than the standards set. Dr. Chittenden, a teacher, was able after some training to keep in protein equilibrium on 40 grams of protein per day. Five of the college athletes he experimented with even made a slight gain of protein (nitrogen) on from 55 to 72 grams of protein daily. Several instances are on record where men of moderate size made a daily use of only from 33 to 50 grams. Unquestionably a protein equilibrium may be maintained, temporarily at least, on much less of an intake than is called for by the. dietary standards.

199. Demands On Protein Supply

Is the protein in the dietary standards in excess of what in a few investigations has been found to be a minimum requirement, necessary or even desirable? Would it be to our advantage to eat less meat, fish, cheese, milk, or eggs? We shall see that when an individual passes from a state of comparative inactivity to severe labor, protein exchange is not materially increased, provided carbohydrates and fats are supplied in sufficient quantity. In other words, the source of muscular energy is not in the destruction of protein compounds, but may come almost entirely from the oxidation of the non-nitrogenous parts of the food. A large protein supply, then, does not appear to be necessary to the laborer, as a means of repairing waste of muscle tissue, although it must be confessed that in ordinary dietetic practice he consumes protein foods somewhat in proportion to the severity of his labor.