THE ABSORPTION AND USE OF FOODS 459 



animal be deprived completely of protein food, although supplied 

 abundantly with fats and carbohydrates, he continues to lose 

 nitrogen from the Body, since tissue breakdown continues, but 

 presently the daily loss of nitrogen becomes constant at a low 

 "starvation level." If this daily loss of nitrogen be measured and 

 an amount of protein sufficient to replace it exactly be added to 

 the food, nitrogen equilibrium is not thereby restored; there is still 

 loss of nitrogen, and to get the animal back into nitrogen equi- 

 librium more protein must be fed. It is hard to explain this except 

 upon the assumption of fuel-proteins already made. The second 

 line of evidence for fuel-proteins as essential to the diet has been 

 gained in feeding experiments with gelatin. Although animals 

 will refuse to eat this albuminoid after a few days, it has been 

 shown that during the period when gelatin is being eaten nitrogen 

 equilibrium can be maintained upon very small quantities of true 

 protein, much less than suffices under any other diet. Since it is 

 certain that albuminoids cannot function as tissue-restorers the 

 conclusion is that the amount of protein required for tissue- 

 restoration is considerably less than the total amount demanded 

 by the Body, and that therefore the Body has a fuel-protein re- 

 quirement which must be met. 



Carbon Equilibrium. For an animal to be in carbon equilibrium 

 only needs that all the fuel taken in be burned, and that no reserve 

 store be called upon. Aside from the temporary storage of carbo- 

 hydrate food as glycogen all the fuel taken into the Body must 

 look forward to one of two fates, either to be oxidized promptly or 

 to be stored in the form of fat for future use. Just as nitrogen 

 equilibrium may be established on a high or a low level so carbon 

 equilibrium can be maintained in the face of variations in the in- 

 take of fuel. It is easily seen, however, that the limits of carbon 

 equilibrium must be narrower than of nitrogen equilibrium. The 

 actual protein requirement of the Body is so much less than the 

 usual protein intake that considerable variations in the protein 

 consumed can be made without affecting the nitrogen equilibrium; 

 but the energy requirement of the Body is quite definite, varying 

 with the work done rather than with the food eaten. Thus it fol- 

 lows that the fuel intake and the energy requirement are harder to 

 keep balanced than are the nitrogen intake and outgo. It may 

 easily be a matter of astonishment how successfully the Body, un- 



