go. PRINCIPLES OF ANIMAL NUTRITION. 
Schulze * claims that this increase in the proteid metabolism of 
the fasting animal is not, in all cases at least, due to lack of fat or 
other non-nitrogenous material to protect the protein from destruc- 
tion. He advances the hypothesis that the loss of protein incident 
to the fasting so injures the cells that finally many of them die 
and the protein of their protoplasm becomes part of the circula- 
tory protein of the body and is rapidly decomposed, thus giving 
rise to an increased excretion of nitrogen. 
While it is not impossible that this ingenious hypothesis has 
some basis of fact, Kaufmann,f in a quite full review of the litera- 
ture of the subject, together with original experiments, shows 
that it can by no means supplant Voit’s explanation. He points 
out in particular that the time when the increase in the proteid 
metabolism begins seems to bear no relation to the loss of protein 
which the body has sustained, while, on the other hand, it coin- 
cides quite closely with the time when the supply of visible fat is 
nearly exhausted.} 
Summary.—In the light of the facts set forth in the foregoing 
paragraphs we may sketch the general outlines of the fasting meta- 
bolism somewhat as follows: 
In the early stages of fasting, particularly if the previous food 
has contained an abundance of proteids, the proteid metabolism 
may be considerable. As the effect of the previous food disappears, 
however, and the store of “circulatory protein” in the body is ex- 
hausted, the proteid metabolism speedily falls to the minimum 
amount required for the vital activities of the protoplasm, and the 
remaining demands of the body for energy are supplied by the 
metabolism of the stored-up fat. If the latter is fairly abundant, 
this stage may last several days, the total metabolism remaining 
nearly constant and the proteids supplying a nearly constant. pro- 
portion of the necessary energy (according to E. Voit about 15-16 
per cent.). Sooner or later, however (unless in a very fat animal), 
the supply of fat from the adipose tissue begins to flag. The de- 
mand for energy, however, remains unabated, and as the fat-supply 
falls off, more and more protein is metabolized in its place, until at 
* Arch. ges. Physiol., 76, 379. + Zeit. f. Biol., 41, 75. 
{Compare also E. Voit’s critique of Schulze’s investigations. (Zeit. f. 
Biol., 41, 550.) 
