88 PRINCIPLES OF ANIMAL NUTRITION. 
The great majority of cases gave values lying between 10 and 
17 per cent. 
Errrect or Bopy Fat.—Both from the summary on p. 87 and 
from the individual results cited by Voit, it is evident that while 
the proportion of energy supplied by the metabolism of pro- 
teids in the fasting animal is normally small and varies only within 
rather narrow limits, it is still subject to relatively considerable 
variations. The most important cause of these variations in the 
fasting animal under uniform external conditions appears to be 
the ratio of fat to protein in the body. 
C. Voit * appears to have first noted that when fasting is pro- 
longed sufficiently to nearly exhaust the reserve of visible fat in 
the body, the proteid metabolism, after remaining nearly constant 
or decreasing slightly for some days, as in the examples just given, 
begins to increase somewhat rapidly. This increase Voit attrib- 
uted to the exhaustion of the fat, the oxidation of which had hith- 
erto partially protected the organized proteids of the body. Sub- 
sequent investigations, particularly Rubner’s,t have in general 
confirmed Voit’s observation, while giving it a somewhat more 
general form. 
E. Voit { has recently reviewed the available experiments upon 
fasting metabolism in their bearing on this question. From the 
experimental data he computes or estimates, first the ratio of pro- 
teid to total metabolism (expressed in terms of energy), and “second 
the ratio of proteids to fat in the body on the several days of each 
experiment. A comparison of these ratios shows a very marked 
correspondence, a high ratio of proteids to fat in the body coin- 
ciding with a large proteid metabolism compared with that of fat, 
and vice versa. The graphic representations of the relations as 
given by Voit are especially convincing. Moreover, the results 
show that the extent of the proteid metabolism does not depend 
directly upon the duration of the fasting. With different animals, 
or with the same animal under different conditions, a certain ratio 
of proteid to total metabolism is attained whenever the correspond- 
ing ratio of proteid tissue to fat in the body is reached, whether this 
be early or late in the experiment. 
The growing ratio of proteid to total metabolism in the fasting 
* Zeit. f. Biol., 2, 326. } Loc. cit. See p. 86. + Zeit. f. Biol., 41, 502. 
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