THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 105 
age becomes available also as a source of energy to the organism, 
and the metabolism of fat is correspondingly diminished. In 
effect, then, the proteids are simply substituted for more or less of 
the body fat asa source of energy, and Rubner, in a scries of experi- 
ments which will be considered in Part IJ, has shown that the sub- 
stitution takes place, under the condition of these experiments, ap- 
proximately in proportion to the amount of available potential 
energy contained in the proteids and fats respectively. That is, 
if the extra proteids metabolized can supply a certain amount, 
100 Cals., e.g., of energy to the organism, the fat metabolism is 
diminished by a corresponding amount, so that the total expend- 
iture of energy by the body remains unchanged, being simply 
drawn from different sources in the two cases. 
Amount REQUIRED TO PRropucE Carson EquiLisprium.—iIn 
the experiments by Pettenkofer & Voit cited above, the quantity 
of food proteids which resulted on the average in nitrogen equili- 
brium produced substantially an equilibrium also between the 
supply and excretion of carbon. The earlier experiments of Bidder 
& Schmidt * gave similar results. Later experiments, however, 
have given divergent results, nitrogen equilibrium appearing to 
be reached with an amount of proteids which is far from supplying 
sufficient energy for the organism, so that while the stock of pro- 
teids in the body is maintained, its store of fat is still drawn upon. 
We have seen that the proteid metabolism in the normal fast- 
ing animal amounts to 10-14 per cent. of the total metabolism, 
while according to E. Voit (p. 96) the food proteids required for 
nitrogen equilibrium are, roughly, 2} to 3 times the fasting proteid 
metabolism. It follows, then, that an amount of proteids con- 
taining from 25 to 42 per cent. of the total available energy expended 
by the fasting organism will maintain its store of proteids, and this 
being so, the remaining 58-75 per cent. must necessarily be sup- 
plied by the metabolism of body fat. Thus with the dog on which 
E. Voit’s main experiment was made, nitrogen equilibrium was 
approximately reached with 12.05 grams of nitrogen in the food,t 
equivalent to 75.31 grams of protein (NX6.25) and containing 
* Compare Atwater & Langworthy; Digest of Metabolism Experiments; 
U.S, Dept. of Agr., Office of Experiment Stations, Bul. 45, 388. 
+ Loc. cit., p. 69. 
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