THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 149 
tially as a source of energy only may be replaced either by body fat 
or by other food nutrients, and this supposition is borne out by the 
observed facts. 
By Body Fat.—In considering the total metabolism of the fast- 
ing animal in Chapter IV, we saw that the fat of the body has a 
marked effect in protecting the body proteids from metabolism, and 
that with the progressive impoverishment of the body in fat, more 
and more of the proteids are substituted for the latter as a source of 
energy. In $1 of the present chapter it was further shown that 
the food proteids, or their non-nitrogenous residue, may be oxi- 
dized in the organism in place of the stored fat of the body. 
It is clear, however, that the same experiments may equally 
well be regarded from the converse point of view as showing that 
the body fat may be oxidized and serve as a source of energy in 
place of the proteids of the food or of the body. In other words, 
it is possible, within quite wide limits, for the animal organism to 
draw its supply of energy, according to circumstances, either from 
food or body proteids or from its stored-up fat. 
By Fats and Carbohydrates of Food.—When, in addition to its 
reserve of fat, a supply of non-nitrogenous nutrients is afforded in 
its food, this range of choice by the organism is still further widened. 
In considering the effects of non-nitrogenous nutrients upon the 
proteid metabolism, and particularly in the discussion of the mini- 
mum of proteids, it became evident incidentally that fat or car- 
bohydrates may to a large extent be substituted for proteids in 
the food. A certain minimum of proteids was shown to be essential 
to the maintenance of the proteid tissues of the body, but proteids 
supplied in excess of this amount undergo nitrogen cleavage and 
serve substantially as a source of energy. This excess of proteids, 
as we have seen, can be replaced in the food by non-nitrogenous 
nutrients, particularly the carbohydrates, at least without damage 
to the proteid nutrition, as is shown by Voit’s results there 
cited (p. 134). The later respiration experiments of Pettenkofer 
& Voit show that this is true also as regards the total metab- 
olism. As appears from the table on p. 109, a ration of 1500 
grams of lean meat sufficed to maintain the dog experimented 
upon approximately in equilibrium as regards the income and 
outgo of both nitrogen and carbon. When, however, a con- 
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