148 PRINCIPLES OF ANIMAL NUTRITION. 
hydrates seems to be to cause a slight decrease, but the chief effect 
is upon the carbon metabolism, increasing rations of carbohydrates 
resulting in a progressive reduction of the amount of body fat meta- 
bolized. 
The effect of starch or sugar when added to an insufficient pro- 
teid diet may be illustrated, as in the case of fat, by a comparison 
of Pettenkofer & Voit’s results, cited on p. 150, with those on pro- 
teids alone: 
Number Food per Day. ates Boe 
ager 
ments. | Meat, | Fat, | Starch, |Dextrose,|Nitrogen,} Carbon, 
Grms. |Grms.| Grms. | Grms. Grms. Grms. 
Proteids alone ....... 6 |) B00 | va) cee) agg | ee tod 
«and starch... 8 500 | 5.3 200 ai —1.8; + 9.0 
se “dextrose . 3 500 ae 200 —1.3 | + 7.2 
Mutual Replacement of Nutrients.—The facts which have been 
considered in the foregoing pages show a remarkable degree of 
flexibility in the animal organism as regards the nature of the mate- 
rial consumed in its vital processes. The amount of proteid mate- 
rial necessarily required for the metabolism of the mature animal 
we have seen to be relatively small. Aside from this minimum, the 
metabolic activities of the body may be supported, now at the ex- 
pense of the stored body fat, now by the body proteids, and again 
by the proteids, the fats, or the carbohydrates of the food. What- 
ever may be true economically, physiologically the welfare of the 
mature animal is not conditioned upon any fixed relation between 
the classes of nutrients in its food-supply, apart from the minimum 
requirement for proteids. The possibility of a mutual replacement 
of the several classes of nutrients in the food follows almost neces- 
sarily from the power of the organism to utilize them all indiffer- 
ently (in a qualitative sense at least). 
RpPLACEMENT OF ProTErps.—It has been shown that proteids 
in excess of the minimum demand can be used by the organism to 
take the place of body fat previously metabolized. Furthermore, 
as we have just seen, the non-nitrogenous nutrients of the food may 
likewise be substituted for body fat. It is natural to suppose, there- 
fore, that that portion of the proteid supply which serves substan- 
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