THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 151 
a similar illustration of this effect of non-nitrogenous nutrients. 
Assuming average figures for the nitrogen and carbon content of the 
food materials used, he obtained the following results: 
Food per Day. Gain or Loss by Body. 
Meat, Fat, Nitrogen, | Carbon, 
Grms. Grms. Grms. Grms. 
Proteids ny (1 experiment)...... 350 ae +1.66 | —2.69 
and fat (2 experiments)... 80 30 —0.08 | +4.46 
The possibility of such a substitution of non-nitrogenous nutri- 
ents for the food proteids as is illustrated in the foregoing experi- 
ments seems, indeed, almost a necessary corollary of the facts con- 
cerning proteid metabolism considered on previous pages. We 
have seen that, beginning with the fasting metabolism, the effect 
of successive additions of proteids to the food is to stimulate the 
proteid metabolism. Only a relatively small proportion of the 
added proteids is employed by the organism for constructive pur- 
poses, the larger part of it undergoing very promptly nitrogen 
cleavage and thus constituting, to all intents and purposes, an ad- 
dition to the supply of non-nitrogenous material available for 
metabolism. It appears quite natural, then, that the portion of 
the proteid supply which thus serves substantially as fuel to the 
organism should be replaceable in the food by non-nitrogenous 
materials which are capable of serving the same purpose. 
Fats AND CARBOHYDRATES.—The apparent identity of the func- 
tions of the fats and carbohydrates as sources of energy which has 
been shown in the preceding paragraphs necessarily implies the 
possibility of their mutual replacement in the food. Rubner* has 
completed the chain of evidence by showing experimentally that fat 
and dextrose may thus replace each other, A dog received for 
twelve days a ration of 300 grams of lean meat and 42 or 50 grams 
of fat, with the exception of three days, on which varying amounts 
of dextrose were substituted for the fat. On six days the respi- 
ratory products were determined. Averaging the results for all 
the days on which the food was the same, and assuming the lean 
* Zeit. f. Biol., 19, 370. 
