THE METABOLISM OF FAT 551 



fat, it must be noted that upon the whole, under normal conditions, 

 it is their specificity of composition rather than their dependence 

 upon the composition of the fat mixture in the food which is the 

 striking fact, and undue weight can easily be given to the results 

 of feeding experiments where great quantities of quite foreign fats 

 are administered. When small quantities of fats very far removed 

 in their properties from the normal fat of an animal are given 

 in the food, they are either completely utilized before reaching 

 the fat depots, or transformed into normal body- fat, since no change 

 whatever can be detected in the latter. If they have been utilized, 

 then it may be that a corresponding amount of fat, formed, say, 

 from dextrose, has been laid down in the fat stores. If this fat is 

 formed from dextrose, it will, of course, be the kind of fat which 

 the particular animal is accustomed to form from dextrose that 

 is, the fat characteristic of the animal. If the foreign fat is itself 

 transformed into body-fat when given in small amount, this same 

 feat can without doubt be gradually accomplished in the case of 

 the surplus of foreign fat laid down in the depots when a large 

 quantity of it is given in the food. 



/ Formation of Fat from Other Sources than the Fat of the Food-^ 

 From Carbo-Hydrates.4-It has been foun^fihatlthe addition of 

 protein to a diet of fat, and especially to a diet of carbo-hydrate, 

 in larger amount than is just necessary for nitrogenous equilibrium 

 (p. 594), leads to a more rapid increase in the carbon deficit that 

 is, in the fat put on than if the minimum quantity of protein 

 required for nitrogenous equilibrium had been gi ven j From this it is 

 inferred that the carbonaceous residue of the broken-down protein is 

 shielded from oxidation by the fat, and to a still greater extent by 

 the carbo-hydrates, and so retained in the body as fat. And there 

 is little doubt that the high repute of carbo-hydrates as fattening 

 agents is in part due to their taking the place of proteins and fats 

 in ordinary ' current ' metabolism, and so allowing body-fat to be 

 laid down from these. Voit, indeed, has gone so far as to assert 

 that this is the only sense in which carbo-hydrates can be said to 

 form fat, and that, in carnivorous animals at least, a direct con- 

 version never occurs. But the experiments of Rubner have shown 

 that in a dog fed with a diet rich in carbo-hydrates, and containing 

 but little fat and no proteins at all, the carbon deficit was greater 

 than could be accounted for by the proteins being broken down in 

 the body and the fat of the food. In the pig and goose, too, the 

 direct formation of fat from carbo-hydrates has been demonstrated. 



For example, in an experiment by Tscherwinsky two young pigs 

 of the same litter were taken. They weighed respectively 7,300 grammes 

 and 7,290 grammes. One was killed, and the amount of fat and nitrogen 

 in its body directly estimated. From the nitrogen the maximum 

 quantity of protein which could be present was calculated. The other 

 pig was fed for four months with barley, which was analyzed. The 

 excreta were also analyzed to determine the amount of unabsorbed 



