524 A MANUAL OF PHYSIOLOGY 



perhaps derived from broken-down proteins, may be reabsorbed, 

 and take its place among the fat ' put on.' 



Formation of Fat from other Sources than the Fat of the 

 Food (i) From Carbo-hydrates. It has been found that the 

 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. 529), 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 given. 

 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 conversion 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 fat and protein. At the end of the four 

 months the pig was killed. It now weighed 24 kilogrammes, and 

 contained 2^52 kilogrammes protein and 9-25 kilogrammes fat. 

 Subtracting the protein (0-96 kilogramme) and fat (0-69 kilogramme) 

 originally present, 1*56 kilogrammes of protein and 8'56 kilogrammes 

 of fat must have been put on. The amount of protein taken in the 

 food was 7 '49 kilogrammes, and of fat cr66 kilogramme. Therefore, 

 5-93 kilogrammes of protein must have been used up, and 7*90 kilo- 

 grammes of fat laid on. At least 5 kilogrammes of this fat must 

 have come from the carbo-hydrate of the food. Only a small amount 

 of the fat put on could possibly have come from the protein. 



It is probable that in the formation of fats the carbo-hydrates 

 are first split up to some extent, and that the fats are then con- 

 structed from their decomposition products, oxygen being lost 

 in the process, since fat is poorer in oxygen than carbo-hydrate. 

 The production of wax by bees, which used to be given as a proof 

 of the formation of fat from sugar, is not decisive, for in raw 



