458 A MANUAL OF PHYSIOLOGY 



of carbo-hydrate, in larger amount than is just necessary 

 for nitrogenous equilibrium, leads to a more rapid increase 

 in the carbon deficit that is, in the fat put on than if the 

 minimum quantity of proteid required for nitrogenous equi- 

 librium had been given. From this it is inferred that the 

 carbonaceous residue of the broken-down proteid 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 it is certain that the high repute of carbo-hydrates 

 as fattening agents is in part due to their taking the place 

 of proteids 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 proteids at all, the carbon deficit was 

 greater than could be accounted for by the proteids 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. It is probable that the carbo- 

 hydrates are first split up to some extent, and that the fats 

 are then constructed 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 honey proteids 

 are present ; and even when bees fed on pure honey or sugar 

 manufacture wax, it may be derived from the broken-down 

 proteids of their own bodies. 



As to the ultimate fate of the fat, from whatever source it 

 may be derived, our knowledge may be compressed into a 

 single sentence : Sooner or later it is oxidized to carbon dioxide 

 and water, its energy being converted into heat or, directly or in- 

 directly, into mechanical or chemical work; much of the fat 

 absorbed from the intestine rapidly undergoes this combustion 

 without entering the fat-cells of the adipose tissue. 



Summary. At this point let us sum up what we have 



