METABOLISM. 345 



was demonstrated in the classical experiment of Lawes and Gilbert. 

 These observers took two young pigs from the same litter. One was 

 killed, and the amount of fat and protein in its body was determined ; 

 the other was fed on a diet containing known quantities of protein, 

 fat, and carbohydrate, and after some weeks it was killed, and the 

 amount of fat in its body was ascertained. 



After deducting the amount of fat taken in the food from that 

 present in the animal's body when it was killed, a large residue remained, 

 which must have been formed either from the protein or carbohydrate 

 of the food. It could not have been formed from protein, since the 

 amount of fat in the animal was larger than the amount of protein 

 food consumed. The greater part of the fat must, therefore, have 

 been formed from carbohydrate ; and there is no doubt that in herbivora, 

 whose diet consists mainly of carbohydrate, the bulk of the fat is 

 formed in this way. There is no evidence that fat can be formed from 

 protein, and animals, e.g. dogs, fed on a purely protein diet do not put 

 on any fat. 



The process by which carbohydrate is converted into fat in the body 

 is not known, but it is probable that the carbohydrate is first broken 

 down into some simple substance such as acetic aldehyde (CH 3 . CHO), 

 or pyruvic acid (CH 3 . CO . COOH), which are known to be formed from 

 carbohydrate, and that by the linking up of molecules of these bodies 

 the fatty acids are synthesised. 



Their possible formation from aldol is thus shown : 



(1 ) CH 3 . CHO + CH 3 . CHO = CH 3 . CHOH . CH 2 . CHO (aldol) 

 (2) CH 3 . CHOH . CH 2 . CHO + |T>0 = CH 3 . CH 2 . CH 2 . COOH + H 2 



Butyric acid. 



The fat formed from carbohydrate contains a large proportion of 

 stearin and palmitin, and has a higher melting point than that 

 usually deposited in the tissues from the food. For this reason the fat 

 of cattle is much firmer than that of omnivorous animals, which may 

 be practically fluid at the body temperature. 



The fat in the fat depots cannot be used directly by the muscles or 

 other tissues, but has first to undergo certain changes in the liver; 

 these consist in the conversion of saturated into unsaturated fats by 

 the removal of hydrogen. A saturated fat, e.g. stearin, is one in 

 which the affinities of all the carbon atoms are satisfied, whereas an 

 unsaturated fat, such as olein, is one in which- the affinities of two or 

 more of the carbon atoms are unsatisfied and the carbon atoms are 

 united by a double bond. 



