552 METABOLISM, NUTRITION AND DIETETICS 



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 kilo- 

 gramme) 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 

 0-66 kilogramme. Therefore, 5-93 kilogrammes of protein must have 

 been used up, and 7-90 kilogrammes of fat laid on. At least 5 kilo- 

 grammes 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. 



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 

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

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

 of their own bodies. 



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 much poorer in oxygen than carbo-hydrate. 

 But the chemistry of the transformation as it takes place in the body 

 is still imperfectly known, and all that can be done here is to indicate 

 one or two of the ways in which chemists conceive that it may occur. 



The formation of the glycerin component of the neutral fats from 

 carbo-hydrates would appear to present little difficulty. In dis- 

 cussing the formation of glycogen from glycerin (p. 528), it was stated 

 that two molecules of glycerose (glycerin aldehyde), a triose or sugar 

 with three carbon atoms, can be condensed to form a hexose or sugar 

 with six carbon atoms like dextrose, from the condensation or union 

 of a number of molecules of which, with abstraction of water, glycogen 

 is built up. The reaction can be worked equally well in the reverse 

 direction that is, from the hexose dextrose two molecules of glycerin 

 aldehyde can be formed, and then from each molecule of the alde- 

 hyde, by reduction, a molecule of the alcohol glycerin. As a matter of 

 fact, it has been demonstrated that glycerin is produced when the cor- 

 responding aldehyde is brought into contact with minced liver. 



As regards the fatty acid components of the fats, it will be seen from 

 the schematic representation of the katabolism of dextrose on p. 536 

 that acetic acid, a fatty acid, is represented at one of the stages as being 

 formed by the oxidation of a molecule of acetaldehyde. Lactic acid 

 is represented in the same scheme as a previous stage in the decom- 

 position of dextrose, and lactic acid can be converted into acetaldehyde 

 and formic acid, the lowest of the same series of fatty acids of which 

 acetic acid is the next highest member. Thus : 



H.COOH 



Lactic acid. Acetaldehyde. Formic acid. 



Aldehydes (as well as ketones) have a great capacity for entering into 

 reactions with other substances, and their molecules show also a marked 

 tendency to combine with one another, forming new compounds by 

 their condensation. Thus, from two molecules of acetaldehyde one 



