212 PHYSIOLOGICAL CHEMISTRY 



a reserve fuel, and it also protects the body from loss of heat, 

 like a subcutaneous blanket, for it is a poor conductor. 



If an animal is fed excessive amounts of carbohydrate food, 

 it will lay on reserves of fat. There is evidence thus that carbo- 

 hydrates may be converted into fat in the body, although it 

 seems that the reverse process does not take place, at least it is 

 very questionable. Since amino acids may be converted into 

 carbohydrates, they also may be fat formers. 



Recent work has made it evident that there is considerable 

 difference in the character of the fat-like substances in different 

 tissues. The fats of the great inert fat deposits are mostly true 

 neutral fat, whereas the "fats" in the active tissues are prob- 

 ably largely in some other form, perhaps lecithin or allied sub- 

 stances. On hydrolysis, these yield a much lower percentage 

 of fatty acids. The "fats" of the liver are intermediate be- 

 tween depot and tissue fats. Blood "fat" probably is largely 

 lecithin or some similar substance. It is likely that the depot 

 fats, when mobilized, are converted in the liver into lecithin 

 or something similar, and also to a certain extent the acids 

 converted into unsaturated acids. This process may be regarded 

 as preparatory to the use of the fats by the cells. 



Light has been thrown upon the mechanism of fat oxidation 

 in the body in various ways. Ordinarily the fatty acids are 

 burned completely to C0 2 and H 2 0. By introducing into the or- 

 ganism compounds in which a fatty acid side chain is attached 

 to a benzene ring, the last step in this destruction of the side 

 chain is prevented and the nature of the resulting substance may 

 be studied. Such compounds in which the side chain has an 

 even number of carbon atoms are oxidized, and the side chain 

 destoyed with the exception of two carbon atoms. From 

 phenylbutyric acid, phenyl acetic is produced. This substance 

 is conjugated with glycocoll and excreted as phenyl aceturic 

 acid, a compound of phenyl acetic and glycocoll analogous to hip- 

 puric acid (q.v.). If the original side chain contains an uneven 

 number of carbon atoms, the side chain is oxidized away, and 

 benzoic acid remains, which is conjugated with glycocoll and 



