5j6 METABOLISM, NUTRITION AND DIETETICS 



in cold climates consume with impunity much greater quantities 

 of fat than any diabetic organism.* 



N II. THE METABOLISM OF FAT. 



Chemistry of Fats. The fats are compounds (esters) of an alcohol 

 with fatty acids, and can be split, with assumption of water, into these 

 constituents by the action of acids or alkalies or of enzymes (lipases). 

 In the majority of the ordinary fats, and in all those which are of 

 physiological importance (the triglyce rides), the alcohol is glycerin. 

 The fatty acid components which may be united with the glycerin are 

 very numerous, and the physical properties of the different fats e.g., 

 their melting-points and solubilities are closely related to the physical 

 properties of the corresponding fatty acids. Thus palmitic and 

 stearic acids are solid at ordinary temperatures, and so are palmitin 

 and stearin, the glycerin esters or fats formed with these acids. Olcic 

 acid, on the contrary, is fluid at the ordinary temperature, and the 

 corresponding fat, olein, is a liquid fat or oil. On the chemical side 

 the fatty acids can be distinguished as saturated and unsaturated. 

 The fatty acids of the series CnH^+x-COOH are saturated acids. 

 Where n is o we have formic acid, H.COOH; where n is i, acetic acid, 

 CH 3 .COOH; where n is 2, propionic acid, CH 3 .CH a .COOH ; where 

 n is 3, butyric acid, CH 3 .CH 2 .CH a .COOH, and so on, each acid in the 

 series differing from the one immediately preceding it in possessing an 

 additional CH a group. In the case of the higher members of the series 

 these carbon chains become very long. In palmitic acid, for instance, 

 CH S .(CH 2 ) 14 .COOH, there are fourteen CH a groups, and in stearic acid, 



CHs.tCH^.COOH, sixteen. Oleic acid, C8 2 17 "X=C/ . COOH 



is a representative of a series of unsaturated fatty acids whose general 

 formula is CnH^-i-COOH. As the formula of oleic acid shows, the 

 unsaturated fatty acids contain hi their molecule two carbon atoms 

 united by a double link, and one of these valencies can be occupied by 

 halogens (e.g., chlorine) or by oxygen. Erucic acid, a fatty acid occur- 

 ring in certain vegetable oils for example, in rape oil also belongs to 

 this series, and linolic acid, found in linseed oil, to another series of 

 unsaturated fatty acids. Then there are the so-called oxyfatty acids, 

 which in their turn comprise saturated and unsaturated acids. They 

 differ from the ordinary fatty acids in containing one or more OH 

 groups. Thus a dioxystearic acid, C 17 H 33 (OH) 2 .COOH, in which two 

 of the H atoms in stearic acid are replaced by OH, is found in castor-oil. 

 It is clear, from the great variety of the fatty acids, that by their union 

 with glycerin (with loss of water) a very large number of different fats 

 can be formed . Thus, when all the OH groups in the trivalent alcohol are 

 replaced by palmitic acid we have tripalmitin ; when they are replaced 

 by stearic acid, tristearin ; when they are replaced by oleic acid, triolein ; 

 and so on. As a group such fats may be termed homo-acid fats, since all 

 the OH groups are replaced by the same fatty acid. Thus 



CHj.OH C^H^.COOH 

 CH.OH + CH 



CHj.OH C^H 



Glycerin. 3 molecules stearic acid. Tristearin. \Vatet. 



* The importance of controlling the diet, even to the extent of introducing 

 periods of fasting, so as to keep the diabetic free from acidosis and glycosuria, 

 has been recently emphasised (Allen) . 



