ANIMAL METABOLISM 



335 



Fat storage 



The main purpose of fat metabolism is to provide energy by oxidation 

 of the fat. However, before this occurs a large part of the fat eaten 

 is temporarily deposited in the fatty tissues of the body. This deposit 

 provides a reserve of energy for the organism far greater than that in 

 the form of glycogen for not only is a much greater quantity of fat 

 deposited, but it has an energy value of 9 Calories per gram as compared 

 to only 4 for carbohydrate. A certain amount of stored fat is also 

 desirable as a protective covering for certain organs, especially the kidney. 



Dynamic State of Stored Fat. Until rather recently it was supposed 

 that stored fat was more or less inert metabolically — excess food laid 

 away and left undisturbed until needed. This viewpoint was entirely 

 changed by the experiments of Schoenheimer, who fed animals fatty 

 acids containing deuterium, an isotope of hydrogen, in place of some 

 of the hydrogen atoms ordinarily present. He found that after four 

 days about half of the deuterium was present in the stored fats and that 

 much of the isotope had been shifted to several other fatty acids besides 

 the one fed. Also, when water containing deuterium was injected into 

 mice, much of the isotopic hydrogen quickly appeared in the body fats. 

 He concluded that the stored fat was normally in a constant state of 

 flux, even in adult animals having a substantially constant weight and 

 total fat content. About one-half of the body fat is synthesized and one- 

 half broken down each week. 



Nature of Stored Fat. In general, each animal species tends to lay 

 down a type of depot fat characteristic of the species, but the nature 

 of this fat is also greatly influenced by the kind of food eaten. This is 

 true because the animal possesses only a hmited ability to transform 

 one fatty acid into another. 



With the aid of isotopic tracers, chiefly deuterium, it has been demon- 

 strated that the animal can shorten or lengthen the chain of saturated 

 fatty acids. Thus stearic acid, for example, can be converted into pal- 

 mitic and myristic acids, and palmitic can be changed back into stearic 

 again. Animal tissues also contain enzymes which can change saturated 

 acids into certain unsaturated ones, for example, stearic into oleic acid. 

 This process, however, is limited to the introduction of one double bond 

 at the 9,10-position. Desaturation at the a,/?-position also probably 

 occurs during beta oxidation (p. 336). 



That the animal cannot synthesize more highly unsaturated fatty acids 

 such as linoleic or linolenic is shown by the fact that these are essential 

 components of the diet (p. 79) . 



The tissues of animals are able to bring about a saturation of a,^-un- 

 saturated acids. However, if the food fats are more highly unsaturated 



