918 NUTRITION AND HEAT REGULATION. 



oxyacid may be derived theoretically, at least, from proteins as 

 well as fats. As has been shown in the previous pages, the amino- 

 acids formed from the proteins may suffer deaminization in the 

 body, leaving behind an oxy- or ketonic acid which may then 

 behave as do the similar acids formed by oxidation of the higher 

 fatty acids. Experiments have demonstrated that in this way 

 oxybutyric acid may be derived from a protein source. 



Origin of the Body Fat. — The views upon the origin of body 

 fat have undergone a number of changes in the last fifty or sixty 

 years, illustrating in an interesting way how development of our 

 experimental methods leads often at first to half-truths which are 

 corrected later by more extensive work. Dumas and others (1840) 

 held to the natural view that the fat of the body originates directly 

 from the fat of the food. Liebig, applying his more exact methods, 

 demonstrated that in some cases at least this source is insufficient 

 to account for all the fat. The fat yielded by the milk of a cow 

 for instance, may be greater in quantity than the fat contained 

 in the food. He also pointed out that the fat of each species of 

 animal is more or less peculiar, the fat of the sheep having a higher 

 melting point than pork fat, and both differing in composition from 

 the fat taken as food. "In hay or the other fodder of oxen no 

 beef suet exists, and no hog's lard can be found in the potato refuse 

 given to swine. " He was led to attribute the source of body fat 

 chiefly to the carbohydrate food, and this belief agreed well with 

 the experience of agriculturists as to the use of such foods in fatten- 

 ing animals for market. This view, in turn, was displaced by the 

 theory of Voit, supported by elaborate feeding experiments. Voit 

 believed that the fat of the body is formed mainly or entirely from 

 the protein of the food, the carbohydrate and the fat of the diet 

 being useful only to protect a part of this protein from oxidation. 

 Voit's experiments have been shown by Pfliiger* to have been 

 based upon erroneous analyses of the meat used in his experiments. 

 The modern point of view is that the fat of the body originates 

 partly from the fat of the food, particularly in carnivora, and 

 partly from the carbohydrate of the food, especially in herbivora, 

 in whose diet this foodstuff forms such a large part. The pos- 

 sibility that fat may also be formed from protein food must be 

 accepted in accordance with what has been stated above con- 

 cerning the intermediary metabolism of the protein. So far 

 as the amino-acids formed from the food protein during diges- 

 tion are not reconstructed into the body-protein of the animal, 

 they are deaminized, and the organic acid grouping left may 



* Pfliiger's "Archiv f. die gesammte Physiologie," 51, 229, 1892, and 77, 

 521, 1899. 



