926 NUTRITION AND HEAT REGULATION. 



for the fatty acids found in milk. It is to be borne in mind that 

 butyric acid and its 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 deami- 

 nization 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. Voit 

 assumed that in this meat the ratio -Q- is equal to 1.34 to 1.37, while 

 Pfliiger showed that it is lower,* 1.33. 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 possibility that fat may also be formed 

 from protein food must be accepted in accordance with what has 

 been stated above concerning the intermediary metabolism of 

 the protein. So far as the amino-acids formed from the food pro- 

 tein during digestion are not reconstructed into the body-protein 



*Pfluger's"Archiv f . die gesammte Physiologic," 51, 229, 1892, and 77, 

 521, 1899. 



