METABOLISM 20I 



part of the molecule, as such a conversion of glycerol into glu- 

 cose would be much simpler than of the entire fat molecule. 

 This view is not, however, in accord with the results indicating 

 the transformation of fat into carbohydrate by the production 

 of a respiratory quotient below 0.7. Taken all together, there- 

 fore, while the evidence of the conversion of fat into carbo- 

 hydrate is not as conclusive as that indicating the reverse for- 

 mation of fat from carbohydrate, it is, nevertheless, pretty well 

 established, at least so far as body fat is concerned. 



While the greater part of the fat of the body and of milk is 

 formed from carbohydrate food, yet food fat itself also yields 

 body fat. The proportion of the food fat that is utilized 

 directly for the production of energy varies with the diet and 

 with the energy requirements of the animal body. Probably 

 in most cases, except when body fat is increasing or milk fat 

 is continually produced, the greater part of the fat food is 

 almost directly converted into heat energy, for, as we shall 

 find, fat is the highest energy food constituent of the three. 



METABOLISM OF PROTEINS 



The metabolism of the protein food is much more compli- 

 cated than that of either of the other constituents and there are 

 many points not yet made clear and concerning which there is 

 more or less discussion and difference of opinion. We shall 

 therefore not pretend to take up the subject in its entirety or 

 to consider any point of it in very great detail. 



The difference between proteins and carbohydrates or fats, 

 as has been repeatedly stated, is that they only, of the three 

 organic food constituents, contain the element nitrogen. The 

 complicated reactions connected with the metabolic changes of 

 this element nitrogen present exceedingly difficult problems which 

 have not yet been fully explained and proved by investigation. 



Amino-acids. — The results of the combined processes of 

 digestion upon protein food constituents is their cleavage into 

 the ultimate products of protein hydrolysis, viz. the amino- 



