CARBOHYDRATES, FATS AND PROTEINS 107 



the lactic acid is brought about by some process requiring 

 the presence of oxygen.* Accompanying the activity there 

 is a decrease in the amount of glycogen in the tissue. 



The removal of lactic acid requires the presence of oxygen, 

 but it does not necessarily lead to the oxidation of lactic 

 acid. The lactic acid may be reformed into glucose or 

 converted into fats or amino acids. One portion may be 

 oxidised in order to furnish the energy to convert another 

 portion into some of the substances mentioned above. 



If glycosuria is produced, as for example by the injection 

 of phloridzin, the amount of sugar excreted in the urine can 

 exceed by a large quantity the amount of carbohydrate 

 stored in the body. This sugar must be formed from other 

 substances, such as amino acids or fats. 



During phloridzin glycosuria .various substances can be 

 administered. If an increase in the excretion of sugar occurs 

 after the administration of any of these substances it is 

 probable that sugar can be formed from those furnishing 

 such an increase. In the case of nitrogenous substances a 

 study of the increased nitrogen excretion furnishes a basis 

 for the comparison of how completely the substances are 

 converted into carbohydrate. 



The results obtained from such experiments show that 

 carbohydrate can be formed from glycerine, lactic acid, 

 propyl alcohol, glycin, alanin and asparagin.f 



Beyond the formation of lactic acid from glucose the 

 further fate of carbohydrates has not yet been determined. 



Hexoses may give rise to pentoses and pentoses may be 

 converted into hexoscs. 



METABOLISM OF FATS 



The oxidation of fats is preceded by the hydrolysis of the fats 

 into glycerine and fatty acid. The glycerine can form carbo- 

 hydrate, or it may be oxidised completely, thus we can confine 

 our attention to the oxidation of the fatty acids. Their oxida- 

 tion depends upon the chemical constitution of the acids. All 

 the fatty acids found in nature contain an even number of 

 carbon atoms which suggests that the building up and destruc- 

 tion of acids involves, as a rule, two carbon atoms at a time. 



Thus, if we combine fatty acids with a phenyl group, which 

 is not oxidised in the body, we can show the removal of two 



* W. M. Fletcher and F. G. Hopkins, Journ. Physiol., 1907, vol. 

 35. P- 247. 



t Graham Lusk, Phlorhizinglucosuria, Ergeb. d. Physiol., 1912, vol. 

 12, pp. 315-392. 



