NUTRITION 273 



acids combine in a similar way with reactive aldehydes or ketones, in the organism. The 

 formation of pyruvic and glyceric aldehydes in carbohydrate metabolism, as we shall see later, 

 is practically certain. 



Knoop (1910) showed that an a-ketonic acid injected into an animal was converted into 

 the corresponding amino-acid. This is the reverse process to one of the modes of de-amination 

 of amino-acids, as we saw above, and is, apparently, together with the similar conversion of 

 a-hydroxy-acids, the first step in resynthesis of protein. 



Many facts relating to this part of our subject have been referred to previously, 

 in an incidental manner. It will have become clear that the great function of 

 carbohydrate food is to afford energy. This applies not only to that given off 

 by muscle in contracting, about which more details will be given in Chapter XIV., 

 but also to that required to bring about endothermic reactions, an example of 

 which we have met with in the case of nitrifying organisms. 



The consumption of sugar in the active heart has been shown by Locke and Rosenheim 

 (1907), Rohde (1910), and confirmed by others ; in the intestine by Rona and Neukirch (1912). 

 In these cases, the particular kind of sugar supplied is not a matter of indifference. Dextrose, 

 mannose, and galactose are utilised by the intestinal muscle, and increase its activity. 

 Fructose is not consumed, and has no effect in increasing activity. 



In what follows I must assume that the reader is familiar with the elementary 

 facts relating to the properties and stereochemistry of the ordinary carbohydrates ; 

 they will be found in the book of Bunge and Plimmer (1907, pp. 106-130) and 

 from some aspects in that of L. J. Henderson (1913, pp. 222-232). The work 

 of Emil Fischer (1884-1908) has been the chief means of our information of the 

 constitution of the sugars ; as we have seen, that of the purines and proteins is 

 also due to him. 



Since the organism must have a supply of material for energy purposes, if the 

 more appropriate carbohydrates and fats are absent, protein is used up. This 

 function of carbohydrates as sparers of protein is shown even in starvation, where 

 the nitrogen output falls to one-third of its previous amount if cream and starch 

 are given (Cathcart, 1909). But carbohydrate is more effective than fat; the 

 nitrogen output, diminished by carbohydrate, goes up again on fat only. The 

 fact is probably one aspect of the essential function of carbohydrate for protein 

 synthesis. 



PRODUCTS OF CARBOHYDRATE METABOLISM 



Although the ultimate products of carbohydrate metabolism in the organism 

 are, of course, carbon dioxide and water, it is a matter of interest, as well as of 

 importance, to know the stages passed through, since these result in the production 

 of reactive substances, which play an essential part in various physiological 

 phenomena, including the processes of synthesis. 



The knowledge we possess is due mainly to the work of Embden with his 

 co-workers and of Dakin with his co-workers. To simplify description, a diagram, 

 taken in the main from the results of these investigators, will be of service : 



c?-Glucose 



H 



Active glyceric aldehyde ^! Glycerol 



H 



Pyruvic aldehyde ^ o?-Alanine 



H 



e?-Lactic acid 



H 



Pyruvic acid 



* 



Aceto-acetic acid -<- Acetaldehyde ^ Alcohol 



i ' 



(Acetic acid) 



I 

 Carbon dioxide and water 



18 



