CHAPTER XLVIII. 



NUTRITIVE HISTORY OF CARBOHYDRATES AND FATS. 



The Carbohydrate Supply of the Body.— The available carbo- 

 hydrate material of the body consists of the glycogen found in the 

 tissues, especially in the liver (1 to 4 per cent, or more) and mus- 

 cles (0.5 per cent.), and the sugar formed from this glycogen and 

 present constantly in the blood to the amount of 0.1 to 0.15 per 

 jent. The supply of glycogen under normal conditions is main- 

 tained chiefly by the carbohydrate food. As was explained in the 

 section on Digestion, the starches, sugars, gums, etc., which con- 

 stitute the carbohydrate foodstuffs are eventually absorbed into 

 the blood as simple sugars, chiefly dextrose, but probably also 

 some levulose and galactose. These simple sugars constitute 

 the important glycogen formers. With regard to the proteins 

 there is still some difference of opinion as to whether all of them 

 are capable of yielding glycogen to the body. Accepting the 

 modern view that the proteins in digestion are split into their 

 constituent amino-bodies the question of the relation of the pro- 

 tein-food to sugar-formation may be approached most readily by 

 investigating the effect of feeding the different amino-acids sepa- 

 rately. A convenient method of studying this point is to feed the 

 amino-acids to dogs rendered diabetic by the use of phlorhizin (p. 

 '913). In such animals the amino-acids, if converted to sugar, will 

 appear in the urine as sugar and can be detected without difficulty. 

 Experiments of this kind* indicate clearly that a number of amino- 

 acids can yield sugar in the body, for example, glycin, alanin, as- 

 partic, and glutaminic acids (p. 821). We may believe, therefore, 

 that the proteins giving rise to these amino-acids during digestion 

 may serve as glycogen-formers. The store of glycogen in the body 

 is about equally divided between the liver and the muscular tissues, 

 and it is estimated that in man each of these depots may contain, 

 at a maximum, about 150 gms. The regulation of the supply of 

 sugar to the blood is usually attributed to the liver. This regu- 

 lation is adjusted so that the percentage of sugar in the blood is 

 kept astonishingly constant, between 0.1 and 0.2 per cent., not 



* Lusk, "American Journal of Physiology," 22, 174, 1908; also Ringer and 

 Lusk, "Zeit. f. physiol. chem., 66," 106, 1910. 



907 



