CHAPTER XL VIII. 

 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 

 cent. In addition it is believed that during starvation glycogen 

 or sugar may be made from the protein tissues of the body, and 

 possibly also from the body fat, although this latter source is 

 disputed. The supply of glycogen under normal conditions is 

 maintained chiefly 'by the carbohydrate food. As was explained 

 in the section on Digestion, the starches, sugars, gums, etc., 

 which constitute 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. There is still some 

 difference of opinion as to whether all proteins are capable of 

 yielding glycogen to the body. Some physiologists believe that 

 after the nitrogen is split off to form urea, the non-nitrogenous 

 portion of the molecule may be converted to glycogen in the 

 liver. Others hold that only those proteins, such as egg-albu- 

 min, which contain a carbohydrate grouping in the molecule 

 are capable of yielding glycogen in the body.* The store of glyco- 

 gen 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 regula- 

 tion of the supply of sugar to the blood is usually attributed to 

 the liver. This regulation is adjusted so that the percentage of 

 sugar in the blood is kept astonishingly constant, not only during 

 the conditions of ordinary living, but under such an abnormal con- 

 dition as prolonged starvation. It is assumed that this regulation 

 is effected mainly by an enzyme formed in the liver cells which 

 converts the glycogen to dextrose in proportion as the sugar of the 

 blood is used up by the tissues. 



Regulation of the Sugar-supply of the Body. This regula- 



* See Pfliiger. in "Archiv f. die gesammte Physiologic," 96, 1, 1903, for 

 literature and discussion. 



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