SOURCE OF GLYCOGEN 443 



THE METABOLISM OF CARBOHYDRATES. 



Energy Value. The nutritive function of carbohydrates in the body 

 is to serve as a source of energy. They are oxidized, with the ultimate pro- 

 duction of carbon dioxide and water, and must liberate the same amount 

 of energy as when burned outside the body, i.e., 4.1 Calories per gram. 

 A given weight of dextrose, therefore, furnishes a little less than half the 

 energy of a corresponding weight of fat. 



Carbohydrates are strictly energy-formers and may be regarded as the 

 immediate source of the energy of oxidations, while fats are reserves drawn 

 on only after the carbohydrates are used up. Dextrose is a constant constitu- 

 ent of the blood to the extent of about o.i to o . 15 per cent. When this per- 

 centage is increased above o. 25, the dextrose is either stored as glycogen, i.e., 

 in the case of the portal blood during the absorption of a carbohydrate meal, 

 or eliminated by the kidney, i.e., as in diabetes. 



The Formation of Glycogen Glycogenesis. The important fact 

 that the liver normally forms sugar, or a substance readily convertible 

 into it, was discovered by Claude Bernard in the following way: He fed a 

 dog for seven days with food containing a large quantity of sugar and starch; 

 and, as might be expected, found sugar in both the portal and hepatic blood. 

 But when the dog was fed with meat only, to his surprise, sugar w r as still 

 found in the blood of the hepatic veins. Repeated experiments gave in- 

 variably the same result. No excess of sugar was found in the portal vein 

 under a meat diet, if care was taken to prevent reflux of blood from the he- 

 patic venous system. Bernard found sugar also in the substance of the liver. 

 It thus seemed to him certain that the liver formed sugar even when, from the 

 absence of saccharine and amyloid matters in the food, none could be brought 

 directly to it from the stomach or intestines. 



Bernard subsequently found that a liver removed from the body, and 

 from which all sugar had been completely washed away by injecting a stream 

 of water through its blood vessels, contained sugar in abundance after the 

 lapse of a few hours. This post-mortem production of sugar was a fact 

 which could be explained only on the supposition that the liver contained a 

 substance readily convertible into sugar. This theory was proved correct 

 by the discovery of a substance in the liver allied to starch, termed glycogen. 



Bernard's brilliant researches led him to announce the theory that the 

 carbohydrate which is periodically absorbed in large amount is stored in the 

 liver only to be reconverted to dextrose and discharged back into the blood 

 stream whenever the percentage falls below a certain level. He regarded the 

 liver as a storehouse which regulated the blood dextrose to a constant level. 

 This is the glycogenic function of the liver. 



Source of Glycogen. The greatest amount of glycogen is produced 

 by the liver upon a diet of starch or sugar, but a certain quantity is, or at 



