416 METABOLISM, NUTRITION, AND DIET 



shows conclusively that fat is synthesized from carbohydrate. It requires 

 about 2.7 grams of dextrose to form i gram of fat, and this condensation 

 takes place with the formation of carbon dioxide and water and the libera- 

 tion of about 15 per cent of the available heat of oxidation. 



Persistent excess of carbohydrate food produces an accumulation of fat, 

 which may not only be an inconvenience causing obesity; but may interfere 

 with the proper nutrition of muscles, produce a feebleness of the action of 

 the heart, and other troubles. 



The formation of fat from proteid is discussed on page 410. 



Obesity is a condition of excessive storage' of fats. In many of these 

 cases there is persistent storing of fat in the presence of a diet of low energy 

 value and with considerable physical labor. It seems that such persons 

 must have a very economic protoplasmic metabolism, a biological factor that 

 lacks sufficient explanation. 



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. A given 

 weight of dextrose, therefore, furnishes much less energy than a correspond- 

 ing weighc 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 i to 1.5 per cent. When this percentage 

 is increased above 2.5, 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., 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 was 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 hepatic 

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

 thus seemed 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. 



