CARBOHYDRATES AND FATS. 913 



in one of four general ways, which may be tabulated briefly as 

 follows : 



1. Conversion of sugar to glycogen (liver) breaks down in 

 alimentary glycosuria. 



2. Conversion of glycogen to sugar (liver) breaks down in 

 injuries to the central nervous system, excessive internal secretion 

 by adrenal gland, etc. 



3. Glycolysis of sugar (muscles and other tissues) breaks down 

 in diabetes mellitus and pancreatic diabetes. 



4. The normal impermeability of the kidney breaks down in 

 phlorhizin diabetes. 



Functions of the Carbohydrate Food. The general value of 

 the carbohydrate food to the organism may be summarized as 

 follows: (1) It furnishes a source of energy for the needs of the 

 tissue cells and particularly for muscular work. It will be remem- 

 bered that the glycogen of a muscle disappears in proportion 

 to the work done by the muscle, and, indeed, prolonged muscu- 

 lar work, especially during starvation, may wipe out quickly the 

 entire store of glycogen in the body, in the liver as well as in 

 the muscles. It is usually believed, therefore, that the oxida- 

 tion of the sugar furnishes energy which by the machinery of 

 the muscles is utilized to do work, that is, to cause muscular 

 contractions. It seems probable that under normal conditions this 

 material furnishes the main, if not the sole source of energy for 

 muscular work. (2) The oxidation of the sugar furnishes an im- 

 portant part of the constant supply of heat needed by the body. 

 Each gram of sugar on oxidation yields 4 Calories of heat, and, 

 since the carbohydrates form the largest part of our diet and are 

 easily oxidized in the body, they must be regarded as an especially 

 available material for keeping up the supply of animal heat. The 

 largest part of the energy liberated by the oxidation of sugar in the 

 muscles during contraction takes the form of heat, and even dur- 

 ing muscular rest the condition of tone is probably attended by a 

 constant oxidation of this material. (3) The oxidation of the sugar 

 protects the protein of the body. Attention has already been 

 called to the fact that an animal may be kept in nitrogen equilibrium 

 on a relatively small protein diet provided carbohydrates (or fats) 

 are also eaten. One may say, in fact, that as the carbohydrate food 

 is increased the protein food may be diminished, down to a certain 

 irreducible minimum which is probably the amount necessary 

 for the reconstruction of new tissue. From the chemical com- 

 position of carbohydrates it is evident that they alone cannot serve 

 to build up protoplasm. An animal fed on carbohydrate food 

 alone, no matter how abundant the supply, would eventually 

 starve to death. Within certain limits, however, the carbohy- 

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