CARBOHYDRATES AND FATS. 795 



work. It will be remembered that the glycogen of a muscle dis- 

 appears in proportion to the work done by the muscle, and indeed 

 prolonged muscular 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 

 oxidation 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 proteid of the body. Attention has already been 

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

 on a much smaller proteid diet provided carbohydrates (or fats) 

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

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

 irreducible minimum. From the chemical composition of carbo- 

 hydrates it is evident that they alone can not serve to build up pro- 

 toplasm. Some proteid food is absolutely necessary for the repair 

 of tissue or the production of new tissue during growth. An ani- 

 mal fed on carbohydrate food alone, no matter how abundant the 

 supply, would eventually starve to death. Within the limits speci- 

 fied, however, the carbohydrates are proteid-sparers ; the energy 

 provided by their oxidation keeps up the supply of heat and enables 

 the muscles and probably the other tissues to function normally, 

 and to this extent protects the living proteid from consumption and 

 enables us to reduce the proteid material in our diet. The im- 

 portance of the carbohydrates in nutrition is illustrated in a strik- 

 ing way by the facts in diabetes. Diabetes occurs in man (diabetes 

 mellitus) as a pathological condition which in some cases at least 

 is referable to a lesion of the pancreas. As already described, it 

 may also be produced in dogs and other animals by removal of the 

 pancreas (pancreatic diabetes). Under both conditions when com- 

 plete the carbohydrates eaten are not oxidized in the body, but are 

 eliminated as sugar in the urine. Corresponding to the loss of this 

 source of energy an increased amount of proteid is necessary in the 

 diet, and adding carbohydrate to the food in such cases does not 



