CARBOHYDRATES AND FATS. 923 



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- 

 drates are protein sparers; the energy provided by their oxidation 

 keeps up the supply of heat and enables the muscles and the other 

 tissues to obtain the energy necessary for their special kind of 

 work, and in this way, chiefly, the carbohydrates protect the living 

 protein from consumption and enable us to reduce the protein 

 material in our diet. Experiments show, in fact, that carbo- 

 hydrate is much more efficient as a sparer of protein than fat. 

 An animal fed on carbohydrates alone loses less protein from the 

 body than when kept on a fat diet containing the same amount of 

 heat energy, and the minimal amount of protein upon which the 

 body may be kept in nitrogen equilibrium is much lower when 

 the protein is combined with an abundant supply of carbohydrate 

 than in the case of a diet of protein and fat together. It would 

 seem that the body must always have sugar to oxidize. If this 

 material is not furnished in the food, it is obtained by breaking 

 down the body protein itself, as is indicated by the continued 

 formation of sugar in diabetes and also by the fact that even in 

 prolonged starvation the sugar contents of the blood are kept at 

 a normal level. (4) Any excess of carbohydrate, taken as food, 

 beyond the power of the tissues to store as glycogen may be 

 synthesized to form fat. Nutritional experiments, described 

 below, leave no doubt that the fat of the body may be formed 

 from carbohydrate food. It is stated that the fat of the body 

 having this origin, so-called carbohydrate fat, is of a more solid 

 consistency than the fat derived from other sources. (5) To some 

 extent carbohydrate may be utilized in constructive processes. 

 Nucleic acid contains a carbohydrate group, and we have evidence 

 from the experiments of Osborne and Mendel on growing rats that 

 the body can make its own nucleic acid. A carbohydrate group 

 forms part of the molecular complex of some proteins and is a con- 

 stant constituent of the cerebrosides found so abundantly in the 

 central nervous system. A carbohydrate, lactose, is a normal con- 

 stituent of the secretion of the mammary glands. In these and 

 similar cases it is not clear whether the carbohydrate group is 

 derived from the carbohydrate of the food or is constructed from 



