CARBOHYDRATES AND FATS. 915 



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 composition of carbohydrates 

 it is evident that they alone cannot serve to l)uild up protoplasm. 

 An animal fed on carbohydrate food alone, no matter how abun- 

 dant the supply, would eventually starve to death. Within cer- 

 tain limits, however, the carbohydrates 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 con- 

 sumption and enable us to reduce the protein material in our 

 diet. Experiments show that carbohydrate is much more effi- 

 cient as a sparer of protein than fat. An animal fed on carbo- 

 hydrates 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 ecjuilibrium 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 ma- 

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

 down the body i^rotein itself, as is indicated by the continued 

 formation of sugar in dialjetes and also b}' 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 



