1014 EXPERIMENT STATION RECORD. 



As the characteristic elements of all foods and the liquid and solid 

 excretory products are nitrogen and carbon, the characteristic element 

 of the gaseous excretory products is carbon. Therefore, if the income 

 and outgo of nitrogen and carbon are determined the data are secured 

 for measuring approximately the total income and outgo of material ' 

 since this may be expressed in terms of nitrogen and carbon. 



If tlie amount of carbon consumed in the food is greater than that 

 excreted in the respiratory products, urine, and feces there is a gain 

 of carbon in the organism, and vice versa. If carbon is gained it is 

 ordinarily assumed that it is stored in the body in the form of fat 

 since this is the characteristic carbon compound present in any con- 

 siderable amount in the body. If the outgo of carbon is greater than 

 the income it is commonly assumed that the food is insufficient and 

 some fat of the body has been utilized. The percentage of carbon in 

 fat is known, and from the gain or loss of carbon the gain or loss of 

 fat can be calculated. As previously stated, the gain or loss of lean 

 can be calculated from the gain or loss of nitrogen, as the percentage 

 of nitrogen in lean meat is also known. These factors, together with 

 the total gain or loss in weight of the subject, furnish data forjudging 

 of the physiological effect of a ration and the comparative value of 

 the gains made on different rations. 



The source of fat in the animal body is a question which lias been 

 much discussed. As already mentioned, the commonly accepted theory 

 is that fat is formed from the excess of fat and carbohydrates con- 

 sumed in the food. It has been maintained that when an excess of 

 protein is consumed fat may be formed from protein ; that is, the pro- 

 tein is broken down in the body into simpler nitrogenous and nitrogen- 

 free substances. While the nitrogen is excreted, some of the carbon is 

 retained and stored in the form of fat. The early experiments which 

 were regarded as proofs of the formation of fat from protein were made 

 with a respiration apparatus. The subject, generally a dog, was fed 

 lean meat, which was assumed to be free from fat and glycogen. All 

 the nitrogen consumed was excreted. Some carbon was retained. The 

 conclusion was therefore drawn that it was retained as fat; that is, that 

 fat had been formed from protein. The discussion of the accuracy of 

 this deduction and the theories based upon it is still going on. The 

 possibility of the formation of fat from protein is rendered probable 

 by experiments of an entirely different nature. 2 



It has been found that muscular work increases the metabolism of 

 carbon; that is, increases the excretion of carbon dioxid in the breath. 



'In this discussion no meution has been made of mineral matter. Iu many exper- 

 iments the balance of mineral matter has been determined either in terms of total 

 ash or of chlorin, phosphorus, sulphur, etc. Though the mineral constituents of the 

 food are of undoubted importance, their function, aside from the formation of the 

 skeletal framework of the body, is not definitely understood. They undergo little 

 cleavage in the body, and hence are of practically no value as sources of energy. 



-N. Zuntz. E. S. R., 7, p. 538. 



