METABOLISM. 49 1 



A body can be maintained by proteid food alone in a state of 

 nitrogen equilibrium. If, however, you add nonproteid foods, it is 

 seen that the amount of proteid necessary to nitrogen equilibrium can 

 be lessened ; hence the nonproteid foods are sparers of prdteid. Hence 

 you decrease the proteid food and increase the nonproteid food, yet the 

 body does not lose more proteid than' before, and nitrogen equilibrium 

 continues as before. The proteids develop energy by oxidation, 

 especially that form manifested in the shape of heat, and also recon- 

 stitute the protoplasm. But the nonproteid foods can also develop 

 heat and work, and thus can substitute for the proteid foods in part. 



Hence an animal may be kept in nitrogen equilibrium on a much 

 smaller amount of proteids, provided fats or carbohydrates are eaten. 



When a fat animal takes proteid in large amounts, then the 

 destruction of fat is increased; and if there is hardly any fat in the 

 food, the fat stored up in the animal will lessen. 



Each increase of proteid ingested produces a rise in proteid metab- 

 olism; hence a nitrogen equilibrium can be obtained on the many 

 different amounts of proteid. The body is unable to store any large 

 quantity of proteid. 



If a person living on a diet which will keep up equilibrium with 

 the proteids undergoing complete metabolism, then increasing the 

 nonnitrogenous food in the diet will cause some of the protein to 

 remain stored in the body. If with sufficient diet a large quantity of 

 carbohydrates be dispensed with in the diet the protein destruction is 

 increased; the body then seizes upon its own protein and the stored 

 fats and carbohydrates. 



Other conditions being equal, the amount of nitrogen eliminated 

 depends upon the richness of fat in the tissues ; a fat animal eliminates 

 less nitrogen than a lean one. In case of a food exclusively of fat 

 there is a very small diminution in the elimination of nitrogen. In the 

 case of a food exclusively of carbohydrates there is always an appreci- 

 able elimination of nitrogen. Hence the body eliminates nitrogen 

 whatever may be the food qualitatively or quantitatively. Hence, to 

 maintain, a body in nitrogenous equilibrium it is necessary to give 

 proteid. 



The quantity of meat which an animal can decompose in 

 nitrogenous equilibrium has two limits, a lower limit and a superior 

 limit, which represent the greatest quantity of meat which the animal 

 is able to ingest, digest and absorb without causing trouble. These two 

 limits are difficult to fix, as the lower limit depends on the state of 

 fat and carbohydrates in the tissues and the superior limit upon the 



