Chemical Changes in Animal Organism. 281 



ordinary constituents of foods the proteins are the most 

 expensive, and consequently the diet with the smallest 

 proportion of these substances which will fully supply the 

 bodily needs of the organism will be the most economical. 

 It must be remembered that proteins are necessary in the 

 adult individual to make good the bodily waste which is 

 continually taking place. If the protein supplied does not 

 fulfil the purpose, then, even if the caloric value of the 

 diet is more than sufficient for the energy needs, the 

 organism will continually lose some of its own protein 

 material. In other words, more nitrogen will be excreted 

 in the urine and the fseces than was ingested in the food. 

 When the output of nitrogen balances exactly the intake, 

 the body is in "nitrogenous equilibrium." By adminis- 

 tration to an individual of a mixed diet of fats, carbo- 

 hydrates, and protein, the caloric value of which is 

 sufficient, and by determining the nitrogen balance by the 

 estimation of the total nitrogen in the food and excreta, it 

 is possible to ascertain whether the amount of protein is 

 sufficient. If this is the case, there will not be an excess 

 of nitrogen excreted over that ingested. With any given 

 protein or mixture of proteins it is possible to determine 

 the minimum which will maintain the body in nitrogenous 

 equilibrium. The diet necessary for different individuals 

 will vary greatly with circumstances, such as the weight 

 of the subject, the amount of muscular work to be per- 

 formed, and other factors. It has been estimated that 

 an ordinary workman, working for 89 hours per day, 

 will require a diet of caloric value of about 3,000, 

 containing about 118 grammes of protein, which will 

 correspond to an output of somewhere about 16*5 grammes 

 of nitrogen in the urine. Recent experiments indicate, 

 however, that the bodily needs can be sufficiently supplied 

 with very much smaller amounts (less than half), and ex- 



