784 NUTRITION AND HEAT REGULATION. 



usually neglected except in observations upon conditions in which 

 muscular activity has been a prominent feature. As a rule, the 

 amount of nitrogen is determined by some modification of the Kjel- 

 dahl method. In principle this method consists in heating the 

 material to be analyzed with strong sulphuric acid. The nitrogen 

 is thereby converted to ammonia, which is distilled off and caught 

 in a standardized solution of sulphuric acid. By titration the amount 

 of ammonia can be determined, and from this the amount of nitrogen 

 is estimated. Nitrogen forms a definite percentage of the proteid 

 molecule (about 16 per cent.) ; so that if the weight of nitrogen is 

 multiplied by 6.25 the weight of proteid from which it is derived 

 is obtained. If, on the other hand, the nitrogen is determined in 

 the food eaten during the period of the experiment it is evident that 

 a balance may be struck which will determine whether the body 

 is receiving or losing nitrogen. If the balance is even the body is 

 in nitrogen equilibrium, that is, it is receiving in the food as much 

 nitrogen (or proteid) as it is metabolizing and eliminating in the 

 excreta. If there is a plus balance in favor of the food it is evident 

 that the body is laying on or storing proteid tissue, while if the 

 balance is minus the body must be losing proteid. During the period 

 of growth, in convalescence, etc., the body does store proteid, and 

 under these conditions the balance is in favor of the food nitrogen. 

 But throughout adult life under normal conditions our diet is so 

 regulated by the appetite that a nitrogen equilibrium is maintained 

 through long periods. Under experimental conditions, involving, 

 for instance, a special diet, it often becomes necessary to make 

 the analyses for nitrogen in order to determine whether or not the 

 individual is losing or gaining proteid or is in equilibrium. 



It is important also to bear in mind that nitrogen or proteid 

 equilibrium may be established at different levels. If, for instance, 

 a man is in nitrogen equilibrium on a diet containing 10 gms. of 

 nitrogen, what will happen if the proteid in this diet is doubled? 

 Our experience teaches us that the extra 10 gms. of nitrogen or 

 62.5 gms. of proteid is not stored in the body indefinitely. As a 

 matter of fact, the extra proteid is metabolized in the body and 

 nitrogen equilibrium becomes established at a higher level. Whereas 

 under the first condition 62.5 gms. of proteid were eaten, and 62.5 

 gms. of proteid were lost from the body either in the form of nitrog- 

 enous excreta or in the feces as undigested proteid, under the 

 second condition 125 gms. of proteid are eaten and 125 gms. of pro- 

 teid are lost. The total mass of proteid tissue in the body may 

 remain the same, or if any increase takes place at the beginning of 

 the change in diet it soon ceases. Experimentally it is found that 

 there is a certain low limit of proteid which just suffices to maintain 

 nitrogen equilibrium, and between this level and the capacity of 



