GENERAL METHODS HISTORY OF PROTEIN FOOD. 809 



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 

 protein molecule (about 16 per cent.) ; so that if the weight of nitro- 

 gen is multiplied by 6.25 the weight of protein from which it is de- 

 rived 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 protein) 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 protein tissue, while 

 if the balance is minus, the body must be losing protein. During 

 the period of growth, in convalescence, etc., the body does store 

 protein, 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 condi- 

 tions, involving, for instance, a special diet, it often becomes neces- 

 sary to make the analyses for nitrogen in order to determine whether 

 or not the individual is losing or gaining protein or is in equilibrium. 

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

 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 protein in this diet is doubled ? 

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

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

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

 nitrogen equilibrium becomes established at a higher level. Where- 

 as under the first condition 62.5 gms. of protein were eaten and 62.5 

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

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

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

 tein are lost. The total mass of protein 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 protein which just suffices to maintain 

 nitrogen equilibrium, and between this level and the capacity of 



