n6 THE PHYSIOLOGY OF PROTEIN METABOLISM 



due to the absence from the tissues of some material which directly, or 

 indirectly, caused its retention. Investigation showed that the creatine, 

 present in the urine as the result of a fast, always disappeared, if carbo- 

 hydrate food were given, but not if either protein, or fatty food with- 

 out carbohydrate, were given. Fats given for several days, however, 

 may bring about the disappearance of the creatine from the urine of a 

 dog, or at least a reduction in the amount present, but so far this has 

 not been observed in man. It is probable that the canine meta- 

 bolism as regards fats is more adaptable than the human. Pari (317) 

 has also obtained certain experimental data from feeding dogs with 

 fat after starvation which might also be interpreted as evidence of the 

 possibility of adaptation taking place. 



It is believed that these experiments showing the appearance of 

 creatine in the absence of carbohydrate and its disappearance when 

 carbohydrate is added to the diet are good evidence in support of the 

 hypothesis that a re-utilization of nitrogenous substances set free by 

 the decomposition of tissue protein does normally take place. It is 

 not meant to be inferred that the form in which the nitrogen is re- 

 utilized is actually creatine, in all probability, it is a more complex 

 molecule containing perhaps creatine, or a creatine precursor, as a 

 constituent which takes part in the resynthesis, and when resynthesis 

 does not occur, owing to the absence of carbohydrate, this complex 

 material is decomposed with a concomitant appearance of creatine 

 in the urine. The appearance of creatine under these conditions is 

 merely to be regarded as an index of faulty metabolism in general, 

 and perhaps more particularly of the non-occurrence of resynthesis. 



The Part Played by Carbohydrates. 



That carbohydrates play an exceptionally important part in the 

 utilization of protein generally has been repeatedly demonstrated. 

 The best example of this is found in the feeding experiments with 

 abiuret digestion products where, unless there be an abundant supply 

 of carbohydrate, there is no retention of nitrogen. The experiments 

 of Lesser (see p. 35), who attempted to repeat the work of Loewi, give 

 a most excellent demonstration of this fact. It will be remembered 

 that this worker was unable to confirm the findings of Loewi, but an 

 examination of his protocols showed that he used fats only to make 

 up the caloric deficiences of his dietary, omitting carbohydrate com- 



