74 THE PHYSIOLOGY OF PROTEIN METABOLISM 



The Influence of a " Pure " Diet. 



Apart from this difference in composition, other factors influence 

 the food, particularly the protein requirements of the body to a 

 great extent. Experiments in which one form of protein has been 

 given as the sole source of nitrogen for a long period demonstrate that, 

 in spite of the abundance of nitrogen in the diet, the animal ceases to 

 thrive. 1 This may be due to the lack of certain " minimum " nitrogen- 

 ous substances in the food, but other factors apparently equally potent 

 play a part. Much of the earlier work in this connexion was faulty 

 owing either to the manner in which the experiments were carried 

 out, or to the fact that the diets could not be regarded as " pure," 

 i.e. the protein used was not absolutely free from impurities. Not- 

 withstanding this it has been found that if animals be kept for a pro- 

 longed period on one diet they invariably die in spite of an abundant 

 caloric intake. The early work of Lunin (267) in which the diet con- 

 sisted of caseinogen, fat, cane sugar, and the ash of milk, showed that 

 death took place much earlier if no ash were present. Further, mice 

 fed on this artificial diet died in from twenty to thirty-one days, 

 whereas mice fed on simple dried milk were still living at the end of 

 two and a half months. Hall (162) who also fed mice on a mixture 

 consisting of caseinogen, fat, starch, cellulose and ash of milk, found 

 that death resulted within forty days. All his animals fed greedily 

 at first. Steinitz (384) fed dogs on the same form of diet, but 

 after a fortnight or three weeks vomiting supervened, and the ex- 

 periment had to be broken off. Rohmann (339) also gave a so-called 

 pure diet, but with greater variation in the non-protein part. He 

 found that under these conditions his animals (mice) remained well 

 for weeks. The diets he used cannot unfortunately be considered as 

 " pure ". The animals were practically fed on an ordinary mixed diet. 

 He made the suggestion that the various proteins might vary greatly 

 as regards their nutritive value and that in this variation lay the ex- 

 planation of many of the poor results previously obtained. Jacob 

 (210) found that both pigeons and rats could be kept for long periods 

 (one of the rats lived 124 days) on a diet of caseinogen, fat, starch and 

 salts, but that eventually death resulted. Falta and Noeggerath (125) 

 fed rats on mixtures of absolutely pure proteins (egg albumin, casein- 



1 Recently Osborne and Mendel have published a long series of experiments which, if 

 they be confirmed by future work, are of fundamental importance. They found that pure 

 proteins like edestin or excelsin when fed to rats with protein-free milk suffice to keep the 

 animals in good condition over a long period. (Carnegie Report, 1911.) 



