PROTEIN REQUIREMENTS 75 



ogen, blood albumin, fibrin, haemoglobin and blood globulin) pure fat, 

 and pure carbohydrate, with a definite amount of salts. They also 

 found that in spite of the variety it was impossible to keep the animal 

 alive for any length of time. Knapp (221) fed rats on a food contain- 

 ing seven varieties of pure protein, together with cholesterol, lecithin, 

 carbohydrate, fat, and salts, but despite this fact the animals died in 

 a few weeks (nine and a half to sixteen weeks). At first they greedily 

 consumed their food, but in the end they gradually pined and died. 

 Finally he confirmed the statement that rats at least could not sub- 

 sist on milk powder, and that even on a fat-free horse flesh or dog 

 biscuit diet there was a tendency to early death. Death in Knapp's 

 opinion was due partly to the diminished intake of food, the result of 

 a diminished appetite from the monotony of the food, and partly to the 

 concomitant defective digestion and absorption of protein. 



Stepp (385), on the other hand, suggested that much of the former 

 work on pure proteins was defective, and that the failure to maintain 

 life was due to the lack of "lipoid" and not to the nature of the pro- 

 tein. He carried out a long series of experiments which clearly 

 demonstrated that mice died when fed on milk bread, which had been 

 extracted with alcohol, ether or sometimes with chloroform as well, 

 whereas mice fed on the unextracted bread lived and thrived. The 

 animals fed on the extracted bread ate with avidity for the first fort- 

 night, but thereafter their appetites failed and they died. Another 

 series of mice were fed on extracted bread plus the extracted material, 

 but although the animals lived longer than mice fed on extracted 

 bread alone they did not survive like the control animals. He held 

 then that a certain amount of fat " lipoid " which is not lecithin 

 was necessary in the diet. In this connexion the suggestion of Glikin 

 is of interest (157) that lecithin is a material of first-class importance 

 in the tissue metabolism, especially in connexion with growing tissues. 



Folin (132) has suggested the existence of a special tissue meta- 

 bolism which in all probability would necessitate a further supply of 

 special food products for the maintenance of nitrogenous equilibrium 

 in the living tissues. He suggests, for example, that creatine may be 

 such a substance and that in consequence muscle tissue is found to 

 be rich in this material. 



Hopkins (200), from his experiments on feeding with zein, has 

 suggested that certain materials taken in the food are essential to the 

 organism and are capable of utilization without in the slightest degree 

 contributing to the tissue formation or structural maintenance; the 

 formation of adrenaline from some of the aromatic nuclei of the pro- 



