32 FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 



because of the greater store of fat, etc. This helps to explain the 

 long period of survival noted by various earlier investigators who 

 have usually employed older (mature) animals for their feeding trials. 



THE CASEIN DIET. PRELIMINARY TRIALS. 



More experiments of this sort have been carried on with casein 

 than with any other protein as the sole nitrogenous component. It 

 can readily be isolated in a state of comparative purity; and inas- 

 much as it contains phosphorus in an organic complex, known by 

 long experience to be assimilable, one of the problematical features 

 pertaining to most other proteins is eliminated by its use. Our 

 efforts have been directed toward rinding a diet containing casein 

 as the sole protein and which might meet the requirements of a long- 

 continued experiment. In this we have been to a large degree suc- 

 cessful in the case of mature rats. 



Our earlier trials were conducted with a variety of food-mixtures 

 containing casein. It was soon apparent that the protein require- 

 ment of the animals can be satisfied with comparative ease ; accord- 

 ingly the ration was prepared with a nitrogen concentration of about 

 2.5 per cent. The necessity for the use of much fat to insure the 

 requisite paste consistency (and thus avoid scattering of the food) 

 has put distinct limitations on the range of choice. We have tried 

 without success to avoid the use of so much fat. To indicate the 

 variety and proportion of inorganic elements which we have 

 attempted to introduce, some of the mixtures are given in table XII. 



Table XII. 

 Salt mixture I (Rohmann). Salt mixture II (McCollum). Salt mixture III. 



grams. grams. 



Ash of milk 60.6 NaCl 33.4 



Ca :i (P0 4 ) 2 24.2 KC1 33.4 



NaCl 12.1 Bone ash 25.1 



grams. 



Ca a (P0 4 ), 10.0 



K 2 HP0 4 37.0 



NaCl 20.0 



Na citrate 15.0 Fe citrate 3.1 Na 2 C0 3 6.7 



Mg citrate 8.0 Fe citrate 1.4 



Ca lactate 8.0 100.0 



Fe citrate 2.0 100.0 



100.0 



It is still debatable whether any "roughage," such as cellulose, 

 is absolutely necessary. McCollum* fed egg-yolk alone to white 

 rats for 18 weeks without unfavorable results. Nevertheless we 

 have introduced an indigestible residue as conforming more nearly 

 to the usual alimentary experience of the animals; and agar-agar 

 was selected because it is more easily manipulated than cellulose 

 and because experience with other animals has shown us how efficient 

 it is for this purpose. f 



*McCollum: American Journal of Physiology, 1909, xxv, p. 127. 



fCf. Saiki: Journal of Biological Chemistry, 1906, II, p. 251. Swartz: Transactions 

 of the Connecticut Academy of Arts and Sciences,i9ii,xvi, p. 247. Mendel and Swartz: 

 American Journal of Medical Sciences, March, 19 10. 



