366 CARNEGIE INSTITUTION OF WASHINGTON. 



lusted 77 days. The animals consumed practically all of the food 

 offered, so that the food intake of each scarcely varied more than 

 2 or 3 gm. We thus found that the average total gain in 11 weeks 

 upon the diet containing 14.8 per cent of lactalbumin was 122 gm., 

 in contrast with a gain of 105 gm. and 95 gm. on similar diets con- 

 taining 16.2 per cent of casein and 16.7 per cent of edestin respec- 

 tively. With 19.8 per cent of casein or 20.5 per cent of edestin the 

 total gains were below those obtained on the same dail}' intake of 

 food contaimng 9.9 per cent of lactalbumin. 



Since the food intakes were strictly comparable and the trials 

 extended over long periods, the superior nutritive efficiency of lact- 

 albumin is thus demonstrated. Incidentally, inasmuch as the animals 

 all received the same amount of food on corresponding days, the 

 rapidly growing ones, fed with the superior growth-promoting pro- 

 teins, were actually at a disadvantage. 



Wlien casein was supplemented by cystine, so as to make the 

 quantity of this amino-acid equivalent to 3 per cent of the protein, 

 a smaller quantity of casein sufficed for a given gain in weight; thus, 

 whereas on a diet containing 8 per cent of casein the total gain during 

 11 weeks was 71 gm., in contrast with 77 gm. gained on an otherwise 

 comparable food containing 8 per cent of lactalbumin, the addition 

 of cystine to an 8 per cent casein diet led to growth equal, while the 

 experiments lasted, to that on the 8 per cent lactalbumin diet. It is 

 unlikely that this result is due to a stimulating action of the cystine; 

 for the substitution of alanine, under precisely similar conditions, failed 

 to induce more rapid growth. 



The only strict basis for com.parison is afforded by experiments in 

 which the animals receive the some amount of food during the same 

 period of time and make the same gain in xceight. In order to eliminate, 

 as far as possible, inequalities in the rate of growth of different 

 individuals and also to be certain that no protein would be 'Svasted" 

 by feeding more than the individual could actually utilize in the 

 metabolism of growth, the concentration of protein in the rations of 

 the present series was lowered so that with a sufficient total energy 

 intake, the protein furnished was insufficient to permit average normal 

 growth. For these experiments a diet containing 8 per cent of 

 lactalbumin Avas taken as a standard with wliich edestin and casein 

 (alone or with cystine or alanine) could be compared. Under these 

 conditions of nutrition, identical except as regards the quantity of 

 protein eaten, lactal]:)umin proved to be superior to both casein and 

 edestin. Thus there were required to produce the same gain in body- 

 weight 12 per cent of casein (50 per cent more than of lactalbumin) 

 and 15 per cent of edestin (nearly 90 per cent more). AVith the 

 lower concentrations of casein or edestin the rate of growth was in 

 every case noticeably behind that produced by the standard lactal- 

 bumin food. In this series, likewise, the addition of cystine, equiva- 



