368 CARNEGIE INSTITUTION OF WASHINGTON. 



unskimmed and skinmied milk, respectively. Furthermore, foods made 

 with desiccated skimmed milk, containing at best only traces of the milk 

 fats, have proved much inferior to those made with desiccated whole 

 milk, since they have led to nutritive failure in every case, though as 

 a rule at a much later interval than is observed with our usual mixtures 

 of isolated foodstuffs containing lard as the only fat. The decline can 

 be at once stopped by the admixture of butter-fat to the dietarj'. 



We have now raised a large number of rats to adult size and have 

 maintained them far longer than a year in perfect health on a diet con- 

 sisting of a single purified protein, or a mixture of two such proteins, 

 together with starch, lard, butter-fat, and natural "protein-free milk." 

 Some of these have produced normal young, which in turn have also 

 thriven on the same food. These experiments emphasize the fact that 

 the need for protein is essentially one for suitable amino-acids and that 

 either variety or quantity in the protein aspect of the diet apparently 

 serves merely to insure a suitable supply of all the essential ones. 



The nutrition-promoting factor in the butter-fat, which seems to be 

 so essential for long-continued normal growth, is not contained in that 

 component represented by the solid glycerides least soluble in alcohol. 

 B}^ fractioning butter-fat we have now been able to demonstrate that 

 the more liquid portions, or what we may term the butter oil, contain 

 the effective ingredient. That it is not universally present in natural 

 oils has also been demonstrated by the inability of fats like almond oil 

 and olive oil to replace the butter oil in promoting resumption of growth. 

 On the other hand, the fat of the egg-yolk and cod-liver oil are efficient 

 in the same way as butter oil. The nutrition-promoting properties of 

 cod-liver oil were tested in great detail, owing to the widespread popu- 

 lar and medicinal use of this product. Our experiments afford, we 

 believe, direct evidence that cod-liver oil is something more than a 

 mere nutrient. At present experiments are in progress to ascertain how 

 much butter oil is necessary to prevent nutritive disaster, or to induce 

 restoration where decline has resulted. The quantities appear to be 

 surprisingly small, as little as 1 per cent of butter-fat in the diet 

 sufficing for more than nine months to prevent the much earlier decline 

 hitherto experienced when lard alone was used. Experiments are also 

 in progress to test the efficiency of the tissue fat of the cow in compari- 

 son with the fat of the manamary secretion. Beef fat now appears to 

 be not entirely devoid of the advantageous properties found in butter- 

 fat. Many of the experimental animals develop an infection of the 

 eye during the periods of their nutritive decline. This has been noted 

 by other investigators. The simple addition of butter-fat or cod-liver 

 oil to the diet without other change leads to a prompt disappearance 

 of the eye disease. 



Professor L. F. Rettger, who earlier cooperated with us in studying 

 the intestinal bacteria of the rats on our experimental dietaries, has 



