74 



GROWTH 



think of protein, fat, and carbohydrate. Although each group 

 is unquestionably a desirable nutrient, it is quite another matter 

 to say that each is a necessary nutrient, and so we may well ask 

 whether or not any of them may be dispensable. 



We may say at once that proteins are obviously indispen- 

 sable. The necessity of fat in the diet is, however, much less cer- 

 tain. In the first place, the technical difficulties in planning a 

 ration satisfactorily meeting the requirements of such a test are 

 not inconsiderable. The ration must not only be free from fat, 



Figure 47. Growth on diets almost entirely free from pre- 

 formed fat. After Osborne and Mendel. Courtesy of the 

 National Academy of Sciences. 



but in addition must include all other substances known to be 

 essential. That class of vitamins commonly called fat-soluble 

 must obviously be included. In view of these considerations, 

 some of the earlier reports on this topic must be regarded with 

 caution. 



The best available evidence on this point has been supplied 

 by Osborne and Mendel. 6 



In one series the ration was: 



Meat residue 23 per cent 



Starch 72 per cent 



Salt mixture 5 per cent 



The vitamins were supplied in tablets, each one containing 

 0.4 gram yeast, 0.2 gram alfalfa, and 0.2 gram cane sugar, 



