VITAMIN B (B^) 99 



In the earlier studies on the distribution of vitamin B, notably those 

 of Osborne and Mendel, of Steenbock, and of McCollum, comparisons 

 were made on the basis of the amounts of foods required for approxi- 

 mately normal growth in young rats on an artificial basal ration free 

 from vitamin B and adequate in all other respects : and it is largely 

 through such experiments that estimates such as those quoted in the 

 preceding section were obtained. Difficulty in securing sufficient con- 

 sumption of the food being tested without displacing too much of the 

 basal diet, together with uncertainty as to what constitutes normal 

 growth, has led some investigators to adopt maintenance of weight 

 over a given period rather than gain in weight as the criterion. Dunham 

 (1921) described such a method as follows: "Albino rats from 4 to 

 5 weeks old were fed ad libitum a basal ration of casein 20, starch 61, 

 butter fat 15, and salt mixture 4 per cent until growth had ceased 

 and in some cases a slight decrease in weight had occurred. The vege- 

 table to be tested, air-dried and pulverized, was then fed in decreasing 

 amounts apart from the daily ration until the weight became constant 

 at some point between 75 and 100 grams, when the rat was 100 to 130 

 days old. The feeding of the basal ration plus this amount of vitamin 

 was continued for 30 days, during which time the weight should re- 

 main constant with an allowable variation of ± 4 per cent. At the end 

 of this period 500 milligrams daily of dried yeast was added to the 

 diet in place of the substance tested. Immediate restoration of growth 

 would indicate that the lack of growth was due to insufficiency of 

 vitamin B." 



An important point overlooked in much of the earlier work was 

 that the feces of rats on the experimental diets may contain appre- 

 ciable amounts of vitamin B and that unless care is taken to prevent 

 the rats from consuming their feces the results of feeding experiments 

 may be of httle quantitative value. Steenbock, Sell and Nelson (1923) 

 called attention to tliis and their conclusions were confirmed by Dutcher 

 and Francis (1924), Smith, Cowgill and CroU (1925) and others. 

 Salmon (1925) showed that when conditions are favorable for bac- 

 terial growth in the intestines the feces of rats may be rich in vitamin 

 B even when the diet is deficient in this vitamin. Cages with raised 

 screen floors of sufficiently large mesh to allow the feces to drop 

 through are now used in practically all vitamin B studies. 



Feeding Method. — Healthy young rats, 25 to 29 days of age, and 

 weighing usually 40 to 55 grams are placed in individual metal cages 

 with raised wire screen floors and given ad libitum distilled water and 

 a basal vitamin-B-free diet now known to be practically free from 



