258 THE BIOCHEMISTRY OF B VITAMINS 



of other dietary constituents on bacteria. Thus, fecal elimination of most 

 of the B vitamins in rats is more of a criterion of the dietary protein level 

 than of any other factor, 53 this being particularly true for biotin, panto- 

 thenic acid, and nicotinic acid. Protein levels likewise influence hepatic 

 storage of B vitamins, making the liver an uncertain tissue for study. On 

 diets in which factors other than the B vitamins are held constant, it has 

 been found 54 that the thiamine content of feces is quite constant and 

 independent of the intake. Similarly, the biotin in the combined urine 

 and feces of humans on low, moderate, and high biotin intakes, respec- 

 tively, is about nine, three, and one to five times the dietary level, or 

 approximately constant. 55 



Milk which is generally quite available for study is unfortunately also 

 influenced in its B vitamin content by a variety of factors other than 

 dietary vitamin levels 55 (p. 347), and has not as yet proved of great 

 value in this regard. It has been shown, however, that there is some 

 correlation between the thiamine levels in human blood and in milk, other 

 factors being constant, 57 and that a daily intake of 1.5 mg of thiamine 

 produces a level of about 20 fig per cent in the milk. Moreover, both the 

 thiamine and riboflavin levels in milk seem to vary with urinary excre- 

 tion, indicating some possibility of studies of the requirement during 

 lactation. 58 This type of study has not, however, been extensive as yet. 



A variety of other materials has been found somewhat more satis- 

 factory for assessing tissue vitamin sufficiency. It seems well established 

 that pork generally reflects the dietary thiamine level of the hog, but 

 studies have not as yet been reported on the assessment of porcine thia- 

 mine requirements by such a method. 59 This technique of assessing vita- 

 min requirements, like the urinary excretion method, lends itself best to 

 those cases where more direct means are not practical. 



A typical example of this approach is the work of Czaczkes and Gug- 

 genheim 60 on the riboflavin requirements of the rat. On diets containing 

 no riboflavin or 5 fig of riboflavin per day, the riboflavin content of liver, 

 kidney, muscle, and urine steadily decreases. On a level of 7.5 fig per 

 day, however, balance is maintained, while on 10 /xg, the tissue content in- 

 creases. Thus, it seems logical to conclude that for these 50-gm rats, the 

 riboflavin requirement is about 7.5 fig per day. These workers point out 

 that the blood content remains constant, while the kidney content does 

 not show an excess, but readily reflects a deficit. Both liver and muscle, 

 however, were good indicators of the nutritional status. 



Not only must the validity of the particular tissue as an indicator be 

 established as above, but while employing any tissue, all other dietary 

 constituents must be kept constant. Thus, it has been found that vitamin 

 C controls riboflavin storage to a marked degree, 61 as does protein. More- 



