392 RIBOFLAVIN 



was given intravenously, Selye® studied bilaterally nephrectomized rats to 

 learn more about the role of the gastrointestinal tract in the absorption and 

 excretion of riboflavin. He noted that excess riboflavin was rapidly excreted 

 into the small intestine, especially the duodenum. Destruction of ribo- 

 flavin proceeded slowly, if at all, in an isolated loop of duodenum, but 

 quickly in an isolated large intestine. The bile does not function in the elimi- 

 nation of this vitamin. If the intestinal canal and kidneys are removed, 

 the tissues of the rat cannot destroy or eliminate any significant percentage 

 of large doses of intravenously administered riboflavin. 



The riboflavin lost in sweat under tropical conditions'^ has been considered 

 of nutritional importance. However, the amounts which can be proved pres- 

 ent in sweat are too small to be significant.^- ^ 



Riboflavin is excreted predominantly in the feces, which contain not only 

 the part contributed by the intestinal walls but also that which is synthe- 

 sized by intestinal bacteria.^'' 



Urine contains riboflavin, ^^ riboflavin phosphate,^- and a compound called 

 uroflavin,^^ a derivative which has been reported to be more soluble and to 

 contain more oxygen than riboflavin. The methods of analysis ordinarily 

 used for the estimation of riboflavin in urine do not distinguish between 

 these compounds, since they have similar fluorimetric and microbiological 

 activities. 



The amount of riboflavin in the urine will vary with the recent dietary 

 intake and with tissue storage. The correlation between urinary excretions 

 and dietary intake is discussed on p. 401. Urinary excretion of riboflavin 

 will also be affected by marked alterations in nitrogen balance. ^^'^^ Less 

 is excreted in the urine, on a given intake, when tissue growth is rapid, as 

 during convalescence after severe trauma, ^^ during lactation, ^^ or after 

 administration of testosterone propionate ;^^ more is excreted after severe 



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