METABOLISM OF THE B VITAMINS 365 



sion will be in the nature of a summary of the general picture at present 

 available of the levels of excretion themselves. 



Levels in Urine. Table 28 summarizes the major known end products 

 of vitamin metabolism found in the urine. In cases where a variety of 

 products exist, variations occur in the relative amounts of the products, 

 and in some cases at least these variations have been extensively studied. 

 For the most part, however, little is known of the significance of the 

 various forms and their relative amounts. 



Table 28. Known Urinary Products of Vitamin Metabolism 



B Vitamin Urinary form 



Thiamine Thiamine (but not pyrophosphate) 



Pyramin 

 Riboflavin Riboflavin (and phosphate) 



Uroflavin 

 Nicotinic acid Nicotinic acid (and amide) 



N'-methylnicotinamide 



N'methyl-6-pyridone-3-carboxylamide 



Nicotinuric acid 



Quinolinic acid 

 Vitamin B 6 Vitamin B 6 



4-Pyridoxic acid 

 Pantothenic acid Pantothenic acid 



Biotin Biotin 



Folic acid (group) Folic acid 



p-Aminobenzoic acid Acetyl PABA 



Choline Choline 



Inositol Inositol 



The excretion of the B vitamins and their derivatives via the kidney 

 or skin involves processes considerably more complex than passive diffu- 

 sion. While few data are available on the topic, it appears that the renal 

 and dermal thresholds are maintained, in some cases at least, by inter- 

 mediate formation similar to that which occurs upon the passage of a 

 vast number of other metabolites across biological membranes. Whereas 

 there have been some studies of the renal clearance and glomerular filtra- 

 tion of the B vitamins, 186, 187 this field of endeavor is relatively unex- 

 ploited at present. 



In considering the levels of B vitamin excretion in humans, and their 

 relation to the dietary intake, one could do little better than to consider 

 the data shown in Tables 29 and 30, which are taken from the excellent 

 study of Denko et ai. 23 The subjects were seven healthy men from 23 to 

 28 years of age, who were maintained on a balanced diet for a twelve- 

 week period. Various aspects of these data have been previously discussed 

 (pp. 275 and 300) , and they are sufficiently complete and lucid that an 

 extended discussion of their various aspects at this time would seem un- 

 warranted. The data are in excellent agreement with the great mass of 

 published material on this topic, and have the advantage of being far 



