METHODS OF ASSESSING B VITAMIN REQUIREMENTS 255 



B vitamin metabolism and of the chemical nature of excretion products 

 has done much to remedy this situation (p. 365) . 



Johnson et al. 32 have critically analyzed many of the technical factors 

 involved in this type of study, devoting special attention to the thiamine, 

 riboflavin and N'-methylnicotinamide content of urine. A comparison of 

 fasting specimens, random specimens and samples after an oral loading 

 test seemed to indicate that fasting urinary excretion studies may be far 

 more accurate than studies using a loading test. While there is little doubt 

 that random urine samples are valueless as compared with the fasting 

 samples, it is apparent (p. 351) that the many factors involved in the 

 storage and excretion of the B vitamins have caused many workers to 

 express grave doubt concerning the value of fasting urinary levels of B 

 vitamins in assessing dietary requirements. This doubt, aggravated by 

 extensive individual variability, seems well founded, as judged by most 

 recent studies. 33, 34 Dietary interrelationships may operate to make such 

 errors even greater. Illustrative of this is the demonstration that very 

 high nicotinamide intakes have been shown to increase thiamine excretion 

 by as much as 70 per cent. 35 



In spite of these and other difficulties, much instructive information is 

 obtainable in this manner. Studies on individuals whose vitamin intakes 

 have been cut by 28 to 66 per cent from the normal levels show that the 

 change was generally reflected in the urinary levels within a single week. 

 In these studies, however, physical changes were not observable through- 

 out the test period of five weeks. 36 There can be little doubt therefore 

 that when cautiously interpreted, results from urinary excretion studies 

 may be of extreme value. It is possible to discuss in the limited space 

 available only a few of the more recent applications of this method, prin- 

 cipally as illustrative examples of the possibilities of this approach. 



In an extended series of studies, Oldham and co-workers, on the basis 

 of urinary excretion data obtained from urine samples from young 

 women, 37 have decided that the thiamine requirement is somewhat less 

 than 1.0 mg/day or 20 /^g/kg body weight. 



Michelson et al. SH have studied in detail the problems inherent in 

 assessing the level of thiamine nutrition by this means, and have pointed 

 out several important considerations. The attainment of excretion equi- 

 librium at a given intake level requires considerable time, and this was 

 not realized in much earlier work. They found that a change in daily 

 intake of thiamine is on an average only half reflected in the excretion 

 in ten days. These authors studied both thiamine and pyramin * excretion 

 over prolonged periods, and found large (threefold) variations between 

 individuals in thiamine excretion at high levels of thiamine intake. They 



* 2-methyl-4-amino-5-hydroxymethylpyrimidine. 



