FACTORS INFLUENCING B VITAMIN REQUIREMENTS 269 



fectly valid for fur-bearing animals are not always as significant when 

 applied to man for this reason. Extensive studies have therefore been 

 made upon the increased excretion of B vitamins in sweat. 



Spector, Hamilton, and Mitchell 2G have found that an increase in 

 relative humidity from 65 to 92 per cent at 32.2° C produces an increase 

 in the average net loss of body weight of 176 per cent, but an increase 

 of 221 per cent in the average dermal excretion of pantothenic acid by 

 humans. When the temperature was increased from 28.9 to 38.3° C, the 

 hourly dermal excretion similarly increased from 5.1 to 27.7 tig, and the 

 total urinary and dermal excretion was increased 11.6 per cent. Johnson, 

 Mitchell and Hamilton 27 made similar studies on inositol excretion in 

 sweat, and found that it increased from 27 Atg/hour under comfortable 

 conditions to 118 itg/hour under hot, moist conditions. The general opinion 

 at present resulting from these studies is that B vitamin losses in sweat 

 under tropical conditions are quite small as compared to the normal 

 urinary output, and that there is thus considerable support for the belief 

 that tropical climates do not increase B vitamin requirements in humans. 



With regard to the other extreme of climate, there has been less actual 

 study, perhaps because no one has as yet seriously questioned the 

 obviously increased requirement necessary in cold climates to support 

 the increased caloric requirement. 2711 Johnson and Kark 2S have presented 

 data showing a linear relation between voluntary caloric intake in men 

 (American soldiers), and the environmental temperature, the range being 

 from 92° F and 3100 Calories to -30° F and 4900 Calories! Studies have 

 shown 29 that a nutritional vitamin intake greater than that normally 

 required is without effect in enhancing the ability of men to withstand 

 the harmful effects of repeated exposure to cold climates. 



Effects of Pregnancy and Lactation. Extensive studies have been made 

 of the effects of pregnancy and lactation on vitamin requirements, and 

 there is no doubt that under these circumstances there is a considerable 

 increase in the required dietary level of B vitamins. Thus, Williams 30 

 points out that in Manila the majority of women of the poorer classes 

 show signs of deficiency during the child-bearing age. In many of these 

 same regions, moreover, infantile beriberi is widespread. To a large extent 

 at least this is due to the increased metabolic rate in pregnancy, and to 

 lactation. Since the total metabolism has been shown to be equal to the 

 metabolism of the mother plus the metabolism of the fetus, it might be 

 anticipated that the thiamine requirement under such conditions could 

 be calculated. There is not at present sufficient evidence to assess the 

 validity of such a process, however. 



Siddell and Mull 31 have made a study of urinary excretion in a group 

 of 42 pregnant women. Their results are shown in Table 10. 



