highly variable. For example, one 14-week, double-blind, 2 × 2 × 2 complete factorial experiment examined the effects of restriction of three vitamins—thiamin, riboflavin, and vitamin B⁶—on physical performance in 24 healthy Dutch males (van der Beek et al., 1994). In the thiamin-restricted group, thiamin intake was 0.43 mg/day (analyzed mean value). Thiamin concentration, erythrocyte transketolase activity, and urinary thiamin decreased significantly over the 11-week experimental period, and α-erythrocyte transketolase activity (or activation coefficient) increased. The decrease in thiamin status was accompanied by small but significant decrements in performance as measured during single short bouts of intense exercise, but these could not be attributed to any one of the three vitamins studied.

In another double-blind study, 12 mg of thiamin (15 mg of thiamin nitrate) along with riboflavin and pyridoxine were provided to all 22 subjects in the experimental group for 5 weeks. Although the activation coefficients for transketolase (and other enzymes) decreased in the supplemented group, no change in blood lactate was found after exercise (Fogelholm et al., 1993).

An observational study (Folgeholm et al., 1992) that found comparable erythrocyte transketolase activation coefficients in skiers and nonskiers provided little useful information on the effect of energy expenditure on thiamin requirements. Compared with the nonskiers, the skiers had much higher energy intakes and expenditures along with much higher intakes of all reported nutrients. For both males and females, mean thiamin intakes were 0.8 mg/1,000 kcal for the skiers and 0.7 mg/1,000 kcal for the control subjects.

It was thus concluded that under normal conditions, physical activity does not appear to influence thiamin requirements to a substantial degree. However, those who are engaged in physically demanding occupations or who spend much time training for active sports may require additional thiamin.

Studies were not found that directly compare the thiamin requirements of males and females. A small (10 percent) difference in the average thiamin requirements of men and women is assumed on the basis of mean differences in body size and energy utilization.