There is no information concerning the effects of heat stress on vitamin B₁₂ status. Recent studies have shown that megadoses of vitamin C (500 mg) may detrimentally affect the availability of vitamin B₁₂ from food (Herbert, 1990). Doses of vitamin C of 3 g per day may even result in vitamin B₁₂ deficiency disease. How this occurs is still unclear, but Herbert (1990) states that nutritionists should advise persons taking megadoses of vitamin C to have their blood checked regularly for vitamin B₁₂ status. These findings should be taken into account with regard to the use of vitamin C to reduce heat stress (see section on vitamin C).

Folic acid (pteroylglutamic acid) and folate (pteroylglutamate) are involved with DNA synthesis and nucleotide and amino acid metabolism, and they are especially important in tissues undergoing rapid turnover, such as red blood cells. Folic acid deficiency has been suggested to be the most common vitamin deficiency in humans and can result in anemia (Keith, 1989). No studies have assessed the relationship of folic acid status and exercise performance or the effect of folic acid supplementation on performance.

Biotin acts as a coenzyme for several carboxylase enzymes that are important in supplying intermediates for the Krebs cycle and for amino acid metabolism. It is also important in fatty acid and glycogen synthesis. Biotin deficiencies are rare in individuals consuming a nutritionally sound diet. One study found no difference in blood biotin levels in athletes compared with controls (Nijakowski, 1966). No studies have examined the effect of biotin supplementation on performance.

Many studies have shown that a deficiency of more than one of the B complex vitamins could lead to a decrease in physical performance capacity (for detailed reviews, see Van der Beek, 1985; Williams, 1989). Deficiency of a combination of several B vitamins produced subjective symptoms of fatigue, loss of ambition, irritability, and pain and loss of efficiency during normal work (see Van der Beek, 1985). Most of the studies that evaluated the effects of depletion of several B vitamins were done in the 1940s. More recently, Van der Beek et al. (1988) placed 12 men on a thiamin-, riboflavin-, vitamin C-, and vitamin B₆-poor diet for 8 weeks. After 8 weeks, this diet caused borderline or moderately deficient blood levels of the four vitamins. These deficiencies were associated with a 9.8 percent decrease in and a 19.6 percent decrease in anaerobic threshold. Thus, a restricted diet of 21.3 to 32.5 percent of the Dutch RDA of these B vitamins