Because vitamin D is involved with calcium metabolism, it could be thought to be related to exercise performance. However, existing evidence suggests that vitamin D supplementation does not affect work performance (Keith, 1989). Unique findings have been reported by Bell et al. (1988) who showed that blood levels of Gla-protein, an indicator of bone formation, and vitamin D were higher in subjects involved in muscle building training compared to controls. The authors suggested that the muscle building exercises stimulated (a) osteoblastic bone formation and (b) the production of vitamin D, possibly to provide calcium for newly forming muscle tissue. Whether these data indicate a greater vitamin D requirement for strenuous work where large loads are carried or moved is not known, and the question warrants further investigation.
Exposure to sunlight in a hot environment should be sufficient to prevent a vitamin D deficiency. In fact one study found no cases of vitamin D deficiency rickets in a survey of 224 African infants (Kendall, 1972). Because the mothers spent time in sunlight, and breast feeding is universal in the African population studied, babies get sufficient vitamin D. Also, it has been suggested that tropical vegetable foods contain appreciable amounts of vitamin D (Kendall, 1972).
The major symptom of vitamin E deficiency in animals, which was identified in 1922, is a damping of the reproductive ability. However, muscle wasting or dystrophic muscles have also been noted in vitamin E-deficient animals (Bieri, 1990). It was not until the 1950s that vitamin E was shown to be important for humans as well as other animals.
Vitamin E comprises at least four compounds known as tocopherols. The most active and well known of these is alpha-tocopherol. Vitamin E has been shown to function as an antioxidant of polyunsaturated fatty acids in cellular membranes (Machlin and Bendich, 1987). In this role, vitamin E serves as a free radical scavenger to protect cell membranes from lipid peroxidation. Free radicals are chemical species with one or more unpaired electrons in their outer orbit, which makes them highly reactive. Because strenuous exercise can increase lipid peroxidation (Kanter et al., 1988; Maughan et al., 1989), vitamin E may have important implications for exercise or work capacity.
Plasma or serum tocopherol levels can provide a relatively good index of vitamin E status (Machlin, 1984). Although few studies have assessed vitamin E status of athletes (Cohen et al., 1985; Guilland et al., 1989; Weight et al., 1988), vitamin E deficiencies are considered rare (Kagen et al., 1989). Vitamin E intake among athletes is considered to be more than sufficient (Buskirk, 1981; Clarkson, 1991). High vitamin E intakes were