or above adequate blood levels of vitamin C (for review see Clarkson, 1991). These data provide no evidence to suggest that chronic exercise creates a vitamin C deficiency.
Acute exercise appears to increase blood levels of ascorbic acid. Plasma and lymphocyte ascorbic acid levels increased in nine men who completed a 21-km race (Gleeson et al., 1987). This study also found that the increase in plasma ascorbic acid levels correlated significantly with an increase in plasma cortisol. The authors suggested that exercise may cause ascorbic acid to be released from the adrenal glands into the circulation along with the release of cortisol. Normally, vitamin C inhibits adrenocorticotropic hormone synthesis (Strydom et al., 1976). If chronic stress increases the release of vitamin C from the adrenals, an abnormal release of adrenocorticotropic hormones could occur followed by a ''fatigue'' of the adrenal glands. At this point the adrenals could not function adequately in another stress situation (Strydom et al., 1976).
Van der Beek et al. (1990) assessed the effect of vitamin C restriction on physical performance in 12 healthy men. The subjects ingested a diet providing only 10 mg per day of vitamin C for 3 weeks and 25 mg per day for 4 weeks. During this time, vitamin C levels in the blood decreased significantly. However, no effect of the vitamin C restriction was found on or the onset of blood lactate accumulation. The marginal vitamin C deficiency did not alter exercise performance.
Excellent and comprehensive reviews of studies concerning the effects of ascorbic acid supplementation on performance can be found elsewhere (Keith, 1989; Williams, 1989). Keith (1989) cited 19 studies, many from outside the United States, that have shown a positive effect, and 18 that have shown no effect, of vitamin C supplementation on performance. Although several studies have shown that vitamin C supplementation will enhance performance (for example, Howald et al., 1975), these studies are flawed by poor designs, or the subjects may have been deficient in vitamin C. There are equally as many studies, and often better controlled ones, to demonstrate that vitamin C supplementation has no effect (for example, Keith and Merrill, 1983; Keren and Epstein, 1980).
Smokers have been shown to have a greater requirement for vitamin C, and the RDA for smokers is set at a minimum of 100 mg of vitamin C per day (compared with 60 mg per day for nonsmokers) (National Research Council, 1989). Keith and Driskell (1982) examined whether vitamin C supplementation of 300 mg per day for 3 weeks improved measures of lung function, resting and exercise heart rate, resting and exercise blood pressure, and the amount of work performed during a treadmill test in chronic smokers and nonsmokers. They concluded that vitamin C supplementation had little effect on lung function and exercise performance in either smokers or nonsmokers.