tained exercise in a hot environment would be predicted to have a negative impact on an individual's zinc balance. Note that an intermediate value for zinc absorption was used for these calculations. Zinc absorption from typical foods ranges from 10 to 40 percent (King and Turnlund, 1989); thus the type of meal fed will have a significant effect on the zinc balance of individuals exposed to the above conditions.
Given the frequent observation of exercise-induced hypozincemia and the potentially high amounts of the element that can be lost via sweat, there may be a need for zinc supplementation in situations where prolonged exposure to exercise and heat is anticipated. However, as discussed for iron, caution must be used when advocating zinc supplements because this element at high levels can interfere with copper absorption due to the similar physiochemical properties of zinc and copper (Keen and Hackman, 1986). Chronic (more than 6 weeks) consumption of zinc supplements in excess of 50 mg per day has been linked to the induction of copper deficiency in humans (Fischer et al., 1984; Fosmire, 1990; Prasad et al., 1978; Samman and Roberts, 1988). Lower levels of zinc supplementation have not been reported to result in copper deficiency.
There are considerable data demonstrating an effect of exercise on magnesium metabolism. Rose et al. (1970) reported that serum magnesium concentrations in marathon runners immediately following a race were significantly lower than prerace values, a phenomenon that was attributed to sweat losses of the element during the run. The idea that excessive sweating could result in a high loss of magnesium from the body is consistent with the work of Consolazio et al. (1963) who found that, under normal conditions, sweat loss accounted for over 12 percent of the total daily excretion of magnesium in men working in temperatures of 49° to 50°C. (Typical magnesium losses via sweat are on the order of 3 to 4 mg per liter [Beller et al., 1975; Consolazio et al., 1963].) The observed lowering of plasma magnesium with intense exercise has since been verified by numerous investigators (Beller et al., 1975; Deuster et al., 1987; Franz et al., 1985; Haralambie et al., 1981; Laires et al., 1988; Lijnen et al., 1988; Refsum et al., 1973; Stendig-Lindberg et al., 1987, 1989). The typical reduction in plasma magnesium following intense exercise is on the order of 10 percent. Stendig-Lindberg et al. (1989) reported that low plasma magnesium concentrations can be demonstrated in young men for up to 18 days after strenuous exertion (a 70-km march). In addition to an increased loss of magnesium via sweat, urinary magnesium loss can increase by up to 30 percent following a bout of intense exercise (Deuster et al., 1987; Lijnen et al., 1988). Although