magnesium, potassium, sodium, and zinc—has been suggested to be influenced by exercise. Due to space constraints, this chapter will focus on four elements—iron, magnesium, zinc, and copper—to exemplify concepts involved in exercise-and heat-induced alterations in mineral metabolism and nutrition. However, prior to this discussion a few comments will be made concerning iodine, selenium, and chromium. The reader is directed to Chapters 12 and 13 for information on exercise-and heat-induced changes in sodium and potassium metabolism.
Consolazio (1966) reported that a considerable amount of iodine can be lost via sweat. In that study, 12 adult males were maintained at a temperature of 38.5°C during the day and 33.1°C during the night. During the 24-hour period, the men exercised at a moderate rate on a bicycle ergometer for 1 hour. The average total sweat loss of the men over the 24-hour period was 5576 g, which resulted in an average loss of 146 µg of iodine. Given that the 1989 U.S. recommended dietary allowance (RDA) (NRC, 1989) for iodine for adult men and women is 150 µg per day, and the observation that typical iodine intakes exclusive of iodized salt range from 250 to 170 µg per day for men and women, respectively (Pennington et al., 1989), it is evident that sweat-associated iodine loss can be significant. The above findings suggest that it is critical that iodized salt (which provides >70 µg of iodine per g of salt) be consumed when an individual is in an exceptionally hot area and/or engaged in strenuous activity. Studies examining the influence of combined heat exposure and endurance exercise on iodine metabolism are needed.
As with iodine, there is limited literature on the influence of exercise and heat on selenium metabolism, although it has been suggested that athletes may benefit from selenium supplements due to its role in glutathione peroxidase synthesis. Singh et al. (1991) reported that plasma selenium concentrations decreased in men exposed to a 5-day rigorous training program conducted by the U.S. Navy, despite an increase in dietary selenium intake during the program. Singh and colleagues suggested that the decrease in plasma selenium might have reflected a shift in selenium from the plasma pool to tissues requiring increased antioxidant protection. This hypothesis would be consistent with the observation that exercise can result in increased rates of tissue lipid peroxidation (Davies et al., 1982). Although the above observations suggest that selenium metabolism may be influenced by exercise, to date there is no compelling evidence that selenium supplementation is necessary for individuals engaged in endurance activities (Lane, 1989; Lang et al., 1987).