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sue iron stores, Snyder et al. (1989) and Nickerson et al. (1985) have reported that providing highly bioavailable iron (heme iron in meat) or iron supplements (105 mg per day) can retard the development of low serum ferritin concentrations.

Given the observation that endurance athletes are typically not characterized by a higher than normal frequency of iron-deficiency anemia (see above), many investigators have questioned the significance of the finding of low serum ferritin concentrations in these individuals. However, it is important to note that the occurrence of low tissue iron stores in these individuals could present a problem with regard to recovery from injuries that result in extensive tissue damage or blood loss. It should be noted that marginal iron deficiency resulting in impaired thermoregulation has recently been observed (Beard et al., 1990); thus exercise-induced alterations in iron status may pose particular risks for individuals exposed to extreme temperatures.

As discussed above, an increased rate of sweat loss of iron is thought to contribute to the depletion of iron stores with chronic endurance exercise. Although the loss of iron via sweat is not normally considered to be a major explanation for the iron depletion, sweat iron concentrations can range from 0.1 to 0.3 mg per liter for men and up to 0.4 mg per liter for women (Aruoma et al., 1988; Brune et al., 1986; Lamanca et al., 1988; Paulev et al., 1983). Given these concentrations, sweat can be an appreciable route of iron loss particularly when sweat rates exceed 5 liters per day. The potential interaction between prolonged exposure to high temperatures and vigorous activity with regard to iron status, and an individual's ability to thermoregulate and recover from injury is an area that needs further clarification.

In sum, dietary iron supplementation may in some instances be justified to ensure good health of the individual. However, caution must be used in advocating excessive iron supplementation, given the potential negative side effects that can be associated with its use, including possible gastrointestinal discomfort, and interactions with other metals that have similar physiochemical properties. For example, it has been suggested that high levels of supplemental iron can inhibit the absorption of zinc (Keen and Hackman, 1986; Solomons, 1986). Given that prolonged exposure to a regimen of strenuous exercise and/or exposure to conditions resulting in high rates of sweat loss is associated with marked changes in zinc metabolism (see below), the potential negative effects of excess iron supplementation are clear.

EFFECTS OF EXERCISE AND HEAT ON ZINC METABOLISM

Lichti et al. (1970) first demonstrated that strenuous exercise can result in marked changes in zinc metabolism. They reported a marked increase in plasma zinc concentrations in dogs following short bouts of intense exercise. This observation was extended by Hetland et al. (1975) who found that



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