discussed, only iron deficiency has any significant prevalence in the human population, although selenium deficiency is observed in certain specific regions of China. However, despite the recent evidence showing that peripheral conversion of the thyroid hormone thyroxin (T4) to tri-iodothysonine (T3) is performed by a selenium-dependent enzyme, there is no evidence linking selenium deficiency to poor thermoregulation in humans.
Profound and severe exercise, especially of the types undertaken by military personnel in the course of their activities, can also cause a dramatic alteration in plasma concentrations of some of these micronutrients which may lead to acute transitory deficiency states in particular tissues. This notion has not been the experimental paradigm for any of these studies and, therefore, warrants further investigation.
The body temperature of an animal is dependent on the balance between processes of heat gain and heat loss. These can be described mathematically by the heat-balance equation:
where S = net rate of heat storage, M = metabolic heat production, W = mechanical work transferred to the environment, E = evaporative heat transfer, C = convective heat transfer, K = conductive heat transfer, and R = radiant heat exchange.
In an environment that is below thermoneutrality, the sole means of heat gain for homeotherms2 is metabolic heat production. Heat loss can occur by evaporation, convection, conduction, radiation, and mechanical work. Metabolic heat in mammals comes from the following metabolic sources: basal metabolism, postprandial thermogenesis, diet-induced thermogenesis, shivering thermogenesis, and nonshivering thermogenesis (Gordon, 1993).
Shivering and nonshivering thermogenesis are the main sources of heat production utilized during cold exposure, and both may be affected by micronutrient deficiency states. In the cold, peripheral vasomotor tone is increased via central nervous system control of smooth muscle in arterioles and arteriovenous anastomoses, leading to a decrease in convective and conductive heat loss (Grayson, 1990). There is reason to believe that the failure to control this process is the fundamental reason iron-deficient anemic humans (and animal models) fail to thermoregulate adequately (Beard et al., 1990a, b; Lukaski et al., 1990).