Proper hydration is important for optimal sport performance  and may play a role in the prevention of heat illnesses . Dehydration increases cardiovascular strain and increases core temperature (Tc) to levels higher than in a state of euhydration . These increases, independently  and in combination [3,5], impair performance and put an individual at risk for heat illness . Exercise in the heat in which dehydration occurs before  or during exercise  results in Tc that is directly correlated (r = 0.98)  with degree of dehydration (Fig. 1). The link between dehydration and hyperthermia has shown that independently and additively they result in cardiovascular instability that puts individuals at risk for heat exhaustion .
Despite laboratory evidence linking dehydration with increased Tc, some authors argue that this physiologic phenomenon does not occur in field settings [8-10]. This may be because field studies fail to control exercise intensity [8-11]. Tc is driven by metabolic rate, and when the same subject is tested in a controlled laboratory environment, a higher metabolic rate produces a higher Tc . Without controlling or measuring relative exercise intensity, a hydrated individual could exercise at a higher metabolic rate and drive his or her Tc to the same level as a dehydrated individual working at a lower intensity. Without
*Corresponding author. E-mail address: [email protected] (D.J. Casa).
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Fig. 1. The degree of dehydration that occurs during exercise is correlated with the increase in esophageal (top graph) and rectal (bottom graph) temperatures. Subjects cycled for 120 minutes in a 33°C environment at approximately 65% VO2max while replacing 0% (No Fluid), 20% (Small Fluid), 48% (Moderate Fluid), or 81% (Large Fluid) of the fluid lost in sweat. Subjects lost 4.2%, 3.4%, 2.3%, and 1.1% body weight in the conditions. (From Montain SJ, Coyle EF. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J Appl Physiol 1992;73(4):1340-50; with permission.)
a randomized crossover experimental design that controls exercise intensity, field studies cannot validly conclude that hydration is not linked to Tc.
Field studies disputing relationships between Tc and dehydration also cite that laboratory studies use environments that are too hot, and that the physiologic relationship does not exist in temperate environments (approximately 23°C) often associated with field studies . Laboratory studies have shown that the increase of Tc with dehydration is exacerbated in hot environments, but still observed in cold environments (8°C) . Dehydration impairs thermoregulation independent of ambient conditions, but the effect is seen especially at high ambient temperatures when the thermoregulatory system is more heavily stressed. Laboratory-based studies have clearly shown that when exercise intensity and hydration state are controlled, Tc increases at a faster rate when subjects are dehydrated .
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