1. Introduction
dehydration
DEHYDRATION, HEAT, SECONDARY
The physiological, perceptual, and functional effects of dehydration may depend on how it is incurred (e.g., intense exercise releases endogenous water via glycogenolysis) but this basic notion has rarely been examined. We investigated the effects of active (exercise) heat- vs. passive heat-induced dehydration, and the kinetics of ad libitum rehydration following each method. Twelve fit participants (five females and seven males) completed four trials in randomised order: DEHydration to −3% change in body mass (∆BM) under passive or active heat stress, and EUHydration to prevent ∆BM under passive or active heat stress. In all trials, participants then sat in a temperate-controlled environment, ate a standard snack and had free access to water and sports drink during their two-hour recovery. During mild dehydration (≤2% ∆BM), active and passive heating caused comparable increases in plasma osmolality (PWater is the nutrient that is both most abundant and most rapidly turned over in humans. The regulation of body water is dynamic and complex, whereby its volume, composition, and distribution between fluid compartments are all tightly regulated by an array of homeostatic mechanisms—especially osmotic and hydrostatic pressures [Even for heat-induced dehydration, a long-standing contentious issue is whether different forms of heat stress incur similar physiological effects and hence also similar psychophysical, psychomotor, or cognitive effects. Some studies have reported that dehydration induced by strenuous exercise, and during the exercise rather than before it, appears to have less effect on plasma volume (PV) and extracellular fluid volume relative to other methods of dehydration [The effects of methods of dehydration on PGiven that the dehydration studies underpinning the ∆BM-based guidelines for exercise and recovery have used heat stress methodologies that often differ markedly from the way athletes dehydrate in training and competition, it is important that research using ecologically valid designs be undertaken to reveal the effects of dehydration both while exercising and as a consequence of exercise [A relevant consideration of the different forms of heat stress is their effects irrespective of dehydration (e.g., on PV, PThe information above is relevant to the theory and practice of rehydration. Specifically, if glycogenolysis releases a functional quantity of water but the resynthesis of glycogen also takes approximately one day, then it may be imprudent to rehydrate with the recommended 120–150% of ∆BM in the immediate hours of recovery. Furthermore, appetite for sodium and carbohydrate may differ between modes of heat stress. Therefore, question four was whether rehydration behaviour or efficacy (fluid balance) differed between modes of heat stress. We hypothesised that fluid intake and retention would be greater after passive dehydration (due to higher PFinally, potential carryover effects of dehydration or other methods of heat stress on fluid regulatory measures 24 h afterward were assessed as a fifth, but secondary, question because the literature is equivocal on this issue (reviewed in [