Hydration effects on physiological strain of horses during exercise-heat stress

This study examined the effects ofhyperhydration, exercise-induced dehydration, and oral fluidreplacement on physiological strain of horses during exercise-heatstress. On three occasions, six horses completed a 90-min exerciseprotocol (50% maximal O₂ uptake,34.5°C, 48% relative humidity) divided into two 45-min periods(exercise I andexercise II) with a 15-min recoverybetween exercise bouts. In random order, horses receivedno fluid (NF), 10 liters of water (W), or a carbohydrate-electrolytesolution (CE) 2 h before exercise and between exercise bouts. Compared with NF, preexercise hyperhydration (W and CE) did not alter heart rate, cardiac output (), stroke volume (SV), corebody temperature, sweating rate (SR), or sweating sensitivity duringexercise I. In contrast, afterexercise II, exercise-induceddehydration in NF (decrease in body mass: NF, 5.6 ± 0.8%; W, 1.1 ± 0.4%; CE, 1.0 ± 0.2%) resulted in greater heat storage,with core body temperature ~1.0°C higher compared with W and CE.In exercise II, the greater thermalstrain in NF was associated with significant(P < 0.05) decreases in (10 ± 2%), SV (9 ± 3%), SR, and sweatingsensitivity. We concluded that 1)preexercise hyperhydration provided no thermoregulatory advantage;2) maintenance of euhydration byoral fluid replacement (~85% of sweat fluid loss) during exercise inthe heat was reflected in higher , SV, and SR withdecreased heat storage; and 3) W oran isotonic CE solution was equally effective in reducing physiological strain associated with exercise-induced dehydration and heat stress.