Hydration status of horses performing endurance exercise: I. Evidence for a role of diet.

Water and electrolyte loss from endurance exercise results in physiological disturbances in the horse. The large intestine may serve as a water reservoir and help attenuate dehydration. Dietary constituents may affect the amount of water within the intestine and available for use. This study examined the hydration status of horses fed three common diets for 14 d then subjected to a 60 km exercise test. After an initial training period, horses were assigned to a replicated 3x3 Latin Square. Diets were grass hay (G), 50:50 grass hay:alfalfa hay (GA), and 50:50 grass hay:short-chopped, fibre-based, oil-supplemented mixed feed (GM). Total body water (TBW) tended to be higher (P<0.08) in horses consuming GA and GM than G (65.8±0.8, 65.4±0.8, and 63.9±0.8%, respectively). Body mass (BM) was not different at the start of the exercise test, but when corrected for faecal loss and water intake showed a trend for diet difference during exercise (P<0.08), decreasing more in GM than G (5.1±0.4% vs. 3.4±0.4%; GA 4.2±0.4%). Heart rate was not different except at the end of bout one (after 15 km) when GM was lower than G or GA (P<0.01). Core body temperature, although not different at the start of the exercise test bout, was lower (P<0.05) at the canter in GM. As the GM diet was higher in oil, the increase in oil intake may have been responsible for the differences observed. However, more likely, the higher TBW with the GM and GA diets at the initiation of exercise, associated with more readily fermentable fibre components, may have provided a greater 'pool' of available water for increased thermoregulation via sweating, allowing maintenance of a lower core body temperature during exercise at the expense of increased BM loss. [ABSTRACT FROM AUTHOR]