Hydration and electrolyte balance in horses during an endurance season

Summary Reasons for performing study: Limited information exists about the physiological training‐induced changes in electrolyte balance of horses competing in long distance endurance races. Objectives: To determine the effects of endurance training and racing on hydration and electrolyte balance in horses. Methods: Blood and urine were sampled at rest in 8 endurance horses before training and after two 11 week training periods (T1 and T2). Each training was followed by a 120 km endurance ride and horses were sampled before, during and 2 h after the rides. Blood was analysed for packed cell volume (PCV), total protein (TP), urea, creatinine and electrolyte concentrations. Urine was analysed for pH, specific gravity, creatinine and electrolyte concentrations, which allowed calculation of fractional excretion of electrolytes (FE). Changes associated with training and with the rides were assessed using a Student paired t test (P≤0.05). Results: Plasma TP, urea, creatinine and sodium concentrations increased during T1 and PCV decreased significantly during T2. FECl increased during T1 then decreased. FEK increased significantly during both training periods. Other blood and urine parameters did not show remarkable changes with training. PCV, plasma TP, urea, creatinine and total Ca concentrations increased and plasma Na+ and Cl‐ concentrations decreased during both rides. Urine concentrations of Na+, K+ and Cl‐, FENa and FECl decreased during the rides while urinary creatinine increased. FEK increased during the first part of the rides then decreased. Conclusions: These data contribute to the understanding of changes associated with training and prolonged endurance exercise.