• horse;
  • electrolyte balance;
  • endurance exercise;
  • fluid balance


Factors contributing to water and ion losses due to sweating have not been well studied during prolonged exercise in horses. This study examined temperature (T; °C; range 6–32°C), distance (D; km; 48–161 km), speed (S; km/h; 8–19 km/h) and muddy terrain (M) as factors determining water and ion losses. Jugular venous blood samples (analysed for plasma ions and refractive index) and body mass (BM) were obtained from 176 endurance horses at 13 events at rest, mid-ride and finish. Regression analysis determined correlations between losses of total body water (TBW) and extracellular (ECF) ions and the variables T, M, S and D. Losses of ECF ions and TBW were calculated assuming an ECF volume (ECFV) of 1 litre/5 kg BM. There were significant decreases in BM (range of means 2–7%) and ECFV (range of means 2–14%) at mid-ride and finish; however, these were independent of the variables tested. Losses of Na+, K+, Cl and Ca++ were correlated with T, D, S and M. Inclusion of S in the model eliminated T and M as factors for Na+, Cl and Ca++ loss. Speed was not a predictor of K+ loss. ECF ion losses at the mid-ride were correlated to T, S, D and M. In conclusion, factors affecting ion losses at the finish, in descending order of importance, were speed (except for K+ due to rapid intercompartmental shifts), muddy terrain, temperature, then distance (for Cl). Using the data from the literature models were used to predict ECF ion losses in endurance events.