• horse;
  • electrolyte balance;
  • fluid balance;
  • prolonged exercise


This study calculated extracellular fluid (ECF) and intracellular fluid (ICF) water losses and ECF ion losses from 39 endurance trained horses during a 163 km ride. Jugular venous blood samples and body mass (BM) were obtained pre-ride, at 32, 80 and 163 km of the ride and at 1 h of recovery. Blood was immediately analysed for haematocrit, plasma [Na+], [Cl], [K+], ionised [Ca++] and total solids. ECF water losses were calculated from changes in plasma total solids assuming equilibrium between plasma and remaining ECF. ECF ion losses were calculated from changes in plasma ion concentrations and ECF volume (ECFV). Results were analysed with respect to speed (Group A, 14.7 ± 1.0 km/h, n = 12; Group B, 11.2 ± 1.0 km/h, n = 11; Group C, did not finish, n = 16) and whether or not horses were pre-loaded with electrolytes before the ride. The majority of ECF water losses occurred by 32 km: ECFV decreased by 5.4 ± 1.7 litres in A and B and by 7.8 ± 1.6 litres in C. At 80 km, there were no further losses in A and B, while in C ECFV had decreased by 10.4 ± 1.9 litres. However, total body water and therefore ICFV, decreased progressively until the finish and were only partially restored after 1 h recovery. Most ECF ion losses also occurred within the first 32 km, with losses greater in C (P < 0.05) and β (P < 0.1) than in A (Na+ 5.9 ± 2.1%, K+ 24 ± 4%, Cl 8.4 ± 1.7%, Ca++ 13 ± 2%). Only at 32 km did the preloaded horses (n = 21) show reduced losses of Na+, K+ and Cl. In conclusion, the majority of ECF ion and water losses occurred early in the ride (by 32 km); net loss of TBW and ICFV continued until 80 km; better performing horses (A) were able to maintain ECF ion and water balance during the ride; and in most horses fluid and ion supplementation was effectively inadequate to replace all lost water and ions.