Pre-loading large volume oral electrolytes: tracing fluid and ion fluxes in horses during rest, exercise and recovery

Exercise results in rapid and large extracellular to intracellular fluid shifts, as well as significant sweating losses of water and ions. It is unknown whether ions within oral electrolyte supplements are taken up by muscle (and other soft tissues) and whether oral supplementation can effectively offset sweating losses. Pre-loading with 8 L of a balanced hypotonic electrolyte supplement attenuated extracellular fluid losses, increased exercise duration and increased sweating fluid and ion losses during submaximal exercise. Supplemented electrolytes appear in skeletal muscle within 1 h after administration. Electrolyte supplementation increased exercise performance, improved maintenance of extracellular fluid volumes, and attenuated body fluid losses while maintaining sweating rates.

This study used radioactive sodium (²⁴ Na) and potassium (⁴² K) in a balanced, hypotonic electrolyte supplement to trace their appearance in skeletal muscle, and also quantified extracellular and whole-body fluid and ion changes during electrolyte supplementation, exercise and recovery. In a randomized crossover design, 1 h after administration of 1 to 3 L of water or electrolyte supplement with ²⁴ Na, horses were exercised at 35% VO_2max to voluntary fatigue or, after administration of 8 L of water or electrolyte supplement with ⁴² K were exercised at 50% peak VO₂ for 45 min (n = 4 in each trial). Pre-exercise electrolyte supplementation was associated with decreased loss of fluid and electrolytes from the extracellular fluid compartments during exercise and recovery compared with water alone. The improved fluid and ion balance during prolonged exercise was associated with increased exercise duration, despite continuing sweating losses of fluid and ions. Nasogastric administration of radiotracer ²⁴ Na⁺ and ⁴² K⁺ showed rapid absorption into the blood with plasma levels peaking 45 min after administration, followed by distribution into the extracellular space and intracellular fluid of muscle within 1 h. Following exercise, virtually all Na⁺ remained within the extracellular compartment, while the majority of K⁺ underwent intracellular uptake by 2 h of recovery. It is concluded that pre-loading with a large volume, balanced electrolyte supplement helps maintain whole-body fluid and ion balance and support muscle function during periods of prolonged sweat ion losses.