Endurance riding can lead to significant losses of water and electrolytes, which can cause clinical illnesses related to increased neuromuscular excitability, including cardiac arrhythmia, muscle cramping and twitching, and gut motility changes. When plasma potassium (K+) increases–as it does with increasing exercise intensity–there is a concomitant increase in neuromuscular excitability. Yet, many endurance riders believe that oral potassium supplementation before and during competition is critical to the good health of their horses.

Researchers from the Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Rutgers University, Virginia Intermont College, Rectortown Equine Clinic, and the Waltham Center for Pet Nutrition in the United Kingdom evaluated whether potassium-free oral electrolytes given during endurance work could moderate the expected increase in plasma potassium, decreasing related neuromuscular side effects. They compared the effects of a potassium-free high-sodium electrolyte mixture (EM-K) to a potassium-rich mixture (EM+K) on plasma ions and acid-base status in horses during an endurance ride.

Forty-six horses entered in an 50-mile (80-km) endurance race were used for the study–24 receiving EM-K and 22 EM+K. Rest stops and veterinary inspections were conducted at 21, 37, 56, and 72 km. Electrolyte mixtures were given orally by syringe after each loop.

Seventeen horses in each group finished; the others were withdrawn for various reasons. For all horses, plasma potassium significantly increased from the ride’s start to the 56-km rest stop, then significantly decreased to the end of the ride. Plasma sodium significantly increased from before the ride to 37 and 56 km, and significantly decreased from there to the end of the ride