Susan Piscopo, DVM, PhD
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Seventeen horses in each group finished; the others were withdrawn for various reasons.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.
However, hydrogen ion (H+, a measure of acid-base status ) was found to be significantly lower in EM-K horses compared to EM+K horses. Another significant finding was that plasma potassium was significantly lower at 80 km and during recovery in EM-K horses compared to EM+K horses.
The authors concluded that the decrease in potassium and hydrogen ion in the last stage of the ride in EM-K horses might have been attributable to the absence of potassium and the increase in sodium in the EM-K formula. However, despite the differences, EM+K horses had increases in plasma hydrogen and potassium that were moderate and not likely to cause clinical neuromuscular signs. The authors conceded that the moderate nature of the ride and mild weather were likely the reason for this, as well as the reason that the significant differences between groups were not evident until the end of the ride. The EM-K mixture would therefore likely be most beneficial in faster horses working harder during more strenuous rides.