Does usefulness of potassium supplementation depend on speed?

Reasons for performing study: Electrolyte mixtures given to counter sweat loss usually contain abundant potassium. However, increases in plasma [K⁺] occur with exercise and supplementation may further increase plasma levels, potentially increasing the risk of neuromuscular hyperexcitability and development of adverse clinical sequellae. This proposition requires study. Objectives: To compare effects of a K-rich electrolyte supplement (EM+K) to a K-free one (EM-K) on plasma [K⁺], [Ca⁺⁺] and acid-base status during an endurance incremental exercise test on the treadmill. Methods: The test consisted of 3 bouts (simulating loops in an endurance race) of 12 km performed at 6, then 7, then 8 m/sec with 25 min rest stops (S1, S2) between loops on 13 endurance trained Arabian horses (7 EM-K, 6 EM+K). Electrolytes were supplied orally 60 mins before exercise (Pre) and at each stop. Blood samples were taken before exercise and during exercise, each S and 120 mins of recovery (R). Blood was analysed for pH, PCO₂, packed cell volume (PCV), plasma [Na⁺], [K⁺], [Cl^-], [Ca⁺⁺], glucose, and lactate [La^-]; plasma [H⁺] and osmolality (osm) were calculated. The dietary cation anion difference (DCAD) was calculated to be -27 meq/dose EM-K and 109 meq in EM+K, respectively. Results: Plasma [H⁺] decreased during the 6 and 7 m/sec loops, increased during the 8 m/sec loop, and returned to Pre at S1, S2 and R. Plasma [K⁺] was higher at 8 m/sec and plasma [Ca⁺⁺] was overall lower in the EM+K group compared to EM-K. Other findings included higher overall PCV, overall glucose, and [La^-] during the 8 m/sec loop (P<0.040) in EM+K compared to EM-K horses. Conclusions: EM+K supplementation leads to higher plasma [K⁺] increasing the risk of neuromuscular hyperexcitability during exercise. Acute effects of a lower DCAD in EM-K may have led to higher plasma [Ca⁺⁺]. Potassium-rich electrolytes may have triggered the release of epinephrine, contributing to higher PCV, glucose release and increased lactate production. Potential relevance: Lower plasma [K⁺] and higher plasma [Ca⁺⁺] with EM-K supplementation may help reduce the risk of conditions associated with neuromuscular hyperexcitability occurring especially during higher speeds in endurance races.