Electrolyte imbalance markers and acid–base disturbances in Brazilian male runners during a 45-km mountain ultramarathon

Objective This study investigated electrolyte and acid–base alterations and their associations with performance in 40 Brazilian male runners during a 45-km mountain ultramarathon. Method The following parameters were assessed before and after the race: pH, sodium (Na + ), potassium (K + ), calcium (Ca 2+ ), glucose, carbon dioxide pressure (pCO 2 ), partial pressure of oxygen (pO 2 ), bicarbonate (HCO 3 ), hematocrit and lactate. Results Potassium levels decreased significantly across all performance groups (Fastest: p = 0.004; Moderate-Fast: p = 0.004; Moderate-Slow: p = 0.003; Slowest: p = 0.004), while lactate increased in the Fastest ( p = 0.03), Moderate-Fast ( p = 0.002), and Slowest ( p = 0.03) groups. Sodium increased in the Moderate-Fast group ( p = 0.005), whereas calcium decreased in the Slowest group ( p = 0.005). Both pCO 2 and HCO 3 were significantly reduced across all groups (pCO 2 : p = 0.0003–0.0005; HCO 3 : p = 0.003–0.0004), and pH decreased in the Slowest group ( p = 0.01). Between-group differences were observed between Fastest and Slowest for pH ( p = 0.02), pCO 2 (p = 0.001), and K + ( p < 0.001), and between Fastest and Moderate-Slow for Na + ( p = 0.03). Negative correlations were found between race time and post-race pH ( R = −0.46), Ca 2+ ( R = −0.33), and Na + ( R = −0.31). In the Slowest group, Δ K 2+ was strongly correlated with race time ( R = −0.83; p = 0.003). Conclusion Performance in a 45-km mountain ultramarathon appears to be closely associated with the ability to preserve acid–base and electrolyte homeostasis, particularly regarding potassium, sodium, calcium and pH.