Acid–Base and Electrolyte Balance Responses in the Performance of Female Ultramarathon Runners in a 45 km Mountain Race

<b>Objectives/Background:</b> This study investigated the influence of acid-base and electrolyte balance on the performance of female athletes in a 45 km ultramarathon. The aim was to analyze the impact of these variables on performance, particularly in athletes with varying completion times. <b>Methods:</b> Nineteen female athletes (mean age: 35.9 ± 6.5 years) were divided into three groups based on their race completion times: faster, intermediate, and slower. Blood samples were collected before and after the race to assess biochemical variables and arterial blood gases. <b>Results:</b> Significant differences in potassium (K⁺) levels were found in the intermediate (<i>p</i> = 0.003, adjusted <i>p</i> = 0.01) and slower (<i>p</i> = 0.006, adjusted <i>p</i> = 0.03) groups. Hematocrit (Hct) showed a significant reduction in the intermediate group (<i>p</i> = 0.007, adjusted <i>p</i> = 0.04). In arterial blood gas variables, significant reductions in HCO₃^- and pCO₂ were observed in the faster (HCO₃^-: <i>p</i> = 0.002, adjusted <i>p</i> = 0.008; pCO₂: <i>p</i> = 0.007, adjusted <i>p</i> = 0.02) and intermediate (HCO₃^-: <i>p</i> = 0.005, adjusted <i>p</i> = 0.02) groups. In the slower group, significant reductions in pH (<i>p</i> = 0.001, adjusted <i>p</i> = 0.004) and HCO₃^- (<i>p</i> = 0.001, adjusted <i>p</i> = 0.004) were found. The correlation between post-race Na⁺ levels and performance was significant in the intermediate group (<i>p</i> = 0.01, adjusted <i>p</i> = 0.02). <b>Conclusions:</b> Acid-base and electrolyte imbalances significantly affect ultramarathon performance, with a greater impact observed in athletes with slower times. These findings highlight the importance of strategies to optimize electrolyte and acid-base balance in endurance events.