Objectives/Background: 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. Methods: 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. Results: Significant differences in potassium (K+) levels were found in the intermediate (p = 0.003, adjusted p = 0.01) and slower (p = 0.006, adjusted p = 0.03) groups. Hematocrit (Hct) showed a significant reduction in the intermediate group (p = 0.007, adjusted p = 0.04). In arterial blood gas variables, significant reductions in HCO3- and pCO2 were observed in the faster (HCO3-: p = 0.002, adjusted p = 0.008; pCO2: p = 0.007, adjusted p = 0.02) and intermediate (HCO3-: p = 0.005, adjusted p = 0.02) groups. In the slower group, significant reductions in pH (p = 0.001, adjusted p = 0.004) and HCO3- (p = 0.001, adjusted p = 0.004) were found. The correlation between post-race Na+ levels and performance was significant in the intermediate group (p = 0.01, adjusted p = 0.02). Conclusions: 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.
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