Background <p>The increasing participation of female contestants starting from 2% in the 1900 Paris Olympics vs. ~ 50% in the 2024 Paris Olympic Games indicates the importance of better understanding about female-specific acute responses to exercise and the potential impact on load tolerance, adaptation and performance. The blood lactate concentration (BLC) is invariant or higher at the mid-follicular menstrual cycle phase. This serves as a case for invariant or higher carbohydrate oxidation (CHO) at predicted mid-follicular, and higher fat oxidation (FO) at predicted mid-luteal conditions contradicting results of animal and selected human models.</p> Aim <p>To test the hypothesis that in naturally menstruating women an elevated BLC is compatible with a decreased reliance on CHO at predicted mid-follicular conditions.</p> Methods <p>14 naturally menstruating women (age: 27.0 ± 5.5yrs, height: 1.70 ± 0.06&#xa0;m, body mass: 66.3 ± 9.2&#xa0;kg) performed incremental cycle ergometry tests at predicted mid-follicular and predicted mid-luteal conditions. CHO, FO and the fraction of energy provided via CHO (relCHO) were calculated.</p> Results <p>CHO and relCHO are lower and FO was higher at predicted mid-follicular than at predicted mid-luteal conditions (all <i>p</i> ≤ 0.023, ω² ≥ 0.01). This was combined with a higher BLC (<i>p</i> = 0.003, <i>ω</i>² = 0.03) at predicted mid-follicular conditions indicating a decrease in relCHO of between ~ 6.5% and ~ 4.7% at BLC levels between 1.5&#xa0;mmol&#xa0;l⁻¹ and 4.0&#xa0;mmol&#xa0;l⁻¹.</p> Conclusions <p>A predicted mid-follicular-related elevation of the BLC does not reflect a cycle phase dependent increase in CHO. The predicted mid-follicular-related decrease in CHO combined with increased FO may indicate the potential for optimized management of training load and CHO demand in female athletes.</p>