Purpose: This study aimed to investigate fat utilization during 1 hour of cycling at maximal fat oxidation and, secondarily, to assess menstrual symptoms across different phases of the menstrual cycle. Methods: Fifteen physically active, naturally menstruating women (age = 25 [4] y, VO2max=38.15[7.02]mL·min-1·kg-1) completed a randomized crossover study involving 1 hour cycling at maximal fat oxidation intensity during 3 menstrual cycle phases: early follicular, peri-ovulatory, and midluteal. Phases were identified individually using mobile app tracking, urinary luteinizing hormone tests, and daily basal temperature and body mass recordings. During exercise, ventilatory variables were measured breath by breath with a gas analyzer to estimate fat and carbohydrate oxidation, and heart rate and the rating of perceived exertion (6-20 a.u., Borg scale) were recorded. Data was analyzed using linear mixed effects modeling. Results: There was no main effect of menstrual cycle phase on total fat oxidized (P = .388), total carbohydrate oxidized (P = .265), or total energy expended (P = .839) during exercise. However, a higher mean heart rate was detected in the peri-ovulatory phase compared with the early follicular phase (P = .004), and participants reported higher rating of perceived exertion in the midluteal phase compared with early follicular (P = .003) and peri-ovulatory phases (P = .024). Menstrual symptom scores were higher in early follicular phase compared with peri-ovulatory and midluteal phases (P < .001). Conclusions: Fat oxidation during exercise at maximal fat oxidation intensity was not affected by the phase of the menstrual cycle in healthy, young, physically active, naturally menstruating women. However, variations in heart rate and perceived exertion suggest that physiological and perceptual responses to exercise may differ across phases.
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