Cardiovascular and metabolic responses during HIIT across menstrual cycle phases in eumenorrheic females compared to men

BACKGROUND: During exercise humans use two main energy sources to produce ATP: carbohydrates (CHO) and fats. The use of these two sources is mainly dependent upon intensity of exercise but recently researchers have examined the effect of female reproductive hormones on the use and availability of CHO and fats during exercise. As most exercise metabolism research has been done with males, the exercise response and the hormonal effects that take place in females are highly understudied. PURPOSE: To examine how fluctuations in menstrual cycle hormones affect substrate (CHO and fat) use during high intensity interval training (HIIT) in normally menstruating females compared to males. METHODS: Twenty two physically active adults (12 male, 10 female; age: 20.2±1.6 yrs; BMI 25±2.7) completed a preliminary treadmill test measuring maximal oxygen consumption (VO2 max). Female participants reported consistent menstrual cycles and were not taking any form of hormonal contraceptive. Participants then completed one or three 10 minute HIIT workouts at 90% of VO2 max with one minute recovery between intervals. Heart rate (HR), rate of perceived exertion (RPE), respiratory exchange ratio (RER) and oxygen consumption (VO2) were measured at the end of each running interval during exercise. RESULTS: There were no significant interactions between phase and time during exercise in RER (F18,162=0.91; p=0.57; eta2=0.09), or VO2 (F18,162=0.69; p=0.81; eta2=0.07). When comparing males and females in each phase there was a significant difference between average VO2 during exercise (F 3,38= 3.766; p=0.018). In a one-way anova, there were significant interactions between sexes for VO2 during 7 of the 10 intervals measured (4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min). There were no significant differences between average HR (F 3,38= 0.719; p=0.547) or average RER (F 3,38= 0.066; p=0.978 during exercise when comparing between males and females in each phase. CONCLUSION: There is a call for more significant research to be done surrounding the hormonal fluctuations during a menstrual cycle and substrate availability during exercise, but this study suggests that while hormonal fluctuations may not affect substrate use during exercise, measuring and reporting menstrual cycle phase in exercise research is important. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.