The Effects Of Menstrual Cycle On Resting Metabolic Flexibility: A Proof Of Principle Case Study

The menstrual cycle has distinct phases such as —midfollicular (MF) and midluteal (ML)— with different hormonal profiles. The MF phase features elevated estrogen and low progesterone, while in the ML phase, both hormones are elevated. These hormonal fluctuations affect the ability to switch between carbohydrate (CHO) and fat oxidation (OX), referred to as metabolic flexibility (MetFlex). PURPOSE: To investigate MetFlex differences following a mixed meal challenge between the MF and ML phases. METHODS: This case study examined changes in CHO and fat OX and blood glucose (BG) in a normal-weight, healthy, inactive woman with a regular menstrual cycle. The participant’s cycle phase was confirmed through self-report, with testing on days 7 (MF) and 21 (ML), relative to the first day of menstruation. MetFlex was assessed using indirect calorimetry before and after ingesting a 350-kcal liquid meal (8 g fat, 42 g CHO, 30 g protein). RESULTS: The average respiratory exchange ratio (RER) was higher in the MF (mean ± SEM = 0.76 ± 0.02) than the ML phase (0.73 ± 0.03). RER peaked at 0.80 at 15, 30, and 90 minutes in the MF phase, while in the ML phase, it peaked at 0.76 at 60 minutes post-meal. CHO OX was higher in the MF phase, peaking at 0.24 g/min at 90 minutes, compared to 0.18 g/min in the ML phase. The Area Under the Curve (AUC) for CHO OX in the MF and ML phases was 28.25 vs. 23.425 g·min/min. The ML phase showed greater fat OX and a delayed BG response. Following a mixed meal, BG in the MF phase rose rapidly, peaking at 103 mg/dL at 5 minutes, while in the ML phase, BG increased more gradually, peaking at 112 mg/dL at 15 minutes. The AUC for BG was higher in the MF compared to the ML phase (15,627.5 vs. 15,312.5 mg·min/dL), indicating a higher overall BG response in the MF phase. CONCLUSION: Hormonal fluctuations during the menstrual cycle may influence substrate utilization and MetFlex. We found that during the MF phase, whole-body CHO OX was higher, and during the ML phase, fat OX was more dominant with a delayed glucose response. These findings highlight the importance of considering menstrual cycle phase when studying MetFlex and nutrient utilization in women. We plan to expand this work and evaluate the effects of the menstrual cycle on MetFlex in young women.