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PHASE-DEPENDENT VARIATIONS IN METABOLIC FLEXIBILITY ACROSS THE MENSTRUAL CYCLE – PRELIMINARY CASE STUDY

Abstract

The menstrual cycle has distinct phases, including the midfollicular (MF) and midluteal (ML) phases, each characterized by different hormonal profiles. The MF phase is marked by elevated estrogen and low progesterone levels, whereas both hormones are elevated during the ML phase. These hormonal fluctuations influence metabolic flexibility (MetFlex), or the body's ability to switch between carbohydrate (CHO) and fat oxidation (OX). PURPOSE: This study aimed to examine differences in MetFlex in response to a mixed meal challenge during the MF and ML phases. METHODS: A healthy, inactive, normal-weight woman with a regular menstrual cycle participated in this case study. Cycle phases were confirmed through self-report, with testing conducted on days 7 (MF) and 21 (ML), relative to the first day of menstruation. Indirect calorimetry was used to assess MetFlex before and after the participant consumed a 350-kcal liquid meal (8g fat, 42g CHO, 30g protein). Changes in CHO and fat OX, along with blood glucose (BG) levels, were recorded. RESULTS: Average respiratory exchange ratio (RER) was higher during the MF phase (mean ± SEM = 0.76 ± 0.02 vs 0.73 ± 0.03). In the MF phase, RER peaked at 0.80 at 15, 30, and 90 minutes, while in the ML phase, it peaked at 0.76 at 60 minutes post-meal. CHO OX was also higher during the MF phase, reaching 0.24 g/min at 90 minutes, compared to 0.18 g/min during the ML phase. The Area Under the Curve (AUC) for CHO OX was 28.25 g·min/min (MF) vs. 23.425 g·min/min (ML). Fat OX was higher in the ML phase, and BG response was delayed compared to the MF phase. After the mixed meal, BG peaked rapidly at 103 mg/dL (5 min) in the MF phase, while in the ML phase, it rose gradually to 112 mg/dL (15 min). The AUC for BG was greater in the MF phase (15,627.5 vs. 15,312.5 mg·min/dL), reflecting a higher overall BG response. CONCLUSION: Hormonal variations across the menstrual cycle appear to impact substrate utilization and MetFlex. This study found higher CHO OX during the MF phase and increased fat OX with a delayed BG response during the ML phase. These findings underscore the importance of accounting for menstrual cycle phases in research on MetFlex and nutrient metabolism in women. Future studies will aim to expand this work to evaluate the menstrual cycle’s effects on MetFlex in a larger cohort of young women.

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