CHANGES IN RESTING METABOLIC RATE ACROSS ONE MENSTRUAL CYCLE IN WOMEN

Abstract

Background: Resting Metabolic Rate (RMR) may fluctuate with menstrual cycle hormones, but findings in the literature remain inconsistent. This study aimed to examine the effects of menstrual cycle phases on RMR among healthy women. Methods: Eighteen healthy adults, including 9 females (age: 24.11 ± 1.12 years; BMI: 23.9 ± 1.08 kg/m²) and 9 males (age: 25.55 ± 1.04 years; BMI: 26.95 ± 1.99 kg/m²) participated. Female RMR and temperature were assessed during Early follicular (EF, 1-7 days), ovulatory window (OW, 12-15 days) and mid-luteal phase (ML, 20-22 days) confirmed using LH surge ovulation kits. Male participants served as a comparison group; males were assigned a menstrual phase and were randomly tested every 7- 10 day to mimic the temporal distribution of female menstrual phases. RMR was measured using indirect calorimetry, while core and skin temperatures were recorded using core temp body sensors. Results: While RMR was significantly higher in males, in both males and females no significant differences were found in RMR across the cycles (EF 1823.84±86.28; OW 1864.38±72.94; and ML 1821.34±65.82 in males and EF 1353.91±35.66; OW 1426.56±29.04; and ML 1389.63±49.61, in females). Similarly, core and skin temperatures showed no significant phase-related changes. Conclusion: Despite RMR and thermoregulatory responses remained stable across different phases of menstrual cycle. These findings support the idea that menstrual phase effects on metabolism are subtle and might suggest that when conducting metabolic studies at homeostasis, like when assessing RMR, the menstrual cycle should not be controlled for. Alternatively, the menstrual phase-related difference in RMR in this study might have been masked by individual variability and a small sample size. Future research should explore these differences in a larger study.