Menstrual Cycle Phase Is Associated With Changes in Body Weight, Bioimpedance, and Carbohydrate Intake, but Not in Resting Metabolic Rate in Physically Active Females.

This observational repeated-measures study investigated the effect of three menstrual cycle phases on resting metabolic rate (RMR), body composition, and dietary intake in 21 naturally menstruating, physically active females (age 24.9 ± 4.1 years; height 168.4 ± 7.8 cm). Measurements were scheduled during the early follicular (EFP), ovulatory, and mid-luteal (MLP) phases using menstrual bleeding onset and urinary luteinizing hormone tests. RMR was measured using indirect calorimetry, body composition via multifrequency bioelectrical impedance analysis, and dietary intake through 3-day food records. Linear mixed-effects models revealed no statistically significant differences in RMR across phases (p = .148). Most dietary intake variables remained stable across the cycle (all p ≥ .074); however, absolute carbohydrate intake was significantly higher in the EFP (234 ± 48 g) than in the MLP (209 ± 53 g; p = .037). Body weight significantly decreased from the EFP (65.2 ± 7.4 kg) to the MLP (64.9 ± 7.3 kg; p = .029). Similarly, the extracellular water ratio decreased (0.376 ± 0.004 vs. 0.374 ± 0.005; p = .019), and phase angle increased (5.85 ± 0.45 vs. 5.98 ± 0.53; p = .021) between the EFP and the MLP. Estimates of fat-free mass and body fat percentage did not differ (p > .05). In physically active females, RMR and most dietary intake variables remained statistically stable across the cycle, whereas carbohydrate intake was higher in EFP than in MLP. Observed fluctuations in body weight and bioimpedance metrics likely reflect hydration shifts rather than changes in tissue composition.