BACKGROUND: Bioelectrical impedance spectroscopy (BIS) outcomes of impedance (Z; Ω), resistance (R; Ω), reactance (Xc; Ω), and phase angle (PhA; °) are markers of tissue hydration and cellular health, parameters that may fluctuate across the menstrual cycle. Raw impedance values are also sensitive to acute nutrition and hydration status. This study sought to characterize differences in whole-body raw bioimpedance values across the menstrual cycle. METHODS: Forty-three healthy, moderately active females were categorized by menstrual status: eumenorrheic (EUM; n=21; age: 27.1±6.8 yrs) or hormonal contraceptive users (HC; n=22; age: 24.2±4.0 yrs). Measurements of Z, R, Xc, and PhA were obtained in the follicular phase (FP) and luteal phase (LP) using a multi-frequency BIS device (SFB7 Impedimed, Queensland, Australia) after a 12-hour fast and 48-hour abstention from exercise. Subjects were asked to lay supine, following a 3-5 min equilibration period, with electrodes placed 5 cm apart on the right wrist and ankle. Acute hydration was evaluated via urine specific gravity (USG) and used as a covariate. Between group (EUM vs HC) and phase (FP vs LP) differences were evaluated using mixed model ANOVAs. RESULTS: Xc demonstrated a significant interaction (p=0.03). Pairwise comparisons showed significantly lower Xc values in EUM vs HC in the FP (Mean Difference [EUM-HC] ± Standard Error: -4.22±1.44 Ω; p<0.01), and significantly lower values within EUM for FP compared to LP ([FP-LP] -1.84±0.87 Ω; p=0.04). A significant interaction was also observed for PhA (p=0.04), with pairwise comparisons demonstrating significantly lower values for EUM in FP compared to LP (-0.15±0.06°; p=0.02), but no significant difference between EUM and HC (-0.21±0.11°; p=0.07). No significant differences were observed (p>0.05) for Z or R values. CONCLUSION: The lower Xc and PhA values observed during the FP, specifically in EUM subjects, may be a result of fluid, inflammatory, and cell integrity changes during menstruation. The lack of changes observed in Z and R values, markers of cellular hydration, may suggest alterations in Xc and PhA could be due to loss of erythrocytes, and related iron, rather than simply fluid loss alone. These data suggest the need for future exploration during the FP as a unique window for nutritional support to reduce potential impairments of cellular function.

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