The Effects of Acute Nutrient Intake on Raw Segmental Bioelectrical Impedance Values and Fluid Shifts in Premenopausal Women

Background: Raw bioimpedance measures—impedance (Z), resistance (R), reactance (Xc), and phase angle (PhA)—are emerging tools to assess hydration, cellular integrity, and muscle quality. While multifrequency bioimpedance analysis (MF-BIA) can detect acute physiological shifts, its sensitivity to nutrient intake, especially across hormonal phases in women, remains underexplored. Purpose: This study investigated how essential amino acids (EAA), carbohydrates (CHO), and water (CON) influence postprandial segmental bioimpedance and fluid distribution in healthy women, and whether these responses vary by menstrual hormone phase. Methods: Twenty-two recreationally active women (18–40 years) underwent serial MF-BIA (InBody 970) assessments during high- and low-hormone phases, randomized to one of three interventions: EAA (15 g), CHO (75 g), or water. Bioimpedance (Z, R, Xc, PhA) and fluid measures (total body water [TBW], intracellular water [ICW], extracellular water [ECW]) were collected at baseline and every 30 minutes for three hours postprandially. Results: CHO intake consistently elicited greater increases in Xc and PhA across multiple body segments, particularly arms and legs—compared to EAA and CON (p< 0.05), with a general trend of CHO > EAA > CON. These increases suggest enhanced cellular membrane activity and conductivity following carbohydrate ingestion. Surprisingly, fluid estimates decreased postprandially across all groups (CON > EAA > CHO), contradicting expectations of nutrient-driven intracellular water uptake. Notably, impedance and resistance increased over time, suggesting possible shifts in tissue conductivity unrelated to actual fluid loss. No significant differences were observed between hormone phases (p>0.05), though collapsed hormonal profiles may have masked potential effects. Conclusion: Carbohydrate intake induces significant changes in segmental bioimpedance markers of cellular function, independent of menstrual hormone phase. However, discrepancies between bioimpedance and fluid outcomes highlight potential measurement artifacts, underscoring the need for caution when interpreting MF-BIA responses to nutrient intake. Future studies should pair bioimpedance with gold-standard fluid assessment methods (e.g., isotope tracers) and analyze hormonal profiles separately to clarify nutrient-specific effects on fluid dynamics. These findings have implications for clinical hydration monitoring and performance nutrition but emphasize the need for refined methodologies in acute nutrient-based bioimpedance assessments.