Effects of Acute Essential Amino Acid Intake on Post-Prandial Raw Bioimpedance and Fluid Shifts Between Healthy Young and Older Adults: an Exploratory Pilot Study.

Introduction:Age-related anabolic resistance can lead to reduced muscle mass but can be costly and timely to diagnose. Multi-frequency bioelectrical impedance analysis could potentially be used as a non-invasive tool for the assessment of anabolic resistance as changes in frequency-dependent impedance values may reflect fluid shifts occurring with nutrient uptake. This exploratory pilot study evaluated the effects of acute essential amino acid (EAA) intake on raw bioimpedance and fluid shifts in healthy young adults (YA, n=5) and older adults (OA, n=7). Methods:Participants completed a five-hour protocol with multifrequency bioelectrical impedance analysis (MF-BIA) at baseline and every 30 minutes post-consumption of a 10 g EAA beverage. Whole-body and segmental values for impedance (Z), resistance (R), reactance (Xc), phase angle (PhA), and body water compartments were assessed. Results:YA demonstrated significantly higher Z, R, Xc, and PhA values compared to OA (p<0.05), particularly in the leg segments and at 50 kHz. Time effects revealed declines in R (p=0.013) and Xc (p=0.002) following EAA ingestion, consistent with postprandial fluid shifts. Fluid analysis showed significant group differences only for ECW/ICW ratio (p=0.001-0.004) with OA > YA and increases in TBW, ICW, and ECW over time. Conclusion:Raw bioimpedance values distinguished between age groups and reflected acute responses to nutrient intake. These findings suggest MF-BIA may be sensitive to short-term physiological changes and, with further validation, could support assessments of muscle quality and nutritional responsiveness.