**Background:** Bioelectrical impedance analysis (BIA) is a non-invasive, widely used method for assessing body composition, including hydration status, muscle mass, and fat mass. BIA estimates are based on the resistance of body tissues to an electrical current, which is heavily influenced by total body water (TBW) and its distribution between intracellular (ICW) and extracellular (ECW) compartments. Acute fluid intake can alter impedance measurements, but the time course and differential effects of fluids with varying electrolyte and osmolyte compositions remain incompletely understood. This study aimed to investigate the impact of isotonic sodium chloride, Ringer's solution, and 5% glucose solution on BIA-derived body composition parameters over 120 minutes in healthy individuals.
**Methods:** This single-center, randomized, controlled crossover pilot trial included 18 healthy individuals (10 females, mean age 23.1 ± 1.8 years, height 176 ± 10 cm, body mass 69.5 ± 12.5 kg, BMI 22.2 ± 2.1 kg·m⁻²). Participants attended four trial visits in randomized order: control (CON, no fluid), isotonic sodium chloride (ISO, 0.9% NaCl, 308 mOsm·L⁻¹), 5% glucose (GLU, 278 mOsm·L⁻¹), and Ringer's solution (RIN, 309 mOsm·L⁻¹). At each visit, participants consumed 12 mL per kg body mass of the assigned solution within 60 seconds (mean volumes: ISO 831.6 ± 148.9 mL, GLU 835.3 ± 151.3 mL, RIN 832.7 ± 149.4 mL; p > 0.99), equaling approximately a 2% increase in TBW. BIA measurements (InBody 770) were performed at baseline (t_rest), immediately after consumption (t_0), and every 10 minutes for 120 minutes (t_10–t_120). Participants fasted for ≥12 hours and refrained from strenuous activity for ≥48 hours before each visit. Within-group changes were analyzed using repeated measures ANOVA or Friedman test with Tukey post-hoc tests; between-group analyses used two-way ANOVA or mixed-effects models.
**Key Results:** Baseline values for all body composition parameters showed no significant differences between trial arms (all p > 0.99). For ICW, a significant interaction between fluid and time was found (F[8.51, 135.5] = 3.028, p < 0.01). ISO showed significant ICW increases from t_rest to t_0 and t_10 (p < 0.01), and RIN showed a significant increase from t_rest to t_0 (p < 0.001). At t_0, ICW was significantly higher in ISO and RIN versus CON and GLU (p < 0.01). For ECW, a significant interaction was observed (F[8.76, 139.6] = 4.688, p < 0.0001). Both ISO and RIN demonstrated significant ECW increases throughout the entire 120-minute period versus t_rest (p < 0.01). RIN showed significant ECW increases versus CON from t_0 to t_120 (p < 0.05 to p < 0.001), while ISO showed significant increases versus CON from t_30 onward and at t_70–t_120 (p < 0.01). For TBW, the interaction was not significant (p < 0.08). ISO showed significant TBW increases at t_0 and t_10 versus t_rest (p < 0.0001), and RIN showed significant increases at multiple time points through t_120 (p < 0.05). For SMM, a significant interaction was found (F[8.390, 133.6] = 2.961, p < 0.001), with significant increases in ISO and RIN at t_10 versus t_rest (both p < 0.01). For FM, a significant interaction was observed (F[4.391, 69.80] = 3.517, p < 0.01). GLU led to significant FM increases from t_rest to t_10 (p < 0.0001) persisting through t_40. At t_0 (p < 0.01), t_10 and t_20 (p < 0.05), FM was significantly higher in GLU versus CON and RIN. For VFA, a significant interaction was found (F[9.522, 151.6] = 3.935, p = 0.0001). GLU showed significant VFA increases at t_0–t_20 (p < 0.001) and at t_40 and t_70 (p < 0.05). At t_0 (p < 0.01), t_10 and t_20 (p < 0.05), VFA was significantly higher in GLU versus CON and RIN.
**Clinical Implications:** This study demonstrates that acute fluid intake with osmotically active components significantly impacts BIA measurements of body composition, with effects persisting up to 120 minutes. Electrolyte-containing solutions (ISO and RIN) primarily affected water compartments (ICW and ECW), while glucose solution led to overestimation of fat mass and visceral fat area. The authors note that the observed changes were small—for example, percentage body fat mass fluctuations in the control arm reached up to 0.7%—and may fall within the error of the impedance technique. This suggests that for most clinical applications, strict fasting protocols may not be necessary. However, in special populations where TBW assessment is critical—such as oncological patients, individuals with COPD, or those with renal or cardiovascular disease—standardized conditions including avoidance of recent fluid intake are recommended. The study reinforces that longitudinal BIA measurements should be performed under consistent conditions to ensure reliable interpretation.