Hydration Dynamics: A Model To Study Absorption, Plasma Volume Expansion, And Fluid Retention

PURPOSE: To develop a human model that assesses absorption rate using deuterium oxide (D2O), plasma volume (PV) expansion, and fluid retention of beverages. METHODS: Fasted, euhydrated subjects (7F, 7 M) were tested for responses to 1 L of beverage. Subjects were randomly, using a crossover design, assigned to three test beverages including a water placebo (P), a beverage with 2.5% glucose and 45 mEq/L sodium (GNa), and one with 1.7% glucose and 60 mEq/L sodium (GHNa). Beverages containing 6 g of D2O were ingested in 10 minutes (min). Blood samples were drawn for D2O, hemoglobin (Hb), and hematocrit (Hct) analyses before and post-ingestion sequentially at 5, 10, 15, 20, 25, 30, 45, 60, 90, and 120 min. Urine was collected at 60- and 120-min post-ingestion for a modified beverage hydration index (BHI). The time (min) to peak enrichment (TPk) and time to half of the peak enrichment (T ½) were estimated by modeling the curves of D2O in the blood. Change in plasma volume (PV) was calculated using Hct and Hb with the area under the curve (AUC) calculated as an outcome variable. Analyses of covariance using a 2x2 crossover design adjusted by the difference (period 2 - period 1) in baseline measures as a covariate were applied to PV AUC from 0 to 120 min, cumulative mass of urine excreted for 2-h following beverage ingestion, D2O T ½, D2O TPk. Total urine mass was used for the BHI over 2 h. Mean + SD were calculated. BHI was analyzed using a 1-group t-test. A 2-sided, 0.05 level was used for statistical significance with p-values adjusted for two comparisons to control for the family-wise error rate. RESULTS: GHNa promoted faster absorption with the reduced T ½ (7.4 + 3.5) and T Peak (46.9 + 17.2) compared to the Placebo, respectively, 10.3 + 5.2 (p = 0.027) and 59.6 + 22.9 (p = 0.026). T ½ and T Pk were not significantly different between GNa and water. PV AUC (%•120 min) were 403 + 415, 340 + 362, and 154 + 419 for GHNa, GNa, and P, respectively, and the AUC was significantly greater for GHNa (p = 0.004) and GNa (p = 0.017) compared to that of P. For the 2-h BHI, GHNa (1.25 + 0.30) differed from P (1.00; p = 0.009) with a tendency for a difference between GNa vs. P (1.10 + 0.25; p = 0.145). CONCLUSIONS: The model seems effective for simultaneous measurement of D2O for absorption, PV expansion, and fluid retention confirmed by expected outcomes for glucose-sodium beverages vs. water.