Matthew A. Tucker1, Cory L. Butts1, Nicole E. Moyen1, Evan C. Johnson1,2, J. D. Adams1, Alf Z. Satterfield1, Ashley Six1 & Matthew S. Ganio1. Human Performance Laboratory, University of Arkansas, Fayetteville, Arkansas; 2Department of Kinesiology and Health, University of Wyoming, Laramie, Wyoming.

Given that euhydration should represent an optimal total body water (TBW) level and there is a clear relationship between TBW and BV, it is important to understand how fluid intake may change BV. It is well documented that altered fluid intake changes urinary and circulatory markers of hydration status; however, the degree to which changes in traditional indices of hydration status are simultaneously reflected in measurements of blood volume (BV) remains unknown. PURPOSE: To investigate changes in hydration status and BV in response to 24 h of controlled fluid intake. METHODS: Seventeen healthy male subjects (age 24 ± 3 y, mass 84.4 ± 8.4 kg) were provided food and water over 24 h that had a total water volume of 60 ml/kg fat-free mass. Plasma volume (PV) and BV (via CO rebreathing) along with serum and urine osmolality (Sosm and Uosm, respectively) were measured pre- and post-intervention. Urine was collected over the 24-h period and analyzed for volume (24-Uvol) and osmolality (24-Uosm). Based on BV responses to the intervention, subjects were post-hoc assigned to groups in which BV had either increased (BVInc: n = 9, 362 ± 136 ml) or decreased (BVDec: n = 8, -493 ± 247 ml; p < 0.001). RESULTS: Total fluid intake was not different between groups (3930 ± 322 vs. 3883 ± 468 ml; p = 0.813, for BVInc and BVDec, respectively). The groups started the hydration protocol with similar Sosm and Uosm (both p > 0.05). However, the BVInc group started with a lower PV (3517 ± 481 ml) and BV (6228 ± 653 ml) versus the BVDec group (4005 ± 345 ml and 7081 ± 644 ml, respectively; p < 0.05). With the fluid intervention, BVInc had an increase in BV (362 ± 136 ml) and PV (268 ± 84 ml) while BVDec had a decrease in BV (-493 ± 247 ml) and PV (-272 ± 110 ml). This lead to similar PV (3875 ± 512 and 3734 ± 386 ml, for BVInc and BVDec; p > 0.05) and BV (6590 ± 669 and 6588 ± 661 ml, p > 0.05) at the end of the 24-h fluid intervention. Interestingly, with 24-h of prescribed fluid, the change in Sosm (-1 ± 2 vs. -2 ± 2 mOsm/kg) and Uosm (-133 ± 190 vs. -33 ± 316 mOsm/kg), along with 24-Uvol (2819 ± 978 vs. 3218 ± 711 ml) or 24-Uosm (377 ± 177 vs. 324 ± 81 mOsm/kg) were similar between BVInc and BVDec (all p > 0.05). CONCLUSION: Despite no significant differences in traditional indices of hydration status prior to the intervention, two groups of subjects’ blood volume responded in opposite fashion to the same volume of fluid over 24 h. Interestingly, the changes in blood volume were not reflected in changes in traditional hydration biomarkers. This may suggest that, while they appeared to begin similarly hydrated, the BVInc group had below optimal TBW level, as evidenced by the retention of fluid (and subsequent increase in BV) during a period of prescribed fluid intake.

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