Effect of Fluid Intake on Changing Blood Volume in Healthy Males

Abstract

Given that euhydration should represent an optimal total body water (TBW) volume and there is a clear relationship between TBW and blood volume (BV), it is important to understand how fluid intake may change BV. Fluid intake can alter 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 BV is unknown. PURPOSE: To investigate changes in hydration status and BV in response to 24 h of controlled fluid intake. METHODS: Blood volume (via CO rebreathing) along with serum and urine osmolality (Sosm and Uosm, respectively) were measured before and after a 24 h standardized food and fluid intake in 17 healthy males. Based on BV responses to the intervention, subjects were post-hoc assigned to groups in which BV had either increased (BVInc: n = 6, age 23 ± 3 y, mass 83.4 ± 13.3 kg) or decreased (BVDec: n = 6, age 25 ± 3 y, mass 86.1 ± 3.5 kg, p < 0.05 between groups). Subjects with no change in BV (n = 5) were excluded from the analysis. RESULTS: Total fluid intake during the intervention was not different between groups (3930 ± 322 vs. 3883 ± 468 ml; p = 0.813, for BVInc and BVDec, respectively). The BVInc and BVDec groups started the protocol with similar Sosm (288 ± 3 vs. 289 ± 3 mOsm/kg), Uosm (787 ± 214 vs. 692 ± 375 mOsm/kg), and BV (6445 ± 596 vs. 7012 ± 746 ml), respectively (all p < 0.05). Despite starting at similar traditional markers of hydration status, the BVInc group increased BV 387 ± 147 ml whereas the BVDec group decreased 410 ± 205 ml (p < 0.001). Post-intervention measures of Sosm (287 ± 3 vs. 287 ± 2 mOsm/kg) and Uosm (628 ± 272 vs. 704 ± 117 mOsm/kg) were not different between groups (p > 0.05). However, a trend for a main effect of treatment was observed with Sosm across groups (Pre: 289 ± 3 vs. Post: 287 ± 3 mOsm/kg; p = 0.08). While changes in BV were significant between groups, post-intervention measures of BV were not different between BVInc and BVDec groups (6831 ± 633 vs. 6602 ± 775 ml, respectively; p > 0.05). CONCLUSION: Interestingly, changes in BV were not reflected in changes in traditional hydration biomarkers. This may suggest that, while they appeared to begin and end similarly hydrated, the BVInc group retained fluid (and subsequently had an increase in BV) during a period of prescribed fluid intake.