Urine, Body Mass, and Thirst during Fluid Restriction and Subsequent Rehydration
Authors: Butts Cory L. , Caldwell Aaron R. , Ganio Matthew S. , McDermott Brendon P. 
Published: June 2021

PurposeExamine urine markers, body mass change, and thirst after fluid restriction and rehydration from dehydrating activity.MethodsParticipants (n = 14 males, age 25 ± 5 y, wt 76.7 ± 10.7 kg, body fat 16.9 ± 6.8%) completed two trials (euhydrated, EU, and hypohydrated, HY) separated by ≥28 days. Adequate fluid intake was maintained prior to the EU trial and throughout exercise to prevent >2% body mass (BM) loss (actual loss; −1.4 ± 0.9%). Dehydration consisted of 24‐h fluid restriction prior to the HY trial (BM loss; −2.4 ± 1.3%) with minimal fluid throughout the trial (end trial BM loss; −4.6 ± 1.5%). Prior to each trial, participants completed 3‐day baseline BM and 24‐h urine collection. Trials involved unilateral eccentric knee flexion (23°C, 54%) followed by one hour of treadmill running (~60% VO2max) and 30 minutes of recovery in the heat (33°C, 50%). A rehydration protocol was prescribed in both trials to return participants to the 3‐day baseline BM and urine was collected until a 24‐h follow‐up visit. Hydration measures included perceived thirst, 24‐h and spot urine osmolality (Uosm) and spot urine specific gravity (Usg). Descriptive statistics for hydration measures were evaluated in each trial. Follow‐up calculations of positive likelihood and odds ratios were completed.ResultsFollowing 24‐h fluid restriction, BM change from the 3‐day baseline was −2.4 ± 1.3% (range; −0.4 to − 4.4%). 24‐h Uosm was 750 ± 211 mOsm/kg (range; 459 to 1105 mOsm/kg) while spot Uosm was 1015 ± 133 mOsm/kg (range; 823 to 1264 mOsm/kg). Spot Usg was 1.027 ± 0.005 (range; 1.020 to 1.034) in the HY trial following fluid restriction and thirst was 6.0 ± 1.7. In the EU trial, 24‐h Uosm was 356 ± 130 mOsm/kg, spot Usg was 1.015 ± 0.007, and thirst was 2.4 ± 1.5. When using a combined clinical criterion (meeting two of three clinical cut‐points for hypohydration of >1% BM loss, Usg >1.020, or thirst >3) to evaluate fluid restriction, the likelihood of positively identifying fluid restriction with the combined clinical criteria was 7.0 (95% CI: 1.94, 25.26). The 24‐h rehydration protocol in the EU trial resulted in 24‐h Uosm of 283 ± 88 mOsm/kg (range; 191 to 437 mOsm/kg), spot Uosm of 530 ± 306 mOsm/kg (range; 50 to 1000 mOsm/kg), and spot Usg of 1.014 ± 0.008 (range; 1.001 to 1.027). The 24‐h rehydration protocol from the HY trial resulted in BM change from the 3‐day baseline of −0.3 ± 1.3% (range; −2.5 to 2.3%). Further, the 24‐h Uosm following rehydration was 567 ± 346 mOsm/kg (range; 219 to 1066 mOsm/kg) while spot Uosm was 643 ± 366 mOsm/kg (range; 133 to 1076 mOsm/kg). Spot Usg was 1.017 ± 0.010 (range; 1.006 to 1.033) and thirst was 2.4 ± 1.4 (range; 1 to 5). Further, when spot Usg was >1.016, the likelihood of having BM loss >1% was 2.1 (95% CI: 1.1, 4.0) with an odds ratio of 7.5 (95% CI: 0.7, 76.8).ConclusionsThe assessment method of fluid balance greatly influences clinical utility and the ability to properly identify fluid restriction and adequate rehydration. Spot and 24‐h urine samples, thirst, and a combination of clinical criteria provided an accurate observation of fluid restriction, however, further investigation is warranted to identify optimal markers to detect 24‐h rehydration from a known dehydrating activity.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.