Small Differences in Rehydration Volume Affect 24h Urinary Concentration
Authors: Johnson Evan C. , Huffman Ainsley E. , Yoder Hillary A. , Dolci Alberto , Perrier Erica T. , Larson‐Meyer D. Enette E. 
Published: June 2021

OBJECTIVESub‐optimal water intake may be unavoidable in certain occupations, or when access to water is limited. Additionally, some individuals chronically consume low fluid volumes ad libitum despite free access to water. In all of these situations, low urine volume and high concentration indicate insufficient water intake. Therefore, the purpose of the current study was to evaluate the impact of various volumes of increased water intake on 24h urinary hydration biomarkers (volume, Uvol; specific gravity USG; and osmolality, Uosmo) following three days of water restriction.METHODS115 participants [56 women (31±9 y, 166±7 cm, 65±12 kg, 34±8 %BF); 59 men (32±8 y, 180±7 cm, 83±18 kg, 22±8 %BF)] volunteered to record all food and beverages consumed over 11 days (d). On d 1 through 7, participants consumed food and beverages ad‐libitum. On d 8 through 10, participants were restricted to 1 L of plain water in addition to their habitual food intake. On d 11, participants were randomly assigned to maintain plain water intake at 1 L (Wbev+0) or into one of four intervention groups in which plain water intake increased to 1.5 L (Wbev+0.5), 2.0 L (Wbev+1.0), 2.5 L (Wbev+1.5), or 3.25 L (Wbev+2.25). 24 h urine collections were completed at baseline (d 3), during fluid restriction (d 9 and 10), and during the increased water intake (d 11). Repeated measures ANOVA was utilized to determine the effect of the water restriction and the increased intakes on urinary hydration biomarkers. Within group, paired sample t‐tests were used post‐hoc to determine if urinary biomarkers returned to baseline values.RESULTSNo differences existed between any groups for demographic or habitual intake variables at baseline (all F[4,114] < 1.5, all P >.206). Fluid restriction (d 8–10) resulted in decreased total fluid and total water intake (food & fluids) similarly in all groups (from 2652±1168 to 1006±59 mL·d−1 and from 3326±1224 to 1763±349 mL·d−1, respectively; F[3,330] = 229.7, P < .001 and F[3,330] = 194.3, P < .001). Correspondingly, urinary hydration biomarkers changed equally within all groups (all F[4,110] < 0.7, all P >.653), 24h Uvol (−1068±858 mL), USG (+0.009±0.006), and Uosmo (+340±233 mOsm·kg−1). Significant group X time interactions revealed that different increased water intake volumes resulted in different changes in Uvol, USG and Uosmo (all F[4,110] > 10.7, all P <.001). Follow up t‐tests demonstrated that all urinary biomarkers returned to baseline in Wbev+1.5 and Wbev+2.25 (all |t|< 1.6, all P >.126), while in Wbev+0, Wbev+0.5, and Wbev+1.0 urinary variables remained significantly elevated (all |t|> 2.8, all P <.011).CONCLUSIONThe current analysis quantifies the changes in 24h urinary biomarkers that can be expected in response to different increases in daily water intake when starting from a similar degree of under‐hydration. These results can be used to help educate individuals who notice their urine concentration increasing in order to assist with properly adjusting water intake following days of low water intake or after situations where access to water is limited.Support or Funding InformationFunding for data collection provided by Danone Research, Palaiseau, FranceThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.