Markers of the hydration process during fluid volume modification in women with habitual high or low daily fluid intakes

Purpose Human daily total water intake (TWI) has a large inter-individual range. Recently, water supplementation has been suggested as a potential preventative and therapeutic modality. Thus, we aimed to measure hydration biomarkers in women with high (HIGH) versus low (LOW) daily TWI to determine baseline differences, and the efficacy of these markers during a systematic alteration in TWI. Methods This cohort study identified 14 HIGH [3.34 (0.56) L day −1 ] and 14 LOW [1.62 (0.48) L day −1 ] from 120 women. Next, fluid intake was decreased in HIGH [2.00 (0.21) L day −1 ] while LOW increased [3.50 (0.13) L day −1 ] across 4 days. Body mass, fluid intake, serum osmolality ( S osmo ), total plasma protein (TPP), 24 h urine osmolality, and 24 h urine volume, were measured on each day of modified TWI. Estimated plasma volume ( E pv ) was calculated using measured body mass and hematocrit values. Results At baseline, urinary markers and TPP differentiated HIGH from LOW [7.0 (0.3) versus 7.3 (0.3) mg dL −1 , respectively]. Upon TWI intervention, (1) body mass decreased in HIGH [−0.7 (1.1) kg, p = 0.010)] but did not increase in LOW [+0.0 (0.6) kg, p = 0.110], (2) E pv decreased 2.1 (2.4) %, p = 0.004, (3) urine osmolality increased in HIGH [397 (144)–605 (230) mOsm kg −1 , p