L. E. Armstrong

Relation between urinary hydration biomarkers and total fluid intake in healthy adults

Background/objectives:In sedentary adults, hydration is mostly influenced by total fluid intake and not by sweat losses; moreover, low daily fluid intake is associated with adverse health outcomes. This study aimed to model the relation between total fluid intake and urinary hydration biomarkers.Subjects/methods:During 4 consecutive weekdays, 82 adults (age, 31.6±4.3 years; body mass index, 23.2±2.7u2009kg/m2; 52% female) recorded food and fluid consumed, collected one first morning urine (FMU) void and three 24-h (24hU) samples. The strength of linear association between urinary hydration biomarkers and fluid intake volume was evaluated using simple linear regression and Pearson’s correlation. Multivariate partial least squares (PLS) modeled the association between fluid intake and 24hU hydration biomarkers.Results:Strong associations (|r|⩾0.6; P<0.001) were found between total fluid intake volume and 24hU osmolality, color, specific gravity (USG), volume and solute concentrations. Many 24hU biomarkers were collinear (osmolality versus color: r=0.49–0.76; USG versus color: r=0.46–0.78; osmolality versus USG: 0.86–0.97; P<0.001). Measures in FMU were not strongly correlated to intake. Multivariate PLS and simple linear regression using urine volume explained >50% of the variance in fluid intake volume (r2=0.59 and 0.52, respectively); however the error in both models was high and the limits of agreement very large.Conclusions:Hydration biomarkers in 24hU are strongly correlated with daily total fluid intake volume in sedentary adults in free-living conditions; however, the margin of error in the present models limits the applicability of estimating fluid intake from urinary biomarkers.

Assessment of hydration biomarkers including salivary osmolality during passive and active dehydration

BACKGROUND/OBJECTIVES:Hydration state can be assessed via body mass change (BMΔ), serum and urine osmolality (Sosm, Uosm), urine-specific gravity (Usg) and urine volume (Uvol). As no hydration index has been shown to be valid in all circumstances, value exists in exploring novel biomarkers such as salivary osmolality (Vosm). Utilizing acute BMΔ as the reference standard, this research examined the efficacy of Sosm, Vosm, Uosm, Uvol and Usg, during passive (PAS) and active (ACT) heat exposure.SUBJECTS/METHODS:Twenty-three healthy men (age, 22±3 years; mass, 77.3±12.8u2009kg; height, 179.9±8.8cm; body fat, 10.6±4.5%) completed two randomized 5-h dehydration trials (36±1u2009°C). During PAS, subjects sat quietly, and during ACT, participants cycled at 68±6% maximal heart rate. Investigators measured all biomarkers at each 1% BMΔ.RESULTS:Average mass loss during PAS was 1.4±0.3%, and 4.1±0.7% during ACT. Significant between-treatment differences at −1% BMΔ were observed for Sosm (PAS, 296±4; ACT, 301±4u2009mOsm/kg) and Uosm (PAS, 895±207; ACT, 661±192u2009mOsm/kg). During PAS, only Uosm, Uvol and Usg increased significantly (−1 and −2% BMΔ versus baseline). During ACT, Vosm most effectively diagnosed dehydration ⩾2% (sensitivity=86%; specificity=91%), followed by Sosm (sensitivity=83%; specificity=83%). Reference change values were validated for Sosm, Usg and BMΔ.CONCLUSIONS:The efficacy of indices to detect dehydration ⩾2% differed across treatments. At rest (PAS), only urinary indices increased in concert with body water loss. During exercise (ACT), Sosm and Vosm exhibited the highest sensitivity and specificity. Sosm, Usg and BMΔ exhibited validity in serial measurements. These findings indicate hydration biomarkers should be selected by considering daily activities.

Hormonal and Thirst Modulated Maintenance of Fluid Balance in Young Women with Different Levels of Habitual Fluid Consumption.

Background: Surprisingly little is known about the physiological and perceptual differences of women who consume different volumes of water each day. The purposes of this investigation were to (a) analyze blood osmolality, arginine vasopressin (AVP), and aldosterone; (b) assess the responses of physiological, thirst, and hydration indices; and (c) compare the responses of individuals with high and low total water intake (TWI; HIGH and LOW, respectively) when consuming similar volumes of water each day and when their habitual total water intake was modified. Methods: In a single-blind controlled experiment, we measured the 24 h total water intake (TWI; water + beverages + food moisture) of 120 young women. Those who consumed the highest (HIGH, 3.2 ± 0.6 L·day−1, mean ± SD) and the lowest (LOW, 1.6 ± 0.5 L·day−1) mean habitual TWI were identified and compared. Outcome variables were measured during two ad libitum baseline days, a four-day intervention of either decreased TWI (HIGH) or increased TWI (LOW), and one ad libitum recovery day. Results: During the four-day intervention, HIGH and LOW experienced differences in thirst (p = 0.002); also, a statistically significant change of AVP occurred (main effect of TWI and day, p < 0.001), with no effect (TWI or day) on aldosterone and serum osmolality. Urine osmolality and volume distinguished HIGH from LOW (p = 0.002) when they consumed similar 24 h TWI.

Evaluation of Uosm:Posm ratio as a hydration biomarker in free-living, healthy young women

Background/objectives:Urinary and plasma indices are utilized to assess whole-body water balance in healthy adults, whereas the urine-to-plasma osmolality ratio (Uosm:Posm) rarely is. To explore the efficacy of Uosm:Posm as a hydration biomarker, diet records of 120 college women were analyzed (beverage water+food water=total fluid intake (TFI); 5 days) to identify habitual high-volume (HIGH) and low-volume (LOW) drinkers.Subjects/methods:The experimental protocol first involved two ad libitum baseline days for HIGH (TFI, 3.21u2009l per 24u2009h; n=14) and LOW (TFI, 1.64u2009l per 24u2009h; n=14). During a controlled intervention (days 3–6), mineral water was the only beverage; HIGH consumed less than baseline (TFI, 2.00u2009l per 24u2009h), and LOW consumed more than baseline (TFI, 3.50u2009l per 24u2009h). During ad libitum recovery (day 7), TFI were 3.17 and 1.71u2009l per 24u2009h for HIGH and LOW, respectively. Duplicate Uosm (24u2009h collection) and Posm (morning) samples were analyzed on all days via freezing point depression osmometry.Results:In the evaluation of relative water excess (Uosm:Posm<1.0), 11/13 values occurred for HIGH on days 1, 2 and 7; for LOW, 28/29 occurred on intervention days 3–6. Chi-squared analysis indicated that the treatment and Uosm:Posm were significantly associated (χ21:0.001=23.5, P<0.001). Statistical regression analyses detected a strong, significant relationship between renal free-water clearance (FWC) and Uosm:Posm (r2=0.86, P<0.00001); this was not true for FWC and Posm (r2=0.00, P=0.40) because Posm values were stable across 7 days.Conclusions:These findings support the use of Uosm:Posm as a hydration biomarker.

INTRAVENOUS VS. ORAL REHYDRATION: EFFECTS ON SUBSEQUENT EXERCISE-HEAT STRESS 224

This study compared the influence of intravenous vs. oral rehydration after exercise-induced dehydration during a subsequent 90-min exercise bout. It was hypothesized that cardiovascular, thermoregulatory, and hormonal variables would be the same between intravenous and oral rehydration because of similar restoration of plasma volume (PV) and osmolality (Osmo). Eight non-heat-acclimated men received three experimental treatments (counterbalanced design) immediately after exercise-induced dehydration (33 degrees C) to -4% body weight loss. Treatments were intravenous 0.45% NaCl (iv; 25 ml/kg), no fluid (NF), and oral saline (Oral; 25 ml/kg). After rehydration and rest (2 h total), subjects walked at 50% maximal O2 consumption for up to 90 min at 36 degrees C. The following observations were made: 1) heart rate was higher (P < 0.05) in Oral vs. iv at minutes 45, 60, and 75 of exercise; 2) rectal temperature, sweat rate, percent change in PV, and change in plasma Osmo were similar between iv and Oral; 3) change in plasma norepinephrine decreased less (P < 0.05) in Oral compared with iv at minute 45; 4) changes in plasma adrenocorticotropic hormone and cortisol were similar between iv and Oral after exercise was initiated; and 5) exercise time was similar between iv (77.4 +/- 5.4 min) and Oral (84.2 +/- 2.3 min). These data suggest that after exercise-induced dehydration, iv and Oral were equally effective as rehydration treatments. Thermoregulation, change in adrenocorticotropic hormone, and change in cortisol were not different between iv and Oral after exercise began; this is likely due to similar percent change in PV and change in Osmo.