Hydration Biomarkers Research Articles

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

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. The experimental protocol first involved two ad libitum baseline days for HIGH (TFI, 3.21 l per 24 h; n=14) and LOW (TFI, 1.64 l per 24 h; n=14). During a controlled intervention (days 3-6), mineral water was the only beverage; HIGH consumed less than baseline (TFI, 2.00 l per 24 h), and LOW consumed more than baseline (TFI, 3.50 l per 24 h). During ad libitum recovery (day 7), TFI were 3.17 and 1.71 l per 24 h for HIGH and LOW, respectively. Duplicate Uosm (24 h collection) and Posm (morning) samples were analyzed on all days via freezing point depression osmometry. 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 (χ(2)1:0.001=23.5, P<0.001). Statistical regression analyses detected a strong, significant relationship between renal free-water clearance (FWC) and Uosm:Posm (r(2)=0.86, P<0.00001); this was not true for FWC and Posm (r(2)=0.00, P=0.40) because Posm values were stable across 7 days. These findings support the use of Uosm:Posm as a hydration biomarker.

Interpreting common hydration biomarkers on the basis of solute and water excretion

This investigation evaluated 12 hydration biomarkers, to determine which represent 24-h whole-body water balance (that is, measured as water retention or clearance (WR-C) by the kidneys). Healthy males (n=59; body mass, 75.1±7.9 kg; height, 178±6 cm; age, 22±3 years; body mass index, 23.9±2.4 kg/m(2)) met with a registered dietitian each morning (days 1-11) to optimize completeness and accuracy of food and fluid records, then went about ordinary daily activities. These men visited the laboratory for blood samples and collected all urine produced on days 1, 3, 6, 9 and 12. The reference standard (WR-C) was calculated using 24-h urine volume, 24-h urine osmolality, and serum osmolality (single morning venous sample). Statistical regression analyses indicated that, among the 12 hydration biomarkers, only 24-h urine osmolality (r(2)=0.60, P<0.0001) and 24-h urine specific gravity (r(2)=0.52, P<0.0001) strongly predicted WR-C. The 24-h fluid intake, 24-h body mass change, 24-h urine color and 24-h urine volume were weak (P>0.05) predictors of WR-C, similar to serum osmolality and other single measurements (range of r(2) values, 0.19-0.0001). These observations of healthy, active young men demonstrate that WR-C is strongly related to the 24-h concentration of urine, which in turn reflects the excretion of total solids in the diet. Although morning urine assessments provided information about a single time point, 24-h urine osmolality and 24-h urine specific gravity were the best predictors of 24-h body water balance.

Challenges of linking chronic dehydration and fluid consumption to health outcomes

The purpose of this article is to review the effects of chronic mild dehydration and fluid consumption on specific health outcomes including obesity. The electronic databases PubMed and Google Scholar were searched for relevant literature published from the time of their inception to 2011, with results restricted to studies performed on human subjects and reports in the English language. Key words included the following: dehydration, hypohydration, water intake, fluid intake, disease, and the names of specific disease states. Strength of evidence categories were described for 1) medical conditions associated with chronic dehydration or low daily water intake, and 2) randomized-controlled trials regarding the effects of increased water consumption on caloric intake, weight gain, and satiety. This process determined that urolithiasis is the only disorder that has been consistently associated (i.e., 11 of 13 publications) with chronic low daily water intake. Regarding obesity and type 2 diabetes, evidence suggests that increased water intake may reduce caloric intake for some individuals. Recommendations for future investigations include measuring total fluid intake (water + beverages + water in solid food), conducting randomized-controlled experiments, identifying novel hydration biomarkers, and delineating hydration categories.

Hydration biomarkers in free-living adults with different levels of habitual fluid consumption

Little is known about the impact of habitual fluid intake on physiology. Specifically, biomarkers of hydration status and body water regulation have not been adequately explored in adults who consume different fluid volumes in everyday conditions, without prolonged exercise or environmental exposure. The purpose of the present study was to compare adults with habitually different fluid intakes with respect to biomarkers implicated in the assessment of hydration status, the regulation of total body water and the risk of kidney pathologies. In the present cross-sectional study, seventy-one adults (thirty-two men, thirty-nine women, age 25–40 years) were classified according to daily fluid intake: thirty-nine low drinkers (LD; ≤ 1·2 litres/d) and thirty-two high drinkers (HD; 2–4 litres/d). During four consecutive days, urinary parameters (first morning urine (FMU) on day 1 and subsequent 24 h urine (24hU) collections), blood parameters, and food and beverage intake were assessed. ANOVA and non-parametric comparisons revealed significant differences between the LD and HD groups in 24hU volume (1·0 (se 0·1) v. 2·4 (se 0·1) litres), specific gravity (median 1·023 v. 1·010), osmolality (767 (se 27) v. 371 (se 33) mOsm/kg) and colour (3·1 (se 0·2) v. 1·8 (se 0·2)). Similarly, in the FMU, the LD group produced a smaller amount of more concentrated urine. Plasma cortisol, creatinine and arginine vasopressin concentrations were significantly higher among the LD. Plasma osmolality was similar between the groups, suggesting physiological adaptations to preserve plasma osmolality despite low fluid intake. The long-term impact of adaptations to preserve plasma osmolality must be examined, particularly in the context of renal health.

Hydration Biomarkers During Daily Life

Human fluid-electrolyte regulation involves multiple neuroendocrine responses to the hourly loss and gain of body water. Such dynamic complexity complicates hydration assessment and minimizes the likelihood that any single biomarker will validly and precisely describe hydration status in all life situations. This article describes the biomarkers that are currently used during daily living to assess mild dehydration, plus recent advances in our understanding of invasive and noninvasive techniques. This article also suggests directions for future exploration of novel hydration indices, in the belief that superior biomarkers exist but have not been discovered.

Hydration Biomarkers and Dietary Fluid Consumption of Women

Normative values and confidence intervals for the hydration indices of women do not exist. Also, few publications have precisely described the fluid types and volumes that women consume. This investigation computed seven numerical reference categories for widely used hydration biomarkers (eg, serum and urine osmolality) and the dietary fluid preferences of self-reported healthy, active women. Participants (n=32; age 20±1 years; body mass 59.6±8.5 kg; body mass index [calculated as kg/m2] 21.1±2.4) were counseled in the methods to record daily food and fluid intake on 2 consecutive days. To reduce day-to-day body water fluctuations, participants were tested only during the placebo phase of the oral contraceptive pill pack. Euhydration was represented by the following ranges: serum osmolality=293 to 294 mOsm/kg; mean 24-hour total fluid intake=2,109 to 2,506 mL/24 hours; mean 24-hour total beverage intake=1,300 to 1,831 mL/24 hours; urine volume=951 to 1,239 mL/24 hours; urine specific gravity=1.016 to 1.020; urine osmolality=549 to 705 mOsm/kg; and urine color=5. However, only 3% of women experienced a urine specific gravity <1.005, and only 6% exhibited a urine color of 1 or 2. Water (representing 45.3% and 47.9% of 24-hour total fluid intake), tea, milk, coffee, and fruit juice were consumed in largest volumes. In conclusion, these data provide objective normative values for hyperhydration, euhydration, and dehydration that can be used by registered dietitians and clinicians to counsel women about their hydration status.

Evaluation of Uosm:Posm ratio and Posm as Hydration Biomarkers in Healthy Young Women During Daily Activities

The urine‐to‐plasma osmolality ratio (Uosm:Posm) is used clinically to diagnose renal and metabolic disorders. Uosm:Posm&lt;1 also represents a state of relative water excess. Diet records of 120 college women were analyzed [beverage water + food water = total fluid intake (TFI)] for 5d to identify 14 high volume (HIGH) and 14 low volume (LOW) drinkers (age, 20.3±1.7 y). Experimental protocol: 2 baseline days verified ad libitum TFI status of HIGH (3.21 L/d) and LOW (1.64 L/d). During a controlled intervention (days 3–6) with mineral water (Volvic™), HIGH consumed less than baseline (TFI, 2.00 L/d; H↔L) and LOW consumed more than baseline (TFI, 3.50 L/d; L↔H). On ad libitum recovery day 7, TFI were: HIGH, 3.17, LOW, 1.71 L/d. Duplicate Uosm (24h collection) and Posm (morning) samples were analyzed via freezing point depression osmometry. Mean (± SD) Uosm:Posm were H↔L: d1, 1.3±0.4; d2, 1.4±0.5; d3, 2.0±0.8; d4, 2.1±0.9; d5, 2.1±0.8; d6, 2.0±0.8; d7, 1.2±0.6. L↔H: d1, 2.7±0.7; d2, 2.6±0.6; d3, 0.9±0.2; d4, 0.9±0.3; d5, 0.9±0.1; d6, 1.0±0.4; d7, 1.9±0.7. 11/13 of Uosm:Posm&lt;1 values occurred for H↔L on d1, d2 and d7, and 32/33 occurred for L↔H on d3 to d6. Mean daily Posm values were similar (7d range: H↔L, 292 – 295; L↔H, 293 – 296 mOsm/kg), reflecting neuroendocrine regulation. These data indicate that Uosm:Posm warrants further investigation as an index of human hydration status across days. Funded by Danone Research, France.

Nutritional, Physiological, and Perceptual Responses During a Summer Ultraendurance Cycling Event

Despite the rapid growth of mass participation road cycling, little is known about the dietary, metabolic, and behavioral responses of ultraendurance cyclists. This investigation describes physiological responses, perceptual ratings, energy balance, and macronutrient intake of 42 men (mean ± SD; age, 38 ± 6 years; height, 179.7 ± 7.1 cm; body mass, 85.85 ± 14.79 kg) and 6 women (age, 41 ± 4 years; height, 168.0 ± 2.9 cm; body mass, 67.32 ± 7.21 kg) during a summer 164-km road cycling event. Measurements were recorded 1 day before, and on the Event Day (10.5 hours) at the start (0 km), at 2 aid stations (52 and 97 km), and at the finish line (164 km). The ambient temperature was >39.0° C during the final 2 hours of exercise. The mean finish times for men (9.1 ± 1.2 hours) and women (9.0 ± 0.2 hours) were similar, as were mean gastrointestinal temperature (TGI), 4 hydration biomarkers, and 5 perceptual (e.g., thermal, thirst, pain) ratings. Male cyclists consumed enough fluids on the Event Day (5.91 ± 2.38 L; 49% water) to maintain body mass within 0.76 kg, start to finish, despite a sweat loss of 1.13 ± 0.54 L·h(-1) and calculated energy expenditure of 3,115 kcal·10.5·h(-1). However, men voluntarily underconsumed food energy (deficit of 2,594 kcal, 10.9 MJ) and specific macronutrients (carbohydrates, 106 ± 48 g; protein, 8 ± 7 g; and sodium, 852 ± 531 mg) between 0530 and 1400 hours. Also, a few men exhibited extreme final values (i.e., urine specific gravity of 1.035-1.038, n = 5; body mass loss >4 kg, n = 2; T(GI), 39.4 and 40.2°C). We concluded that these findings provide information regarding energy consumption, macronutrient intake, hydration status, and the physiological stresses that are unique to ultraendurance exercise in a hot environment.