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.

The present study assessed the effects of mild dehydration on cognitive performance and mood of young males. A total of twenty-six men (age 20·0 (SD 0·3) years) participated in three randomised, single-blind, repeated-measures trials: exercise-induced dehydration plus a diuretic (DD; 40 mg furosemide); exercise-induced dehydration plus placebo containing no diuretic (DN); exercise while maintaining euhydration plus placebo (EU; control condition). Each trial included three 40 min treadmill walks at 5·6 km/h, 5 % grade in a 27·78C environment. A comprehensive computerised six-task cognitive test battery, the profile of mood states questionnaire and the symptom questionnaire (headache, concentration and task difficulty) were administered during each trial. Paired t tests compared the DD and DN trials resulting in .1 % body mass loss (mean 1·59 (SD 0·42) %) with the volunteer's EU trial (0·01 (SD 0·03) %). Dehydration degraded specific aspects of cognitive performance: errors increased on visual vigilance (P¼0·048) and visual working memory response latency slowed (P¼ 0·021). Fatigue and tension/anxiety increased due to dehydration at rest (P¼ 0·040 and 0·029) and fatigue during exercise (P¼0·026). Plasma osmolality increased due to dehydration (P,0·001) but resting gastrointestinal temperature was not altered (P¼0·238). In conclusion, mild dehydration without hyperthermia in men induced adverse changes in vigilance and working memory, and increased tension/anxiety and fatigue.Key words: Vigilance: Reaction time: Reasoning: Memory: FurosemideThe most comprehensive studies that evaluated the effects of dehydration on cognitive performance were conducted more than 20 years ago in laboratories located in a hot climate (1,2) . These studies employed a combination of highheat and aerobic exercise to rapidly produce dehydration. Subsequently, at least six other studies have evaluated the influence of body water loss via exercise in the heat (3 -5) and suggested that mild dehydration adversely affects cognitive function at approximately 2 % or more body mass loss, but not at 1 % body mass loss (1,2,5) . However, because body water loss and hyperthermia (i.e. resulting from exercise in a hot environment) were both present in these studies, the effects of dehydration alone could not be examined. Recently, three studies have examined dehydration without inducing hyperthermia (3,6,7) , via passive water restriction or prolonged exercise in a mild environment. These studies induced a narrow range of moderate dehydration (2·6-2·8 %) and therefore did not determine whether more modest levels of dehydration affect cognitive function.The present investigation was designed to assess the effects of mild dehydration, between 1 and 2 % body water loss, on cognitive performance and mood of healthy young males. Few studies have evaluated the effects of such mild dehydration in either men or women (7,8) . To isolate mild dehydration as the sole independent variable, body water loss was induced by mild exercise in a mild environ...

Limited information is available regarding the effects of mild dehydration on cognitive function. Therefore, mild dehydration was produced by intermittent moderate exercise without hyperthermia and its effects on cognitive function of women were investigated. Twenty-five females (age 23.0 ± 0.6 y) participated in three 8-h, placebo-controlled experiments involving a different hydration state each day: exercise-induced dehydration with no diuretic (DN), exercise-induced dehydration plus diuretic (DD; furosemide, 40 mg), and euhydration (EU). Cognitive performance, mood, and symptoms of dehydration were assessed during each experiment, 3 times at rest and during each of 3 exercise sessions. The DN and DD trials in which a volunteer attained a ≥1% level of dehydration were pooled and compared to that volunteer's equivalent EU trials. Mean dehydration achieved during these DN and DD trials was -1.36 ± 0.16% of body mass. Significant adverse effects of dehydration were present at rest and during exercise for vigor-activity, fatigue-inertia, and total mood disturbance scores of the Profile of Mood States and for task difficulty, concentration, and headache as assessed by questionnaire. Most aspects of cognitive performance were not affected by dehydration. Serum osmolality, a marker of hydration, was greater in the mean of the dehydrated trials in which a ≥1% level of dehydration was achieved (P = 0.006) compared to EU. In conclusion, degraded mood, increased perception of task difficulty, lower concentration, and headache symptoms resulted from 1.36% dehydration in females. Increased emphasis on optimal hydration is warranted, especially during and after moderate exercise.

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±2.7 kg/m 2 ; 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|X0.6; Po0.001) 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 ¼ 0.49-0.76; USG versus color: r ¼ 0.46-0.78; osmolality versus USG: 0.86-0.97; Po0.001). Measures in FMU were not strongly correlated to intake. Multivariate PLS and simple linear regression using urine volume explained 450% of the variance in fluid intake volume (r 2 ¼ 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.

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