Few clinical studies have evaluated the impairments induced by mild dehydration on mood and physiological parameters but with contradictory results.1,2 The lack of homogeneity between study designs, including duration of the dehydration phase, time of assessments, and the way to induce dehydration, appears as an important confounding factor. Most of the previous studies used heat or sustained physical exercise or a combination of both to achieve moderate dehydration levels.1,2 These studies do not allow the proper effects of dehydration to be determined because of the combination of several stressors.
The aim of the current study was to find subjective variables and/or physiological parameters that are easy to measure and sensitive to mild dehydration induced by water deprivation over a 24-hour period. We were also interested in determining whether ad libitum water intake could reverse the deleterious effects of dehydration. Fluid intake and energy intake were strictly controlled, and the different parameters were assessed at various time points throughout the 24 hours to avoid the methodological weaknesses described in the previous studies.
MATERIALS AND METHODS
This study evaluated, using a well-controlled dehydration protocol, the effects of 24-hour fluid deprivation (FD) on mood and physiological parameters. Twenty young healthy women participated in this randomized 2-period (dehydrated vs control) crossover study. Caloric intake was strictly controlled in both conditions. In the FD period, final water intake was between 6 and 7 PM, and no beverages were permitted until 6 PM on the next day. Water intake was permitted only at fixed periods during the control conditions. Except FD, both periods were identical and included various self-rating mood scales and several physiological parameters.
Different Mood Scales
The Profile of Mood Scale (POMS)3 assessed tension, depression, anger, vigor, fatigue, and confusion. The Bond and Lader visual analog scale (VAS)4 measured alertness, calmness, and contentedness. The emotional VAS5 gave an indication on emotional stability. The sensation VAS6 measured thirst and headache. The sleep VAS7 was used to assess sleep onset, sleep quality, ease of awakening, and awakening behavior. The Karolinska Sleepiness Scale8 assessed subjective daytime sleepiness. During both periods, the scales were administered at several time points during the day (except sleep VAS administered only at awakening) in order to evaluate the evolution of mood according to hydration state.
Physiological Dehydration Markers
Urine parameters. Urine was collected from day 0 after dinner up to the end of the assessments (6 PM) on day 1. Three urine parameters were recorded at each void: (a) gravity measured with the digital urine specific gravity “pen” refractometer, Pen-Urine SG (Atago Co Ltd, Tokyo, Japan); (b) coloration using the urine color scale9,10; (c) volume of urine (total and for each void).
Plasma osmolality (Posm). Venous blood samples (5 mL) were collected on day 0 at 7 PM and on day 1 at 7 PM and 6 PM.
Saliva osmolality (Sosm). Saliva samples (≥200 μL) were collected on day 0 at 7 PM and on day 1 at 06:30 PM, at 12:30 PM, and at 6 PM.
Analyses of variance (ANOVA) were performed only when means appeared to be clinically different between the 2 fluid conditions (ie, more or less 1.5 SD)11 for at least 1 timepoint. The ANOVA model included fluid condition, time (except for sleep VAS), and sequence as fixed effects, with interaction between condition and time (except for sleep VAS). In the same way, physiological blood and saliva parameters were analyzed using analyses of covariance (ANCOVA) with baseline values as the covariate. The statistical level of significance was fixed at P < .05 (2-tailed).
Particular attention is focused on condition effect in this article and on the influence of the transition from dehydration to an acute rehydration by comparing the scores on the mood scales following the acute water intake at the end of the FD period (time point 6 PM, day 1) versus the scores at 4 PM on day 1 (last FD time point).
Physiological Dehydration Markers
Mean values for gravity and color of urine on day 0 indicated that the subjects started the sessions in a comparable well-hydrated status for both experimental conditions. Conversely, clinically significant mean differences for the 3 urine parameters were observed between the 2 experimental conditions on day 1 (color: 2.71 ± 0.14, gravity: 0.015616 ± 0.000757, volume: −1187.3 ± 78 mL). Also, the total urine volume collected on day 1 was dramatically decreased in the FD condition (mean volume, 529.8 ± 37.4 mL) compared with the control condition (mean volume: 1717.1 ± 68.9 mL) resulting in a statistically significant difference (estimate ± 95% confidence interval [CI] = −1137.8 mL ± 198.11; P < .001).
Although few individual Posm values were outside the normal range (N = 280-300 mOsm/kg) in both experimental conditions, none was considered as clinically abnormal. Despite a large intersubject variability for both parameters, clinically relevant differences could be observed between the 2 experimental conditions at the latest measure (day 1). This difference was statistically significant for Sosm only (estimate ± 95% CI = 9.693 ± 8.454; P = .025).
Scales and Questionnaires
Thirst sensation increased with the duration of FD. The subjects felt significantly more thirsty throughout the FD period compared with the control condition (P < .001). Fatigue (POMS) and sleepiness (Karolinska Sleepiness Scale) scores were significantly higher (P < .001, both), whereas vigor (POMS) and alertness (Bond and Lader VAS) ratings were significantly lower (P < .001 and P < .01, respectively) in FD condition in comparison to control condition.
Also, calmness (Bond and Lader VAS) and happiness (emotional VAS) scores were lower (P < .01 and P < .05, respectively), and confusion (POMS) was higher (P < .001) in FD condition compared with control condition.
Rehydration Effects on Subjective Physiological and Mood Aspects
As expected, thirst feeling was reversed by ad libitum water intake at the end of the progressive FD period (6 PM vs 4 PM in FD condition: −70.918 ± 10.561, P < .001). Ad libitum water intake also reversed all the different mood impairments induced by FD except vigor, fatigue (POMS), and calmness (Bond and Lader VAS) as indicated by the absence of significant differences at 6 PM versus 4 PM observed in the FD condition (estimates ± 95% CI = 0.78 ± 2.47, −1.274 ± 3.094, −3.962 ± 8.062 for vigor, fatigue, and calmness, respectively).
The main objective of this study was to evaluate the effect of an acute FD on mood and on physiological parameters in young healthy females.
The results indicated that the FD was carried out in a well-controlled manner. Indeed, this point was attested by an objective physiological parameter (ie, total urinary volume) as well as by thirst measurements (ie, thirst VAS).
Physiological Dehydration Markers
Fluid deprivation induced early and late clinical changes. Changes observed in urine gravity and in urine color scale were the early physiological markers of dehydration.
Saliva osmolality represents an interesting noninvasive later marker of dehydration. Indeed, this parameter showed statistically significant differences between experimental conditions at the end of the FD period.
Finally, Posm remained within a normal variability interval, and only a trend to increase was observed at the end of the FD period.
Dehydration Effects on Mood
When mood is examined, several parameters such as sleep quantity, testing environment, and time of mood assessment must be controlled.1,12 Calorie intake and caffeine consumption are also known to have an effect on mood assessments but were not systematically controlled in studies dealing with dehydration.13,14
The present study was designed to better control the external factors having an effect on the target parameters (eg, caffeine, nicotine and alcohol consumption, sleep quantity and quality, physical activity).
Results showed that FD had effects on several mood aspects. As previously demonstrated,14–16 the most important effects were those observed on sleep/wake parameters such as higher sleepiness and fatigue scores and lower vigor and alertness scores in the FD condition; the subjects were also more confused, less calm, and less happy.
Rehydration Effects on Mood
Mood was also assessed at 6 PM after an ad libitum fluid intake in both conditions. The results showed that most of the mood impairments induced by the FD were reversed by this acute water intake, with the exception of 3 parameters, which were fatigue, vigor, and calmness. To our knowledge, this is the first study showing that FD has longer-term effects on mood that are not directly reversed by an ad libitum fluid intake.
Even if the likelihood that a 24-hour total FD occurs in everyday life is not realistic, these results are interesting because the first dehydration effects on urine and mood were observed very early in this study (ie, in the morning after 12-16 hours of FD); a dehydration period of 12 to 16 hours is frequent in everyday life. Indeed, this situation would correspond to people going to work or to school without breakfast in the morning or to busy working people skipping lunch time or forgetting to drink during a hard working day for instance.
Overall, during the present study, external factors acting on mood were well controlled. Assessment of Sosm and urine color scale were identified as reliable and noninvasive methods for assessing hydration status. In line with literature, deleterious effects of FD on mood were evidenced. The effects on subjective sleep/wake parameters seem of particular interest for future investigations. Moreover, an improvement of most, but not all, impaired mood scores after acute water intake was observed.
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