Subjects

This observational study involved eight primary schools from various Spanish provinces (Madrid, Córdoba, Segovia and Ciudad Real), including the capital of the each province and a semi-urban/rural city. All schools were chosen randomly, looking to include children from different locations of the Spanish geography. Of the several schools contacted by telephone, eight accepted the invitation to be included in the study. Permission was requested to meet the parents of children in the age group 7–12 years. Once permission was given, the details of the study were explained to parents and all questions answered. Written informed consent was then sought to include their children in the study [20]. After this step, lifestyle information and a 3-day dietary record were collected, and the necessary material to collect 24-hour urine samples (24-h) was provided.

A total of 1,232 children was given the opportunity to participate in the study. Of these, 278 children provided written informed consent; 16 were excluded because they did not collect the urine sample correctly and 20 were excluded because they did not fully complete the lifestyle questionnaire. The final sample, therefore, compromised 242 from 7 to 12 years old; 49.17% of them were females, and 50.83% were males. The fieldwork was performed between February and March 2014, when the average temperatures were 13.5°C and 14.5°C, respectively and the range of temperatures were 3.6–14.7°C in February and 4.6–16.8°C in March.

The inclusion criteria were as follows: living in the provinces of the study, being between 7 and 12 years old and attending the second to sixth grades of primary school, being in compliance with the informed consent via parents or legal guardians, and not presenting any clinical problem that could modify the results, such as liver or kidney diseases, diabetes, and/or hyper/hypokalemia. In addition, the participants had received no pharmacological treatments in the 3 months prior to the study with corticosteroids, insulin or diuretics.

Anthropometric data

All measurements took place at the schools in the mornings and in accordance with norms set out by the WHO (World Health Organization) [21]. The weights and heights were measured using a digital electronic balance (range 0.1–150 kg; precision, 100 g; Alpha; Seca, Igni, France) and a digital stadiometer (70–205 cm; 1mm; Harpenden Pfifter, Carlstadt, NJ, USA). The body mass index (BMI) was then calculated and compared with the IOTF cut-off points [22].

Lifestyle study

A lifestyle questionnaire [23], previously applied in other Spanish populations of children and adults [24, 25, 26], was filled out by the parents or legal guardians (S1 Appendix). The questionnaire included questions to quantify the primary activities that could indicate an active lifestyle and sedentary behaviors, both on weekdays and weekends. This questionnaire collected the frequency of active play, physical activity as a subject in school, extracurricular physical activities and the time spent on each activity. Additionally, questions about the hours spent in front of different displays were used as an indicator of sedentary behavior (personal computer/video game console/television), both on workdays and weekends. Parents, legal guardians or caregivers of schoolchildren collected this information through active observation except for physical activity as subject in school and extracurricular physical activities. In the last two cases teachers provided the information. In the case of active plays, information provided by schoolchildren on the activities carried out during the break has also been taken into account. When all of the information was collected, the reported mean of PA hours per week was calculated (activities that increase the energy expenditure above the BMR (Basal Metabolic Rate) according to the Compendium of Physical Activities (2011) [27]. These activities included: active play, PA performed in school and extracurricular physical activities) and sedentary behaviors (hours spent in front of the PC (personal computer), video game console and TV (television)).

To estimate the time spent on PA, a classification of children into “actives” was created for those participating in greater than the WHO's recommendation of 1 hour or more of performing moderate to vigorous physical activities to maintain health from 5 to 17 years [28]. However, children were classified as “sedentary” when they spent more than 2 hours performing this type of sedentary activities, according to the latest guidelines of the Australian government for children within the same age range [29]. Afterwards, a classification of the study sample was performed by accounting for these two variables to establish the four following different groups: “sedentary active”, “non-sedentary active”, “sedentary inactive” and “non-sedentary inactive”.

Dietetic study

Parents or legal guardians filled out a dietary record [30] about the consumption of food and beverages by the schoolchildren over 3 days (2 weekdays and 1 weekend day). The days chosen to collect the information were Thursday and Friday as weekdays and Sunday and weekend day. The questionnaire collected the amounts of food and beverages consumed and the time of each intake. In the case of lunch on weekdays that was not consumed with parents, the information collected was checked against the menus provided by school catering service. Once collected, the data were processed in DIAL software for nutritional assessment [31]. The results were focused on estimating the dietary water intake (coming from food and beverages, expressed in ml/day), the percentage of water coming from beverages, and the ml/day of different beverages consumed, from water as a beverage, sugary drinks, sweetened drinks, sports drinks, natural juices, commercial juices and nectars, milk and milkshakes. Specifically, water as a beverage is defined as water in the form of tap water, mineral water and sparkling mineral water. The dietary water intake classification was established on the basis of the Adequate Intake (AI) by the European Food Safety Authority (EFSA) [32], which is 1,600 ml/day for 4-8-year-old children, 1,900 ml/day for females 9 to 13 and 2,100 ml/day for males aged 9 to 13.

Urine testing

A 24-hour urine sample was collected on a weekend day, and it coincided with the dietary record. All of the participants received written instructions and individual containers for the correct collection of the sample. All micturitions were stored immediately in preservative-free, 1-L plastic containers at temperatures <-12°C before transfer to the laboratory. The volume as well as the urinary urea [33], sodium, potassium [34] and creatinine [35] were determined. To confirm appropriate collection of 24-h urine, the correlation between urinary levels of creatinine and muscular mass of each subject was taken into account. Fat-free mass was calculated bearing in mind the creatinine excreted over 24 h in urine using the following equation [36]:

The osmolality was calculated according to the following formula: Osmolality = [{sodium (mEq/l) + potassium (mEq/l)}*2 + urea (mg/dl)]/5.6 [37]. The children were considered to have adequate hydration (AH) when the osmolality was equal to or lower than 800 mOsm/kg in accordance with the criteria of other authors [38, 39, 40, 41, 42].

Statistical analysis

The data were analyzed using the statistical software IBM SPSS Inc. (version 21.0). Descriptive analysis of the values shows the mean ± standard deviation (m±sd). The Kolmogorov-Smirnov test was used to test the normality of the variables. Student's t-test was used to study the normal variables, and the Mann-Whitney U test was used for those that were not normal. For the categorical variables, the Z and Chi-Square proportion tests were used. The Bonferroni test was used to adjust the values in multiple comparisons. To evaluate the influence of the lifestyles, a 2-way ANOVA was performed on the urinary osmolality and different biochemical parameters, and the water dietary intake and beverages were explored with a post hoc Bonferroni analysis. The association of an individual’s sex, compliance with the practice of PA and sedentariness (independent variables) and risk of dehydration (dependent variable) were analyzed using a logistic regression analysis to calculate three models of the odds ratio (OR). In addition, the OR adjusted by sex and the OR adjusted by sex and other lifestyle factors were calculated. Statistical differences were assigned for a value of p <0.05.

Physical activity

In accounting for the PA performed by the schoolchildren, more than 60% met the recommendations for total PAs. This percentage falls within similar ranges to those obtained in the ANIBES Study [56] in the 9- to 12-year-old cohort (51.6%; of those, 61.1% were males and 37.9% were females). Males had higher adherence to the recommendations, spending more time performing active play and extracurricular physical activities. Other authors also found statistical differences in these types of PAs according to sex, although comparisons between studies in no easy become there may be cultural aspects that affect boys’ and girls’ PA differently. Ishii et al. [57] studied a sample of Japanese schoolchildren; they observed the same differences in the type of PAs performed and the leisure physical activities based on sex, which were lower in females than in males. Yamamoto-Kimura et al. [58] studied Mexican schoolchildren; their results showed that males practiced more extracurricular physical activities than females.

Other relevant aspects of the results associate compliance with the PA recommendation and being a boy with a higher risk of IH in the study sample. Males may have a higher risk because of their body composition, which can present differences even at these ages, with more body fat and less body water than females [59, 60].

However, adherence to PA guidelines may be a risk factor for IH if the water intake does not increase proportionally to the losses associated with higher PA (in the case of our study, there are no differences in the water intake depending on the lifestyle). Additionally, other socioeconomic factors such as accessibility to beverages may also be influence on the hydration status, so they have to be considered as other potential risks of IH.

As far as we know, the effects of compliance with the PA guidelines on the state of hydration in schoolchildren have not been evaluated. Similar data have been found in the PHYSMED study [61] conducted in a Spanish adult population aged 55 to 88 years old in which, similar to the results obtained in the present study, significant differences were observed in the plasma osmolality between the "sedentary inactive" and "non-sedentary active" groups.

These data highlight the importance of raising awareness about the hydration status in schoolchildren and their environment in which the primary source of hydration should be water as a beverage. It is especially important when it comes to implementing a strategy to promote the performance of PA, with nutritional education playing a key role. Cleary et al. [62] showed a clear example of this promotion; they evaluated a sample of female adolescents who played volleyball as an extracurricular sports activity. These girls had IH on average before and after sports practice. This situation reversed over the period of time (4 weeks) during which they were subjected to nutritional education in the field of hydration for sports. During this training period, the results showed a suitable improvement in the individual hydration state after PA practice due to correct fluid replacement. Unfortunately, once the training was completed, the adolescents again had IH levels before and after sports practice. Kavouras et al. [63] designed an intervention program to increase fluid intake in children who exercise with heat. In this study the children had free access to fluids and water accessibility was facilitated in training, dinning and resting areas. During the intervention period hydration status and performance in an endurance run was improved. In both cases, the fluid replacement regime was developed by the health professionals who were conducting the study due to the lack of standardized hydration guidelines adapted to different sports. In this sense, it is vitally important to bear in mind that post-exercise hydration recommendations may vary depending on the sport and intensity. These specificities are not included in the current hydration guidelines. This finding suggests that fundamental support for this group, in addition to training, should be a commitment by professionals involved in sports, health professionals and the family environment. On the other hand, it is interesting to highlight the need to implement other strategists as promoting water consumption in schools in order to encourage AH in this group [64, 65].

Sedentary behaviors

Our results indicate that sedentary behavior, unlike PA, did not increase the risk of IH, nor did it increase the risk of non-compliance with adequate water intake.

The lifestyle of the studied population included a low percentage of schoolchildren who complied with the sedentary guidelines. These results are consistent with those obtained by Rodríguez-Huertas et al. [66] in Spanish schoolchildren aged 6 to 12 years in which only 30% performed less than 2 hours a day of sedentary leisure activities and data from the ALADINO study [67], which showed that 37.3% of the population dedicates less than 1 hour, 33.3% dedicates approximately 1 hour, and 21.7% dedicates 2 or more hours to these sedentary activities. In addition, our results show that males commit significantly more of their time to PCs/video game consoles/TVs compared to females. De Jong et al. [68] have obtained similar results in their evaluation of the television and computer use habits of two school-aged population groups, from 4 to 8 years and from 9 to 13 years. Their results showed that computer use by this population increased as the age increased and with the presence of computers in the bedroom for males in both age groups.

Regarding the relationship between sedentariness and beverage consumption, it is very difficult to compare with other studies since some of them studied different sedentary behaviors and their relations to the consumption of drinks, but they do not show results for guideline compliance in general. The systematic review by Pearson et al. [69] concluded that television viewing was positively associated with the consumption of energy-dense beverages among other dietary patterns, both in schoolchildren and adolescents. In our country, the ALADINO Study [70] showed that more than 2 hours of screen time per day had a significant association with the increased frequency of consumption of smoothies, fresh fruit and sugary drinks.

It is of special relevance to note that these results may not be applicable to other subjects or population groups, but they provide an interesting line of research to evaluate the influence of lifestyle factors on hydration. Among the strengths of our study, we highlight the size of the study sample, which is a healthy school population, not athletes, and the collection of 24-h urine to determine the hydration status, which makes it possible to quantify both the diuresis in addition to the osmolality. However, self-reported data on PA and sedentary lifestyles are subject to error, as is the 3-day dietary record, and thus, there may have been problems in terms of the over/underestimation of some of the aspects studied here. In addition, the cross-sectional design of the study does not allow us to establish a cause-effect relationship between variables.