The objective of the study is to describe the hydration status and to assess the main food- and/or fluid intake-associated factors in healthy adolescents.
A total of 194 adolescents from the city of Zaragoza aged 12.5–17.5 years (99 males) participating in the 'Healthy Lifestyle in Europe by Nutrition in Adolescence' (HELENA) cross-sectional study were included. First morning urine was collected, and osmolality was determined by freezing point depression osmometer. A self-reported computer-based 24-h dietary recall was applied the same day of the urine collection. Analysis of variance, Kruskal–Wallis procedure or Pearson’s χ2 analyses was used to examine the group associations.
Seventy-one percent of adolescents did not meet the European Food Safety Agency (EFSA) recommendations for average total water intake (TWI), and 68% had high first morning urine osmolality values. TWI and the proportion of those meeting EFSA reference values significantly (P<0.05) decreased with increased osmolality. Males who met the EFSA recommendations consumed significantly (P<0.05) more plain water (1035.13 vs 582.68 ml) and dairy drinks (368.13 vs 226.68 ml) than those who did not. In females, the consumption of water (1359.41 vs 620.44 ml) and sugar-sweetened beverages (214.61 vs 127.42 ml) was significantly higher in those meeting the EFSA recommendations than in those who did not. First morning urine osmolality was associated with vegetables (unstandardized β: −0.6; 95% confidence interval (CI): −1.02 and −0.18) and fruits intake (β: −0.41; 95% CI: −0.63 and −0.19) in males and with dairy drinks (β: −0.39; 95% CI: −0.76 and −0.02) and fruits (β: −0.41; 95% CI: −0.73 and −0.10) in females.
There was a high prevalence of inadequate TWI and high urine osmolality among these Spanish adolescents. Lower urine osmolality was associated with higher consumption of vegetables in males, dairy drinks in females and fruits in both males and females.
Water is essential for all functions of the body.1 Total body water, as a percentage of body mass, varies as a function of body composition (from 50 to 70%).2 Water loss occurs constantly through the lungs, skin, kidneys and gastrointestinal tract,3 and these losses are compensated through metabolic water gain, food moisture and fluid intake.4 Dehydration can cause shrinkage of brain tissue, associated increase in ventricular volume and negative effects on cognitive performance,5 whereas insufficient fluid intake may reflect in metabolic cell stress, increased risk for urolithiasis and constipation or urinary tract infections, among others.6 Various international bodies have set dietary reference intake (DRI) for total water for young populations. However, not all DRIs have been set based on the same considerations. For children between ages of 4 and 13, the European Food Safety Authority (EFSA) bases its DRIs primarily on caloric intake, whereas for adolescents aged 14 through adulthood intake reference values are based on population median consumption and the achievement of a desirable urine osmolality. The current recommendations established by the EFSA are 2100 ml/day for males (9–13 years) and 1900 ml/day for females. Adolescents from 14 years and older are considered as adults with 2.5 l/day for males and 2.0 l/day for females.4 In contrast, the Institute of Medicine DRIs for the United States and Canada correspond to median intake observed in the National Health and Nutrition Examination Survey III for children (1–18 years) and adults. The current recommendation for children (9–13 years) established by the Institute of Medicine is 2400 and 2100 ml/day for males and females, respectively. For adolescents 14 years and older, adequate water intake is 3300 ml/day for males and 2300 ml/day for females.7 However, all health authorities highlight the necessity to establish the water intake recommendation based on the water balance.4
Little is known about water intake and hydration in childhood and adolescence. The available evidence suggests a high prevalence of insufficient water intake in some populations. A German (3–18 years) longitudinal study of children and adolescents reported a mean total water intake of 1642 and 1457 ml in 9- to 13-year-old boys and girls, respectively.6 This falls ~450 ml short of the corresponding EFSA reference values of 2100 ml (boys) and 1900 ml (girls).8 Similarly, intake data collected in two major American cities9 revealed that among 9- to 11-year-old children, median intake before arriving school was only 260–270 ml of total water (from food and fluids) and that only 25% reported drinking water in the morning. These observations were confirmed by a high prevalence (63–66%) of elevated morning urine osmolality (>800 mOsm/kg)9 suggestive of insufficient fluid intake. Urinary hydration biomarkers have been shown to better reflect fluid intake in comparison with serum biomarkers (which better reflect acute dehydration) and are associated with health outcomes such as risk for chronic kidney disease.10
The ‘Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) Cross-Sectional Study’ is a large, multicenter study that obtained food and fluid intake data from adolescents across multiple European countries, using a standardized dietary recall. In a previous report, Duffey et al.11 described mean fluid consumption and energy intake in adolescents from eight European cities. As first morning urine samples were only collected in the adolescents from Zaragoza, we were able to assess their morning hydration status by measuring first morning urine osmolality. The aims of the present study are to describe the hydration status (first morning urine osmolality) and fluid consumption in healthy adolescents from the city of Zaragoza (Spain) and to investigate the association between first morning urine osmolality and food and beverage intake.
Materials and methods
The HELENA Cross-Sectional Study obtained standardized, reliable and comparable data from European adolescents regarding nutrition and health-related parameters. From November 2006 to December 2007, field work took place in 10 European cities (Vienna, Ghent, Lille, Dortmund, Athens, Heraklion, Pecs, Rome, Zaragoza and Stockholm) from 9 countries. A random cluster sampling (all subjects from a selection of classes of all schools in the chosen cities) of 3000 adolescents (aged 13.0–16.99 years) was carried out and stratified by geographical location, age and socioeconomic status. We obtained data related to body composition, dietary intake, physical activity and laboratory information (blood and urine samples). The entire description of the HELENA Cross-Sectional Study was published previously.12
This analysis included only the sample from Zaragoza/Spain, one of the 10 centers involved in the HELENA Study. The initial Zaragoza sample (n=390; age 12.5–17.5 years) was calculated with a confidence level of 95% and ±0.3 error in the worst of situations for the parameter body mass index (BMI); the chosen factor because of the greatest dispersion in the studied population with regard to the problem that was studied.12 However, only 288 participants provided a first morning urine sample. Among these subjects, 194 also provided food and fluid intake data corresponding to the day before the urine collection. Thus, the final number of included adolescents for the current analysis consisted of 99 males and 95 females, representing 49.7% of the original Zaragoza sample. The study adhered to the ethical criteria of the Declaration of Helsinki, informed consent was obtained from all participants and their parents, and the protocol was approved by the Regional Human Research Review Committee of Aragón.13
First morning urine osmolality
We collected the first urine in the morning, during the school period (weekday), while adolescents were under fasting status. Midstream urine samples were collected by the adolescents at home, upon waking, and they brought it to the study center in the same morning. After preparing the aliquots needed for the originally planned analyses, remaining urine was frozen at −20 °C before being used for this study. Previous studies have shown that these procedures are valid to obtain reliable results.14 Urine osmolality was determined by freezing point depression osmometer (Model 3300 Micro-Osmometer; Advanced Instruments, Inc., Norwood, MA, USA). For the purposes of our analysis, hypohydration was defined as a first morning urine osmolality greater than cutoff 800 mOsm/kg. Although no singular consensus exists on the most appropriate cutoff for elevated urine osmolality, values approaching 800 mOsm/kg have been reported previously as the upper limit for euhydration.14, 15, 16
In the present analysis, only one 24-h dietary recall corresponding to food and fluid intake was applied on the day before the urine collection. A self-administered, computer-based, validated17 and culturally adapted18 HELENA-dietary assessment tool (HELENA-DIAT) was used. Participants were guided by the software to introduce their food and beverage consumption on six meal occasions (breakfast, mid-morning snack, lunch, mid-afternoon snack, dinner and after-dinner snack). In addition, the software supported and enhanced the respondent’s recall, by having a number of specific reminders to probe for beverages. The adolescents had the option to manually enter any food or beverage not included in the software. Guidance on quantities was provided through household measurements or pictures of portion sizes. Information collected by the HELENA-DIAT was linked afterward with the German Food Code and Nutrient Data Base (BundeslebensmittelschluSsel or BLS, Federal Research Centre for Organization: Nutrition and Food (BfEL), Karlsruhe, Germany), version II.3.1.,18 which has shown to reproduce accurate data on nutrient food composition in comparison with the corresponding national food composition databases.19
Beverage and food groups
A total of 1723 different food and beverage types obtained in the computerized 24-h recall were recoded into 26 food and 10 beverage categories. Some food groups were further aggregated, such as alcoholic beverages (for example, beer, wine and liquor), complex carbohydrates (for example, pasta, rice and flour), added sugar and other caloric sweeteners (for example, honey), oily fruits (for example, nuts and seeds, and avocado and olives) and some milk products (for example, desserts, creams and ice creams). Some other groups were not included in the analysis because of very low or nonexistent consumption (for example, products for special nutritional use, puddings or margarines and lipids of mixed origins).
Total water intake (TWI) was calculated as the total amount of water provided by fluids and foods. The German Food Code and Nutrient Data Base was applied to obtain the percentage of water contained in each specific food item. Total fluid intake (TFI) was calculated as the total volume of all beverages, including plain water. Beverages were regrouped into five broad categories: plain water (still or sparkling); alcohol (wine, beer, cider and liquors); beverages containing sugars or sugar-sweetened beverages (SSBs; soft drinks, fruit juices, infusions with added sugar and milk–fruit combined products); milk and milk products; and other drinks (infusions without added sugar, vegetable juices and diet soft drinks).20
Body mass and height were measured with an electronic scale (SECA 861) and with a telescopic stadiometer (SECA 225) to the nearest 0.1 kg and to the nearest 0.1 cm, respectively,21 and were used to calculate BMI. These measurements were taken on the same day of the urine collection, and the 24-h dietary recall was administered. Protein intake was also taken into consideration as a statistical covariate, as it determines the obligatory amount of water needed for the urinary excretion of solutes.4
PASW 17.0 for Windows (SPSS, Inc., IBM, Chicago, IL, USA) was used for all analyses. Characteristics of the study sample are presented as means (s.d.), unless otherwise stated. Baseline characteristics between the sexes were compared using Student’s t-test (for normally distributed variables) or the Mann–Whitney U-test (if the normality assumption was violated) for continuous variables. For group comparisons, analysis of variance, Kruskal–Wallis procedure (for non-normal variables) or Pearson’s χ2 analyses (for categorical variables) was used. All the analyses were stratified by sex. Tests for normality were performed using the Kolmogorov–Smirnov test. Associations between first morning urine osmolality and its potential determinants (age, BMI, protein intake and food and fluid intake) were tested with linear regression (unstandardized β coefficients and 95% confidence intervals). For this analysis, non-normal variables were logarithmically transformed. Two-sided significance levels are quoted at 0.05.
A total of 99 males (age: 14.4±1.2 years; BMI 21.2±3.5 kg/m2) and 95 females (age: 14.7±1.1 years; BMI 21.4±3.2 kg/m2) were included in the analysis. With regard to the overall HELENA/Zaragoza sample, there were no significant differences in demographic characteristics, except for age: the subsample in the current analysis was slightly younger than the overall sample (overall Zaragoza sample: age 15.4 years; P=.01). TWI was similar between males and females (1977±692 and 1801±784 ml/day, respectively). Seventy-two percent of males and 69% of females did not achieve sex-specific EFSA recommendations for TWI. TFI was also similar (1286±479 and 1236±602 ml/day). Mean first morning urine osmolality was above 800 mOsm/kg in both groups (882±230 and 840±232 mOsm/kg for males and females, respectively). The prevalence of elevated first morning urine osmolality (>800 mOsmol/kg) was 71% in males and 65% in females. Notably, males consumed significantly more protein than females (113±57 vs 79±46 g/day; P<0.001).
Table 1 describes trends in fluid and protein intake across quintiles of first morning urine osmolality. Among males, TWI (P=0.04) and the proportion of males meeting EFSA recommendations (P<0.01) tended to decrease as first morning urine osmolality increased. Among females, TWI (P=0.01), TFI (P=0.01), intake of plain water (P=0.04) and the proportion of females meeting EFSA recommendations (P=0.03) all decreased as first morning urine osmolality increased.
Reported beverage consumption by level of first morning urine osmolality
Figures 1a and b describe the contribution (in ml) of each beverage group to TFI of the participants by urine osmolality quintile in males and females, respectively. Plain water was the single largest contributor to TFI, followed by milk and milk products in most urine osmolality quintiles for both sexes. The exceptions were the fifth quintile in males, and the third and fourth quintiles in females, in which SSB consumption outweighed milk and milk product consumption. In addition, among females, plain water intake was significantly higher in the third quintile (mean: 1214.49 ml) than in higher quintiles (4th: mean, 692.63; 5th: mean, 679.61 ml).
Reported beverage consumption in those meeting the EFSA recommendations
Males meeting the EFSA recommendations for TWI consumed significantly more plain water (1035±540 vs 583±378 ml; P<0.00), milk and milk products (368±279 vs 227±142.89 ml; P=0.01) than those who did not meet the EFSA recommendation (Figure 2a). In females, the consumption of plain water (1359±747 vs 620±295 ml; P<0.00) and SSBs (215±235 vs 127±158 ml; P=0.03) was significantly higher among those who met the EFSA recommendations than in ones who did not meet these recommendations (Figure 2b).
First morning urine osmolality and EFSA recommendations
First morning urine osmolality was lower in females meeting EFSA dietary reference values for TWI (mean: 770±237 mOsm/kg) than in those who did not meet EFSA reference values (mean: 876±223 mOsm/kg; P=0.034). The difference between males meeting and not meeting EFSA dietary reference values did not reach statistical significance (829±232 vs 905±227 mOsm/kg, P=0.13).
Associations between first morning urine osmolality, and food and beverage consumption
Table 2 describes the associations between first morning urine osmolality and different contributors of TWI (beverages and some water-rich foods), adjusted by age, protein intake and BMI, in both sexes. Significant inverse relationships were found between first morning urine osmolality and vegetables (excluding potatoes) and fruits in males and with dairy drinks and fruits in females. Lower first morning urine osmolality was associated with higher consumption of these foods and beverages.
The study was motivated by recent reports9, 22 of high prevalence of elevated urine osmolality and hyperosmotic stress among children and adolescents. The results from the current analysis confirm previous findings, as a high proportion of the adolescents in the HELENA-Zaragoza sample consumed less than the EFSA recommendations for total water (71% fell below the age- and sex-specific dietary reference values), and 68% had elevated first morning urine osmolality (>800 mOsm/kg).
Compared with the HELENA sample as a whole,11 adolescents in the Zaragoza subsample reported lower total fluid intake (1455 vs 1262 ml/day) than those adolescents belonging to the other cities. From all sources of fluid intake, plain water was the largest contributor to TFI among adolescents, representing 54% and 65% of total fluid intake in males and females, respectively. Slightly lower values were observed in the entire HELENA sample;11 however, girls also reported a higher percentage of plain water (55% of total fluid intake) than boys (45%). One difference between the HELENA sample as a whole and the Zaragoza subsample was that the second largest contributor to total fluid intake in Zaragoza was milk and milk products. This is in contrast with the entire HELENA sample, in which fruit juices were the second largest contributor to TFI.11 This suggests an important cultural component to fluid intake among adolescents, which can be observed in previous studies. In France (12–19 years), plain water and dairy drinks were also recently reported as the first and the second main contributors to TFI, although total fluid intake was lower than in our sample.23 In a longitudinal study of German adolescents (the DONALD study),24 when considering a period of 5 years, regular soft drinks and fruit juices accounted for the second highest proportion of TFI after plain water. Similarly, in a Brazilian study measuring youths from 3 to 17 years,25 plain water intake was followed by soft drinks and dairy drinks. Although the second and third sources of fluid intake may differ, a common result to all of these studies is that plain water remained the largest contributor to total fluid intake.
In our sample, there was a high prevalence of adolescents who did not meet the EFSA dietary reference values for total water (72% of males and 69% of females). This finding was substantiated by the high prevalence of elevated first morning urine osmolality (>800 mOsmol/kg) observed in 71% of males and 65% of females. The aggregate of insufficient intake plus elevated urine osmolality suggests that a majority of adolescents experience hyperosmotic stress on cells during the morning hours. It is important to note, however, that urine concentration is subject to circadian variation (cite http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3714557/), and thus first morning urine osmolality is not representative of 24-h hydration. However, despite this limitation, our findings are consistent with previous studies conducted in Israel (10–12.4 years) and in the United States (9–11 years),9, 22 suggesting that elevated urine osmolality in adolescents is prevalent across multiple countries with different drinking habits. It is interesting to note that, in our study, adolescents who did not meet the EFSA recommendations for TWI had a low consumption of plain water. In addition, as plain water is the largest driver of total fluid intake, consumption of plain water was significantly lower in the highest urine osmolality quintiles in females from our study. This is in line with results from Stookey et al.,9 who showed that the likelihood of elevated urine osmolality was two times higher in children (9–11 years) who do not drink plain water in the morning, even if they consumed other beverages. Together, these findings seem to underscore the importance of plain water to drive increased intake and lower urine osmolality.
We also observed some sex differences. On average, despite consuming slightly more total fluid, males had higher first morning urine osmolality than females, as well as higher protein intake. This is consistent with previous observations (cite DONALD study) and suggests that sex differences in urine concentration begin in childhood.
Studies on the effect of food intake on hydration status are scarce. To our knowledge, this is one of the first studies to quantify the association between food and beverage intake and hydration status, using first morning urine osmolality in healthy adolescents. In this study, fruit intake was inversely associated with urine osmolality (males and females), as well as vegetable consumption in males and dairy drink intake in females. These results are in line with a recent published study26 based on the DONALD ongoing open cohort in children (4–10 year old), among others.27, 28 As occurs with the DONALD cohort,26 our results are opposite to those that hypothesized that when there is a decrease in water intake the consumption of fruits and vegetables is high for compensation. There are few studies6, 26, 29 available relating diet with urinary markers to assess hydration, which suggests that hydration biomarkers were determined through the quality of the diet.
One disadvantage of our study was that only first morning urine samples (not 24-h samples) were available for analysis. As sleeping essentially reflects an overnight water restriction, the relation between first morning urine osmolality and TFI is weak.30 This is also reflected in our study, in which a strong relation between TFI and urine osmolality was not established. However, in a study conducted in southern Israel14 with school-aged children (10–12.4 years), 81% of children who were classified as hypohydrated in the morning (based on first morning urine osmolality) remained hypohydrated at noon. Thus, despite the fact that a single morning urine sample is not the best biomarker to reflect the ‘usual’ day-to-day status of an individual at the population level, it could be considered representative of the ‘usual’ prevalence of elevated urine osmolality at that time of the day.9 Moreover, the urine sample that subjects collected represented midstream urine and not the entire void. Although we may assume that midstream urine concentration would be representative of the concentration of the entire void, this assumption was not tested. Another limitation is a possible lack of representativeness of the sample from Zaragoza, as only about half of the Zaragoza sample had all data points appropriate for this analysis. Because of its cross-sectional nature, we can not draw conclusions about cause-and-effect chains.
A major advantage of this study is that procedures and measurements were validated, standardized and reliable.12 Moreover, it is one of the first studies evaluating the fluid consumption of adolescents in basal conditions.
The main result of this analysis was the observation of low total water intake of this sample with Spanish adolescents (72% of males and 69% of females did not achieve the EFSA dietary reference values) and the correspondingly high urine osmolality values (71% of males and 65% of females had first morning urine osmolality above 800 mOsm/kg), which lend support to the fluid intake reported using the dietary recall. In this sample, the largest contributor to total fluid intake was plain water, representing over half of the total intake for both boys and girls, followed by dairy drinks and SSBs.
Another important contribution of this study is that for the second time in literature,26 positive associations between hydration and vegetable and fruit consumption were described. However, more research is needed to better evaluate these relationships, and, ideally, 24-h urine osmolality should be used. Future research should focus on relationships between fluid intake, hydration status and health outcomes.
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The analysis was supported by a grant from Danone Research. We acknowledge Alexis Klein and Erica Perrier for their help in interpreting the results and reviewing the manuscript. We acknowledge all the adolescents who made possible the HELENA study with their participation.
The authors declare no conflict of interest.
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Iglesia, I., Santaliestra-Pasías, A., Bel-Serrat, S. et al. Fluid consumption, total water intake and first morning urine osmolality in Spanish adolescents from Zaragoza: data from the HELENA study. Eur J Clin Nutr 70, 541–547 (2016). https://doi.org/10.1038/ejcn.2015.203
European Journal of Clinical Nutrition (2020)