Body Composition and a School Day Hydration State among Polish Children-A Cross-Sectional Study

Background: Little is known on the relationship between obesity and hydration level in children. To explore the possible association between children's hydration status and body composition, we conducted this cross-sectional study. Methods: The survey was carried out in 2018 in Preliminary and High Schools from the Malopolska Province, Poland. The study group consisted of 264 children aged 7-15 years. The level of hydration was assessed based on urine osmolality during a school day. The examined had anthropometric tests and body composition assessment (FM, BF%, FFM, TBW, TBW%). Odds ratio (OR) and 95% confidence interval (CI) were calculated using a logistic regression analysis. Results: In the study group, 9.5% of the examined were overweight, 7.2% obese, and it referred more to the country than towns (p < 0.05). Improper hydration was found in 53% of children, and 16.3% of them were severely dehydrated during a school day (urine osmolality > 1000 mOsm/kgH(2)O). The level of dehydration was higher in children with excessive body fat (BF%) than in children with normal BF% [903.00 vs. 775 mOsm/kgH(2)O]. Older age (>10 y) showed inverse association with dehydration [OR 0.52 (95% CI; 0.28-0.99)] and excessed BF% showed 2.3-fold increase in odds of dehydration during a school day [OR 2.39 (95% CI; 1.15-4.94)]. Improper hydration was a risk factor of difficulties with concentration declared by students during a school day OR 2.85 (95% CI; 1.16-6.99). Conclusions: Attention should be paid to appropriate hydration especially in children with excessive body fat content who feature a higher risk of dehydration and fluid demand.