Background and aims: In the case of children, dehydration affects mobility and cognitive functioning at school. We assessed the morning hydration status in a group of schoolchildren aged 7-15 years. 
Methods: The study, conducted in spring 2018 among 331 healthy pupils from Niepołomice and Kraków, measured urine osmolality of two urine samples: the morning sample brought by the child from home and the sample taken at school. To assess nutritional status, BMI interpretations for a child’s sex and age by national percentiles charts were used. We present results regarding 314 pupils (average age 10.36+1.64 years, no statistical differences according to gender), including 175 boys and 139 girls, who gave their second urine sample. Statistical analyses were performed in the Statistica 13 PL program. 
Results: The nutritional status of children did not differ depending on gender and place of residence. 75.8% of all children had normal weight, 7.5% a body mass deficiency, 10.0% were overweight and 6.7% obese. The average osmolality level of the morning urine sample was 801.76 + 232.32 mOsm/kgH2O, and of the second sample 757.0 + 272.38 mOsm/kgH2O. More than a third of the children (36.6%) had urine osmolality above 800 mOsm/kgH2O, while 16.8% had urine osmolality over 1000 mOsm/kgH2O. 
Conclusions: Every second child had evidence of a hydration deficit during morning stay at school.  
The project received a positive opinion from the Bioethical Committee of the Jagiellonian University and was financed by the Polish State Committee for Scientific Research, Grant no K/ZDS/006175.

ASSESSMENT OF MORNING HYDRATION STATUS AMONG SCHOOLCHILDREN FROM MAŁOPOLSKA REGION (SOUTH OF POLAND)