Seasonal changes in hydration in free-living Japanese children and adolescents.

Changes in hydration status occur throughout the day affecting physiological and behavioural functions. However, little is known about the hydration status of free-living Japanese children and the seasonality of this response. We evaluated hydration status estimated by urine osmolality (Uosm) in 349 children (189 boys and 160 girls, 9.5 ± 2.6 years, range: 6-15 years) upon waking at home and during a single school day in spring (April) and summer (July). Further, we assessed the efficacy of employing self-assessment of urine colour (UC; based on an 8-point scale) by children to monitor their hydration status. Early morning Uosm was greater in the spring (903 ± 220 mOsm L -1 ; n =  326) as compared to summer (800 ± 244 mOsm L -1 ; n =  125) ( P  = 0.003, paired t test, n =  104). No differences, however, were observed in Uosm during the school day ( P  = 0.417, paired t test, n =  32). While 66% and 50% of children were considered underhydrated (Uosm ≥ 800 mOsm L -1 ) upon waking in the spring and summer periods, respectively, more children were underhydrated (∼12%) during the school day. Self-reported UC was similar between seasons as assessed in the morning and school day ( P  ≥ 0.101, paired t test), which differed from the pattern of responses observed with Uosm. We showed that a significant number of Japanese children are likely underhydrated especially in the spring period. Children do not detect seasonal changes in hydration from self-assessed UC, limiting its utility to manage hydration status in children.