Background : Evidence on the hydration status of adolescents and seasonal and time variations is scarce. Aims : This study aimed to assess variations in the hydration status and total water intake of adolescents in Cyprus between winter and summer and between the morning and end of the school day. Subjective feelings and cognitive function associated with hypohydration were also examined. Methods : Fifty-three adolescents?(39?boys) with a mean age?(standard deviation, SD) of 15.1?(1.9) years were included in the study. Participants provided urine samples and completed a cognitive function test and a questionnaire on subjective feelings upon arrival at school and at the end of the school day. Data were collected in winter and summer in 2016–2017. Hydration level was determined by urine specific gravity and cognitive function was assessed with a symbol cancellation test. Total water intake was estimated from a food and fluid record kept by the participants. Results : A high prevalence of hypohydration was found in both seasons ranging from 72% to 94%. Hypohydration was associated with perceptions of lower concentration and alertness?(P?=?0.008 and P?=?0.047, respectively). Adolescents had significantly higher mean urine specific gravity in winter than in summer: 1.026?(SD?0.007) versus 1.023?(SD?0.007), respectively?(P?=?0.002). Hydration status was not associated with cognitive function or total water intake. Conclusions: The high prevalence of hypohydration and the associated feelings of lower concentration and alertness in adolescents suggests that educational measures are needed to promote good drinking behaviour and improve hydration.
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