The effect of voluntary dehydration on cognitive functions of elementary school children

Aims: (1) To describe the occurrence of voluntary dehydration in two classes of elementary school students as expressed by their morning and noon-time urine osmolality; and (2) to determine the relationship between the children's scores on cognitive tests and their state of hydration. Methods: Group comparison among fifty-eight sixth-grade students (age range 10.1-12.4 y old) during mid-June at two schools in a desert town. Morning and noon-time urine samples were collected in school, and five cognitive tests were scored in the morning and at noon-time. Main outcome measures: (1) morning and noon-time urine osmolality; (2) scores of five cognitive tests (hidden figures, auditory number span, making groups, verbal analogies, and number addition) that were applied in the morning and at noon-time. Results: Thirty-two students were dehydrated (urine osmolality above 800 mosm/kg H2O) in the morning. An individual's noon-time urine osmolality was highly related to morning osmolality (r= 0.67, p= 0.000). The morning cognitive scores were similar in the hydrated and dehydrated students (p= 0.443). The adjusted mean scores of the noon-time tests, with the morning test scores as covariates, demonstrated an overall positive trend in four of the five tests in favor of the hydrated group (p= 0.025). The effect was mainly due to the auditory number span test (p= 0.024). Conclusion: Voluntary dehydration is a common phenomenon in school-aged children that adversely affects cognitive functions.