Twenty-four-hour urinary sodium and potassium excretion and associated factors in Japanese secondary school students

Data on the sodium and potassium intake using dietary records among schoolchildren are sorely lacking in the Japanese literature. Some evidence indicates that sodium and potassium intake has been correctly measured, but information concerning these associated factors is scarce. The 24-h urine samples and first morning voiding (overnight) samples were collected twice from 68 secondary schoolchildren in Suo-Oshima Town, Japan. Sodium, potassium and creatinine concentrations were analyzed. Body height and weight were measured, and menstruation and physical activity were assessed via questionnaires. We analyzed the 24-h samples with a >20-h collecting period and no missed voiding. The 24-h sodium excretion was 163.2±36.8 and 149.8±45.1 mmol per 24 h for the boys and girls, respectively. Considering daily habits and loss from sweat, intake was assumed to be 10.6±1.2 and 10.0±2.4 g per day for the boys and girls, respectively. The 24-h potassium excretion was 43.4±10.8 and 45.8±14.4 mmol per 24 h for the boys and girls, respectively. Estimated usual potassium intake was 2195±401 and 2330±630 mg per day for the boys and girls, respectively. Sodium excretion was associated with sodium and potassium concentrations in overnight urine samples and physical activity. Potassium excretion was associated with height and physical activity. We described daily sodium and potassium excretion in Japanese secondary schoolchildren. Excretion was associated more with physical activity than with bodyweight. Therefore, the estimation methods used in adults are not applicable for use in adolescents.