Identifying 24-h Urine Osmolality Thresholds for Adequate Water Intake in U.S. Children

Validated pediatric reference values for hydration biomarkers remain limited. This study aimed to identify 24-h urine osmolality (UOsm) thresholds corresponding to the reference water intake values for children as defined by National Academy of Medicine (NAM) guidelines.

In this cross-sectional study, 729 healthy children (3-13 y; 46% female) from Arkansas and Arizona completed two-day dietary records and provided 24-h urine samples. We used logistic regression, receiver operating characteristic (ROC) analyses, and relative risk (RR) analyses to identify optimal UOsm cutoffs for adequate water intake and compare them with previously used candidate thresholds (<500 and <800 mmol/kg).

More than half of children (58%) did not meet NAM water intake reference values. Children meeting the recommendations consumed more plain water than those who did not (P<0.05). Among children aged ≥4 years, not meeting the recommendations was consistently associated with higher UOsm, darker urine, and lower urine volume (all P<0.05). ROC analysis suggested an optimal UOsm cutoff of 595 mmol/kg (AUC=61%; sensitivity=0.72; specificity=0.45; P<0.001). After adjusting for age, sex, body mass index z-score, and physical activity, children with UOsm >600 mmol/kg had a 39% higher risk of not meeting NAM recommendations. Logistic regression revealed that children with UOsm <500 mmol/kg and 500-600 mmol/kg had significantly higher odds of meeting NAM guidelines than those with UOsm >800 mmol/kg (OR: 3.02 and 1.84, respectively; P<0.05).

These findings suggest using approximately 600 mmol/kg as a clinically meaningful 24-h UOsm cutoff for identifying adequate water intake in children. This is slightly higher than the traditional 500 mmol/kg threshold suggested for adults.