Total water intake guidelines are sufficient for optimal hydration in United States adults.

Purpose: Recent studies suggest that 24-h urine osmolality (UOsm) for optimal water intake should be maintained < 500 mmol·kg. The purpose of this study was to determine the total water intake (TWI) requirement for healthy adults to maintain optimal hydration as indicated by 24-h urine osmolality < 500 mmol·kg.

Methods: Twenty-four-hour UOsm was assessed in 49 men and 50 women residing in the United States (age: 41 ± 14 y, body mass index: 26.3 ± 5.2 kg·m). TWI was assessed from 7-day water turnover, using a dilution of deuterium oxide, corrected for metabolic water production. The diagnostic accuracy of TWI to identify UOsm < 500 mmol·kg was evaluated using receiver operating characteristic (ROC) analysis in men and women separately.

Results: Twenty-four-hour UOsm was 482 ± 229 and 346 ± 182 mmol·kg and TWI was 3.57 ± 1.10 L·d and 3.20 ± 1.27 L·d in men and women, respectively. ROC analysis for TWI detecting 24-h UOsm < 500 mmol·kg in men yielded an area under the curve (AUC) of 77.4% with sensitivity, specificity, and threshold values of 83.3%, 64.5%, and 3.39 L·d, respectively. The AUC was 82.4% in women with sensitivity, specificity, and threshold values of 85.7%, 72.1%, and 2.61 L·d.

Conclusion: Considering threshold values in men and women of 3.4 L·d and 2.6 L·d, respectively, maintaining TWI in line with National Academy of Medicine guidelines of 3.7 L·d in men and 2.7 L·d in women should be sufficient for most individuals in the United States to maintain 24-h UOsm < 500 mmol·kg.