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⁻¹. 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⁻¹. 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⁻²). 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⁻¹ 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⁻¹ and TWI was 3.57 ± 1.10 L·d⁻¹ and 3.20 ± 1.27 L·d⁻¹ in men and women, respectively. ROC analysis for TWI detecting 24-h UOsm < 500 mmol·kg⁻¹ 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⁻¹, respectively. The AUC was 82.4% in women with sensitivity, specificity, and threshold values of 85.7%, 72.1%, and 2.61 L·d⁻¹. Conclusion: Considering threshold values in men and women of 3.4 L·d⁻¹ and 2.6 L·d⁻¹, respectively, maintaining TWI in line with National Academy of Medicine guidelines of 3.7 L·d⁻¹ in men and 2.7 L·d⁻¹ in women should be sufficient for most individuals in the United States to maintain 24-h UOsm < 500 mmol·kg⁻¹.