• Lawrence E Armstrong, Evan C Johnson, Amy L McKenzie, and Colleen X Muñoz.
• University of Connecticut, Department of Kinesiology, Human Performance Laboratory, Storrs, Connecticut, USA; University of Connecticut, Department of Nutritional Sciences, Storrs, Connecticut, USA. Electronic address: lawrence.armstrong@uconn.edu.
• Nutrition. 2016 Jan 1; 32 (1): 79-82.

ObjectivesThe physiological regulation of total body water and fluid concentrations is complex and dynamic. The human daily water requirement varies because of differences in body size, dietary solute load, exercise, and activities. Although chronically concentrated urine increases the risk of renal diseases, an empirical method to determine inadequate daily water consumption has not been described for any demographic group; instead, statistical analyses are applied to estimate nutritional guidelines (i.e., adequate intake). This investigation describes a novel empirical method to determine the 24-h total fluid intake (TFI; TFI = water + beverages + moisture in food) and 24-h urine volume, which correspond to inadequate 24-h water intake (defined as urine osmolality of 800 mOsm/kg; U800).MethodsHealthy young women (mean ± standard deviation; age, 20 ± 2 y, mass, 60.8 ± 11.7 kg; n = 28) were observed for 7 consecutive days. A 24-h urine sample was analyzed for volume and osmolality. Diet records were analyzed to determine 24-h TFI.ResultsFor these 28 healthy young women, the U800 corresponded to a TFI ≥2.4 L/d (≥39 mL/kg/d) and a urine volume ≥1.3 L/d.ConclusionsThe U800 method could be employed to empirically determine 24-h TFI and 24-h urine volumes that correspond to inadequate water intake in diverse demographic groups, residents of specific geographic regions, and individuals who consume specialized diets or experience large daily water turnover. Because laboratory expertise and instrumentation are required, this technique provides greatest value in research and clinical settings.Copyright © 2016 Elsevier Inc. All rights reserved.

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