Human Hydration Indices: Acute and Longitudinal Reference Values

TLDR

Hydration status is difficult to describe because free‑living individuals’ fluid intake and excretion patterns are poorly documented and water balance regulation is complex and dynamic. The study aimed to provide reference values for euhydration and compare urinary indices between first‑morning samples and 24‑hour collections. The authors observed 59 healthy, active men over 12 days, recording detailed daily food, fluid intake, and exercise. Results showed that healthy, active men consumed >2.1 L of fluid per day, produced >1.3 L of urine, had urine specific gravity >1.018 and color ≥4, rarely reached SG <1.010 or color 1, and that first‑morning urine was more concentrated and strongly correlated with osmolality; these data establish euhydration reference values and extremes for seven common indices.

Abstract

It is difficult to describe hydration status and hydration extremes because fluid intakes and excretion patterns of free-living individuals are poorly documented and regulation of human water balance is complex and dynamic. This investigation provided reference values for euhydration (i.e., body mass, daily fluid intake, serum osmolality; M ± SD ); it also compared urinary indices in initial morning samples and 24-hr collections. Five observations of 59 healthy, active men (age 22 ± 3 yr, body mass 75.1 ± 7.9 kg) occurred during a 12-d period. Participants maintained detailed records of daily food and fluid intake and exercise. Results indicated that the mean total fluid intake in beverages, pure water, and solid foods was &gt;2.1 L/24 hr (range 1.382–3.261, 95% confidence interval 0.970–3.778 L/24 hr); mean urine volume was &gt;1.3 L/24 hr (0.875–2.250 and 0.675–3.000 L/24 hr); mean urine specific gravity was &gt;1.018 (1.011–1.027 and 1.009–1.030); and mean urine color was ≥4 (4–6 and 2–7). However, these men rarely (0–2% of measurements) achieved a urine specific gravity below 1.010 or color of 1. The first morning urine sample was more concentrated than the 24-h urine collection, likely because fluids were not consumed overnight. Furthermore, urine specific gravity and osmolality were strongly correlated ( r 2 = .81–.91, p &lt; .001) in both morning and 24-hr collections. These findings provide euhydration reference values and hydration extremes for 7 commonly used indices in free-living, healthy, active men who were not exercising in a hot environment or training strenuously.