Urinary Indices during Dehydration, Exercise, and Rehydration

TLDR

This study assessed whether urine color, specific gravity, and osmolality reliably indicate hydration status by comparing them to body water changes. Nine highly trained men completed a 42‑hour protocol of dehydration, exhaustive cycling, and rehydration while urine color, specific gravity, osmolality, and plasma osmolality were measured. Urine color tracked body water changes as well as or better than urine osmolality, specific gravity, urine volume, plasma osmolality, sodium, and total protein, confirming these measures are valid and sensitive indices of hydration status even after marked dehydration, exercise, and rehydration.

Abstract

This investigation evaluated the validity and sensitivity of urine color (U col ), specific gravity (U sg ), and osmolality (U osm ) as indices of hydration status, by comparing them to changes in body water. Nine highly trained males underwent a 42-hr protocol involving dehydration to 3.7% of body mass (Day 1, −2.64 kg), cycling to exhaustion (Day 2, −5.2% of body mass, −3.68 kg), and oral rehydration for 21 hr. The ranges of mean (across time) blood and urine values were U col , 1-7; U sg , 1.004-1.029; U 08m , 117-1,081 mOsm • kg −1 ; and plasma osmolality (P osm ), 280-298 mOsm ⋅ kg −1 . Urine color tracked changes in body water as effectively as (or better than) U osm , U sg , urine volume, P osm , plasma sodium, and plasma total protein. We concluded that (a) U col , U osm , and U sg are valid indices of hydration status, and (b) marked dehydration, exercise, and rehydration had little effect on the validity and sensitivity of these indices.