Urinary indices during dehydration, exercise, and rehydration

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-osm, 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 (orbetter 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.