Influence of Mild Dehydration From Occupational Work on Plasma Volume and Osmolality

Abstract

High intensity exercise changes the volume and concentration of fluid compartments. The effects of mild dehydration are not well understood because previous studies have focused on either severe dehydration or water intoxication. Repetitive lifting is a task performed in military, industrial, and construction settings; such work often is performed under a time constraint without an adequate water supply. PURPOSE: The objective was to examine percent change of plasma volume (%ÄPV) and osmolality during and after a repetitive box lifting (RBL) task. METHODS: Ten healthy male subjects (mean ± SD; age 21±2 y; body mass (BM), 77.8±10.6 kg; height, 177±4 cm; and body fat, 10.7±8.8%) were randomly assigned to: trial F (ad libitum fluid given during exercise) and NF (no fluid) using a crossover design. The environmental conditions were the same for both trials (23°C, 50% relative humidity). A morning blood sample was taken from a Teflon catheter placed in a forearm vein. Subjects then ate a standard breakfast followed by a 2-hour rest/exercise period (30 min rest and 10 min RBL) performed 3 times (120 min total). RBL consisted of self-paced lifting of a 20.5 kg box from the floor to a 1.32 m platform. RESULTS: The exercise was intense, as evidenced by maximal plasma lactate concentration (17.1±2.4 mmol·L-1) and heart rate (190±7 bpm) with no differences between trials. Dehydration was 0.95±0.17%BM and rectal temperature rose to 38.3±0.3oC with no differences between trials. The change of posture from reclining (morning blood draw) to standing for the rest/exercise period altered %ÄPV -10.2±3.8% with no differences between trials. Return to a reclining posture 30 min after the protocol significantly changed %ÄPV +22.7±5.6 and +19.1±4.2%, for F and NF respectively (P<0.05). For both trials, %ÄPV was significantly reduced by 9.7% from rest to after RBL1 and then increased by 4.4% from before RBL1 to after RBL3. Plasma osmolality increased 5.5±1.1% after each RBL and then returned to pre-exercise values during rest for both trials. CONCLUSIONS: Mild dehydration did not change plasma osmolality and %ÄPV during exercise, but it decreased %ÄPV after strenuous occupational work.