Hypohydration and Thermoregulation in Cold Air.

Hypohydration and thermoregulation in cold air. J. App 1. Physiol. 841 185-189, 1998.This study examined the effects of hypohydration on thermoregulation during cold exposure. In addition, the independent influences of hypohydration-associated hypertonicity and hypovolemia were investigated. Nine male volunteers were monitored for 30 min at 250C, then for 120 min at 70C, under three counterbalanced conditions euhydration Eu, hypertonic hypohydration HH, and isotonic hypohydration IH. Hypohydration was achieved 12 h before cold exposure by inducing sweating HH or by ingestion of flirosemide IH. Body weight decrease 4.1 0.2 caused by hypohydration was similar for HH and IH, but differences P 0.05 were found between HH and IH in plasma osmolality 292 t 1 vs. 284 1 mosmolkgH2O and plasma volume reduction -8 2 vs. -1813. Heat debt 349 1 14 among did not differ P 0.05 among trials. Mean skin temperature decreased throughout cold exposure during Eu but plateaued alter 90 min during HH and IH. Forearm- finger temperature gradient tended P 0.06 to be greater during Eu 10.0 1 0.70C than during HH or III 8.9 1 0.70C. This suggests weaker vasoconstrictor tone during hypohydra- tion than during Eu. Final mean skin temperature was higher for HH than for Eu or IH 23.5 1 0.3, 22 6 0 4 and 22.9 1 0.30C, respectively, and insulation was lower on HH than on IH 0.13 1 0.01 vs. 0.15 1 0.01C.W-1.m-2 respec tively, but not with Eu 0.H 1 0.010C.W-.m-2. This provides some evidence that hypertonicity impairs the vasoconstrictor response to cold. Although mild hypohydration did not affect body heat balance during 2-h whole body exposure to moderate cold, hypohydration-associated hypertonicity may have subtle effects on vasoconstriction that could become important during a more severe cold exposure.