Hypohydration and thermoregulation in cold air.

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 25 degrees C, then for 120 min at 7 degrees C, 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 furosemide (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 +/- 1 vs. 284 +/- 1 mosmol/kgH2O) and plasma volume reduction (-8 +/- 2 vs. -18 +/- 3%). Heat debt (349 +/- 14 among) did not differ (P > 0.05) among trials. Mean skin temperature decreased throughout cold exposure during Eu but plateaued after 90 min during HH and IH. Forearm-finger temperature gradient tended (P = 0.06) to be greater during Eu (10.0 +/- 0.7 degrees C) than during HH or IH (8.9 +/- 0.7 degrees C). This suggests weaker vasoconstrictor tone during hypohydration than during Eu. Final mean skin temperature was higher for HH than for Eu or IH (23.5 +/- 0.3, 22.6 +/- 0.4, and 22.9 +/- 0.3 degrees C, respectively), and insulation was lower on HH than on IH (0.13 +/- 0.01 vs. 0.15 +/- 0.01 degree C.W-1.m-2, respectively), but not with Eu (0.14 +/- 0.01 degree C.W-1.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.