The temperature of water ingested before exercise alters the onset of physiological heat loss responses.

This study sought to determine whether the ingested water temperature prior to exercise alters the onset threshold and subsequent thermosensitivity of local vasomotor and sudomotor responses after exercise begins. Twenty males (age: 24±4 y, weight: 75.8±8.1 kg, VO2peak : 52.3±7.7 mL·kg-1 ·min-1 ) ingested 3.2 ml·kg-1 of 1.5°C, 37°C, or 50ºC water, rested 5 min, then cycled at 50% VO2peak for 15 min at 23.0±0.9°C and 32±10%RH. Mean body temperature (Tb ), local sweat rate (LSR) and skin blood flow (SBF) were measured. In a subset of 8 people (age: 25±5 y, weight: 78.6±8.3 kg, VO2peak : 48.9±11.1 mL·kg-1 ·min-1 ), blood pressure was measured and cutaneous vascular conductance (CVC) was determined. The change in Tb was greater at the onset of LSR with 1.5ºC (0.19±0.15ºC) compared to 50ºC (LSR: 0.11±0.12ºC; P=0.04) but not 37ºC (0.14±0.14ºC, P=0.23) water ingestion, but did not differ between trials for SBF (1.5ºC: 0.18±0.15ºC, 37ºC: 0.11±0.13ºC, 50ºC: 0.09±0.09ºC; P=0.07). Conversely, the thermosensitivity of LSR and SBF were not different (LSR: 1.5ºC: 1.11±0.75 mgžmin-1 žcm-2 ž°C-1 , 37 ºC: 1.11±0.75 mgžmin-1 žcm-2 ž°C-1 , 50ºC: 1.34±1.11 mgžmin-1 žcm-2 ž°C-1 ; P=0.46; SBF: 1.5ºC: 717±882%baselineAUž°C-1 , 37ºC: 517±606%baselineAUž°C-1 , 50ºC: 857±904%baselineAUž°C-1 ; P=0.95). After 15 min of exercise, LSR and SBF were greater with 50ºC (LSR: 0.40±0.17 mgžmin-1 žcm-2 , SBF: 407±149%baselineAU,) compared to 1.5°C water ingestion (LSR: 0.31±0.19 mgžmin-1 žcm-2 ; P=0.02; SBF: 279±117%baselineAU; P<0.001), but not 37ºC (LSR: 0.50±0.22 mgžmin-1 žcm-2 , SBF: 324±169%baselineAU). CVC was statistically unaffected (1.5°C:275±81%baselineCVC, 37°C:340±114%baselineCVC, 50°C:384±160%baselineCVC; P=0.30). Collectively these results support the concept of visceral thermoreceptors modifying the central drive for thermoeffector responses.