Temperature of water ingested before exercise alters the onset of physiological heat loss responses

This study sought to determine whether the temperature of water ingested before exercise alters the onset threshold and subsequent thermosensitivity of local vasomotor and sudomotor responses after exercise begins. Twenty men [24 (SD 4) yr of age, 75.8 (SD 8.1) kg body mass, 52.3 (SD 7.7) ml.min(-1).kg(-1) peak O-2 consumption (<(V)over dot>(O2peak))] ingested 1.5 degrees C, 37 degrees C, or 50 degrees C water (3.2 ml/kg), rested for 5 min, and then cycled at 50% <(V)over dot>(O2peak) for 15 min at 23.0 (SD 0.9)degrees C and 32 (SD 10) % relative humidity. Mean body temperature (T-b), local sweat rate (LSR), and skin blood flow (SBF) were measured. In a subset of eight men [25 (SD 5) yr of age, 78.6 (SD 8.3) kg body mass, 48.9 (SD 11.1) ml.min(-1).kg(-1) <(V)over dot>(O2peak)], blood pressure was measured and cutaneous vascular conductance (CVC) was determined. The change in T-b was greater at the onset of LSR measurement with ingestion of 1.5 degrees C than 50 degrees C water [Delta T-b = 0.19 (SD 0.15) vs. 0.11 (SD 0.12)degrees C, P = 0.04], but not 37 degrees C water [Delta T-b = 0.14 (SD 0.14)degrees C, P = 0.23], but did not differ between trials for SBF measurement [Delta T-b = 0.18 (SD 0.15)degrees C, 0.11 (SD 0.13)degrees C, and 0.09 (SD 0.09)degrees C with 1.5 degrees C, 37 degrees C, and 50 degrees C water, respectively, P = 0.07]. Conversely, the thermosensitivity of LSR and SBF was not different [LSR = 1.11 (SD 0.75), 1.11 (SD 0.75), and 1.34 (SD 1.11) mg.min(-1).cm(-2) . degrees C-1 with 1.5 degrees C, 37 degrees C, and 50 degrees C ingested water, respectively (P = 0.46); SBF = 717 (SD 882), 517 (SD 606), and 857 (SD 904) % baseline arbitrary units (AU)/degrees C with 1.5 degrees C, 37 degrees C, and 50 degrees C ingested water, respectively (P = 0.95)]. After 15 min of exercise, LSR and SBF were greater with ingestion of 50 degrees C than 1.5 degrees C water [LSR = 0.40 (SD 0.17) vs. 0.31 (SD 0.19) mg.min(-1).cm(-2) (P = 0.02); SBF = 407 (SD 149) vs. 279 (SD 117) % baseline AU (P < 0.001)], but not 37 degrees C water [LSR = 0.50 (SD 0.22) mg.min(-1).cm(-2); SBF = 324 (SD 169) %baseline AU]. CVC was statistically unaffected [275 (SD 81), 340 (SD 114), and 384 (SD 160) %baseline CVC with 1.5 degrees C, 37 degrees C, and 50 degrees C ingested water, respectively, P = 0.30]. Collectively, these results support the concept that visceral thermoreceptors modify the central drive for thermoeffector responses.