Effects of obesity and mild hypohydration on local sweating and cutaneous vascular responses during passive heat stress in females

The purpose of this study was to evaluate the effect of obesity and mild hypohydration on local sweating (LSR) and cutaneous vascular conductance (CVC) responses during passive heat stress in females. Thirteen obese (age, 24 +/- 4 years; 45.4% +/- 5.2% body fat) and 12 nonobese (age, 22 +/- 2 years; 25.1% +/- 3.9% body fat) females were passively heated (1.0 degrees C rectal temperature increase) while either euhydrated (EUHY) or mildly hypohydrated (HYPO; via fluid restriction). Chest and forearm LSR (ventilated capsule) and CVC (Laser Doppler flowmetry) onset, sensitivity, and plateau/steady state were recorded as mean body temperature increased (Delta T-b). Participants began trials EUHY (urine specific gravity, U-sg = 1.009 +/- 0.006) or HYPO (U-sg = 1.025 +/- 0.004; p < 0.05), and remained EUHY or HYPO. Independent of obesity, HYPO decreased sweat sensitivity at the chest (HYPO = 0.79 +/- 0.35, EUHY = 0.95 +/- 0.39 Delta mg center dot min(-1)center dot cm(-2)/degrees C Delta T-b) and forearm (HYPO = 0.82 +/- 0.39, EUHY = 1.06 +/- 0.34 Delta mg center dot min(-1)center dot cm(-2)/degrees C Delta T-b); forearm LSR plateau was also decreased (HYPO = 0.66 +/- 0.19, EUHY = 0.78 +/- 0.23 mg center dot min(-1)center dot cm(-2); all p < 0.05). Overall, obese females had lower chest-sweat sensitivity (0.72 +/- 0.35 vs. 1.01 +/- 0.33 Delta mg center dot min(-1)center dot cm(-2)/degrees C Delta T-b) and plateau (0.55 +/- 0.27 vs. 0.80 +/- 0.25 mg center dot min(-1)center dot cm(-2); p < 0.05). While hypohydrated, obese females had a lower chest LSR (p < 0.05) versus nonobese females midway (0.45 +/- 0.26 vs. 0.73 +/- 0.23 mg center dot min(-1)center dot cm(-2)) and at the end (0.53 +/- 0.27 vs. 0.81 +/- 0.24 mg center dot min(-1)center dot cm(-2)) of heating. Furthermore, HYPO (relative to the EUHY trials) led to a greater decrease in CVC sensitivity in obese (-28 +/- 27 Delta% maximal CVC/degrees C Delta T-b) versus nonobese females (+9.2 +/- 33 Delta% maximal CVC/degrees C Delta T-b; p < 0.05). In conclusion, mild hypohydration impairs females' sweating responses during passive heat stress, and this effect is exacerbated when obese.