A comparison of thermoregulatory responses to exercise between mass-matched groups with large differences in body fat

We sought to determine 1) the influence of adiposity on thermoregulatory responses independently of the confounding biophysical factors of body mass and metabolic heat production (H-prod); and 2) whether differences in adiposity should be accounted for by prescribing an exercise intensity eliciting a fixed H-prod per kilogram of lean body mass (LBM). Nine low (LO-BF) and nine high (HI-BF) body fat males matched in pairs for total body mass (TBM; LO-BF: 88.7 +/- 8.4 kg, HI-BF: 90.1 +/- 7.9 kg; P = 0.72), but with distinctly different percentage body fat (% BF; LO-BF: 10.8 +/- 3.6%; HI-BF: 32.0 = 5.6%; P < 0.001), cycled for 60 min at 28.1 +/- 0.2 degrees C, 26 +/- 8% relative humidity (RH), at a target H-prod of 1) 550 W (FHP trial) and 2) 7.5 W/kg LBM (LBM trial). Changes in rectal temperature (Delta T-re) and local sweat rate (LSR) were measured continuously while whole body sweat loss (WBSL) and net heat loss (H-loss) were estimated over 60 min. In the FHP trial, Delta T-re (LO-BF: 0.66 +/- 0.21 degrees C, HI-BF: 0.87 +/- 0.18 degrees C; P = 0.02) was greater in HI-BF, whereas mean LSR (LO-BF 0.52 +/- 0.19, HI-BF 0.43 +/- 0.15 mg.cm(-2).min(-1); P = 0.19), WBSL (LO-BF 586 +/- 82 ml, HI-BF 559 +/- 75 ml; P = 0.47) and H-loss (LO-BF 1,867 +/- 208 kJ, HI-BF 1,826 +/- 224 kJ; P = 0.69) were all similar. In the LBM trial, Delta T-re (LO-BF 0.82 +/- 0.18 degrees C, HI-BF 0.54 +/- 0.19 degrees C; P < 0.001), mean LSR (LO-BF 0.59 +/- 0.20, HI-BF 0.38 +/- 0.12 mg.cm(-2).min(-1); P = 0.04), WBSL (LO-BF 580 +/- 106 ml, HI-BF 381 +/- 68 ml; P < 0.001), and H-loss (LO-BF 1,884 +/- 277 kJ, HI-BF 1,341 +/- 184 kJ; P < 0.001) were all greater at end-exercise in LO-BF. In conclusion, high % BF individuals demonstrate a greater Delta T-re independently of differences in mass and Hprod, possibly due to a lower mean specific heat capacity or impaired sudomotor control. However, thermoregulatory responses of groups with different adiposity levels should not be compared using a fixed Hprod in watts per kilogram lean body mass.