A Lower Heat Activated Sweat Gland Density in Larger Individuals Does Not Impair Local Sweat Rates During Hot and Humid Heat Stress

Abstract

While the absolute number of sweat glands is primarily determined in utero, a larger body surface area (BSA) will naturally reduce the density of sweat glands per surface area which may compromise sweat rate per unit BSA, and consequently the maximal skin surface sweat coverage (i.e. skin wettedness: ωmax) during exercise in a hot/humid environment. The purpose of the present study was to compare the heat activated sweat gland density (HASGD), sweat gland output (SGO), local sweat rate (LSR) and ωmax between groups differing greatly in BSA during hot and humid active heat stress. Twelve participants separated into two groups based on body size (Small (SM): n=6, 65±6 kg, 1.8±0.1 m2; Large (LG): n=6, 102±13 kg, 2.3±0.1 m2) exercised on an upright cycle ergometer at an evaporative heat balance requirement (Ereq) of 240 W/m2 and 290 W/m2 for up to 75 min at 36°C, 70% RH. LSR of the upper back was measured throughout and expressed relative to surface area (mg/cm2/min). HASGD in number of sweat glands per cm2 (glands/cm2) was measured medially to LSR using the starch iodine technique every 15 minutes and averaged throughout the trial. Mean SGO was derived from the division of LSR and mean HASGD, and expressed as μg/gland/min. On a separate occasion ωmax was measured using a humidity‐ramp protocol (increase of 0.3 kPa from 2.3 kPa every 7.5‐min) during exercise at a fixed external workload of 100W at 36°C until an inflection of esophageal temperature was observed. LSR was similar at 240 W/m2 (SM: 1.24±0.24; LG: 1.40±0.47 mg/cm2/min, P=0.25) and 290 W/m2 (SM: 1.36±0.18; LG: 1.36±0.36 mg/cm2/min, P=0.48). HASGD was greater in SM compared to LG group at 240 W/m2 (SM: 67±12; LG: 55±6 glands/cm2, P=0.03) and 290 W/m2 (SM: 73±9; LG: 57±11 glands/cm2, P<0.01). SGO was trending to be greater in the LG group at 240 W/m2 (SM: 18.6±5.0; LG: 25.2±10.2 μg/gland/min, P=0.10) while significantly greater in the LG group at 290 W/m2 (SM: 17.3±1.6; LG: 21.2±5.7 μg/gland/min, P=0.03). On the other hand, ωmax was not different between groups (SM: 0.82±0.16; LG: 0.72±0.07, P=0.13). In conclusion, a lower HASGD in people with a greater BSA is compensated by a greater SGO to yield a similar LSR up to an Ereq of 290 W/m2 and a similar ωmax. Future research should attempt to assess how even greater reductions in HASGD due to disproportionally larger differences in BSA (ie morbid obesity) compromise LSR, ωmax, and therefore the level of heat stress that can be physiologically compensated.Support or Funding InformationSupported by a NSERC Discovery Grant (P. Imbeault & O. Jay)