Does Body Surface Area Independently Alter Whole‐body Sweat Loss during Exercise at a Fixed Absolute Evaporative Requirement for Heat Balance?

Abstract

The absolute evaporative requirement for heat balance (Ereq) is the principal determinant of whole‐body sweat loss (WBSL) during fixed‐duration exercise in ambient conditions that permit complete sweat evaporation, regardless of body size. However, exercise at a fixed absolute Ereq in hot/humid conditions may lead to disproportionately greater WBSL among individuals of smaller body surface area (BSA) due to a lower absolute maximum evaporative capacity and thus higher skin wettedness required for heat balance (ωreq) and lower sweating efficiency (Seff). To test this hypothesis, young adult males of small (SM: 1.81±0.07 m2; n=4) or large (LG: 2.28±0.15 m2; n=3) BSA cycled for 70 min at 34±0.3°C, 71.8±1.3% RH at an exercise intensity set to elicit a fixed VO2 of 1.5 L/min, and therefore a metabolic heat production of ~450 W and an Ereq of ~430 W (indirect calorimetry). Body mass was measured at 0, 45, and 70 min to estimate WBSL during non‐steady‐state (0–45 min) and steady‐state (45–70 min) exercise. Seff was calculated as the quotient of the WBSL required to attain Ereq with complete evaporation and the observed WBSL during the steady‐state period, and expressed as a percentage. The attainment of a steady state (i.e., the actual rate of evaporation equaled the required rate of evaporation) from 45 to 70 min was confirmed by stable esophageal and skin temperatures (±0.1°C throughout steady state). During non‐steady‐state exercise, ωreq was not different between groups (SM: 0.81±0.05 g; LG: 0.73±0.08 g; P=0.17), and no between‐group difference in WBSL was observed (SM: 522±81 g; LG: 476±115 g; P=0.56). In contrast, steady‐state ωreq was greater in SM (SM: 0.81±0.03; LG: 0.71±0.04; P=0.02), leading to significantly higher WBSL (SM: 361±49 g; LG: 260±54 g; P<0.05) and diminished Seff (SM: 74.1±12.7%; LG: 96.3±6.4%; P=0.04) in the SM group. In summary, our preliminary data suggest that in hot/humid ambient conditions, a large between‐group difference in BSA may lead to disproportionately greater steady‐state WBSL in smaller individuals despite exercising at the same absolute Ereq due to an independent effect of BSA on Seff.Support or Funding InformationSupported by an NSERC Discovery Grant (O. Jay)