The evaporative requirement for heat balance determines whole‐body sweat rate during exercise under conditions permitting full evaporation

Key points•  A relative exercise intensity (%) protocol is often used to compare absolute whole‐body sweat rates (WBSRs) during exercise between participants of different aerobic capacity. •  Under conditions permitting full evaporation, heat balance theory suggests that exercise intensity should be fixed to elicit the same rate of evaporation required for heat balance ( E req ). •  Whole‐body direct calorimetry was employed to measure WBSRs throughout 90 min of exercise across a range of air temperatures and rates of metabolic heat production. •  Irrespective of ambient temperature and metabolic heat production, E req alone described ∼90% of all variability in WBSR during steady‐state and non‐steady‐state exercise, whereas <2% of variation was independently described by %. •  To perform an unbiased comparison of WBSRs (but not necessarily core temperature) between different individuals/groups under conditions allowing full evaporation, future studies should consider using a fixed E req irrespective of the % incurred.Abstract  Although the requirements for heat dissipation during exercise are determined by the necessity for heat balance, few studies have considered them when examining sweat production and its potential modulators. Rather, the majority of studies have used an experimental protocol based on a fixed percentage of maximum oxygen uptake (%). Using multiple regression analysis, we examined the independent contribution of the evaporative requirement for heat balance ( E req ) and % to whole‐body sweat rate (WBSR) during exercise. We hypothesised that WBSR would be determined by E req and not by %. A total of 23 males performed two separate experiments during which they exercised for 90 min at different rates of metabolic heat production (200, 350, 500 W) at a fixed air temperature (30°C, n = 8), or at a fixed rate of metabolic heat production (290 W) at different air temperatures (30, 35, 40°C, n = 15 and 45°C, n = 7). Whole‐body evaporative heat loss was measured by direct calorimetry and used to calculate absolute WBSR in grams per minute. The conditions employed resulted in a wide range of E req (131–487 W) and % (15–55%). The individual variation in non‐steady‐state (0–30 min) and steady‐state (30–90 min) WBSR correlated significantly with E req ( P < 0.001). In contrast, % correlated negatively with the residual variation in WBSR not explained by E req , and marginally increased (∼2%) the amount of total variability in WBSR described by E req alone (non‐steady state: R 2 = 0.885; steady state: R 2 = 0.930). These data provide clear evidence that absolute WBSR during exercise is determined by E req , not by %. Future studies should therefore use an experimental protocol which ensures a fixed E req when examining absolute WBSR between individuals, irrespective of potential differences in relative exercise intensity.