Whole-body Evaporative Heat Loss Research Articles

Effects of short-term heat acclimation on whole-body heat exchange and local nitric oxide synthase- and cyclooxygenase-dependent heat loss responses in exercising older men.

What is the central question of this study? Does short-term heat acclimation enhance whole-body evaporative heat loss and augment nitric oxide synthase (NOS)-dependent cutaneous vasodilatation and NOS- and cyclooxygenase (COX)-dependent sweating, in exercising older men? What is the main finding and its importance? Our preliminary data (n=8) demonstrated that short-term heat acclimation improved whole-body evaporative heat loss, but it did not influence the effects of NOS and/or COX inhibition on cutaneous vasodilatation or sweating in older men during an exercise-heat stress. These outcomes might imply that although short-term heat acclimation enhances heat dissipation in older men, it does not modulate NOS- and COX-dependent control of cutaneous vasodilatation or sweating on the forearm. Ageing is associated with decrements in whole-body heat loss (evaporative+dry heat exchange), which might stem from alterations in nitric oxide synthase (NOS)- and cyclooxygenase (COX)-dependent cutaneous vasodilatation and sweating. We evaluated whether short-term heat acclimation would (i) enhance whole-body heat loss primarily by increasing evaporative heat loss, and (ii) augment NOS-dependent cutaneous vasodilatation and NOS- and COX-dependent sweating, in exercising older men. Eight older men [mean (SD) age, 59(8)years] completed a calorimetry and microdialysis trial before and after 7days of exercise-heat acclimation. For the calorimetry trials, whole-body evaporative and dry heat exchange were assessed using direct calorimetry during 30min bouts of cycling at light, moderate and vigorous metabolic heat productions (150, 200 and 250W/m2 , respectively) in dry heat (40°C, 20% relative humidity). For the microdialysis trials, local cutaneous vascular conductance and sweat rate were assessed during 60min exercise in the heat (35°C, 20% relative humidity) at four dorsal forearm skin sites treated with lactated Ringer solution (control), NOS inhibitor, COX inhibitor or combined NOS and COX inhibitors, via microdialysis. Evaporative heat loss during moderate (P=0.036) and vigorous (P=0.021) exercise increased after acclimation. Inhibition of NOS alone reduced cutaneous vascular conductance to a similar extent before and after acclimation (P<0.040), whereas separate and combined NOS and COX inhibition had no significant effects on sweating relative to the control site (P=0.745). Our preliminary results might suggest that short-term heat acclimation improves evaporative heat loss, but does not significantly modulate the contributions of NOS or COX to cutaneous vasodilatation or sweating on the forearm in older men during an exercise-heat stress.

Modification of sweat evaporative heat loss in the PMV/PPD model to improve thermal comfort prediction in warm climates

Deficient performance of Predicted Mean Vote (PMV) in estimating thermal sensation in warm environments has always been considered as one of the major drawbacks of this model. This well-known shortcoming is often attributed to PMV overlooking thermal adaptation, but simplistic treatment of thermoregulatory sweating may also partly contribute to this error. In this paper a new approach to estimating sweating heat loss in the PMV model is proposed based on a piecewise fuzzy regression model. The effect of sweating at low physical activity levels and the impact of environmental parameters on whole-body evaporative heat loss are also accounted for. The new modified PMV model is validated with previously published experimental data. The result of our analysis demonstrated that the proposed model performs better than the original PMV model. The modified PMV model predicted thermal sensations more accurately than the original version of the PMV model, even without the expectancy factor being applied. This implies that a considerable part of the PMV error is associated with the simplification of sweating calculations. Ignoring the structural drawback of the original PMV calculations can lead to over-estimation of adaptation (or expectancy) coefficients when adjusting the PMV model.

On the effects of constant and variable work of equivalent average intensity on whole‐body heat exchange

Currently recommended heat exposure guidelines prescribe work‐rest allocations based on the level of heat stress originating from the environment and estimated work intensity with the primary goal of maintaining body core temperature within safe limits. Due to the nature of most occupations, these parameters are rarely, if ever, constant. For this reason, ambient conditions (e.g., temperature and humidity) and metabolic rate (endogenous heat production) are quantified as a time‐weighted average over a given work period. What is unknown, however, is whether time‐dependent alterations in work intensity influence whole‐body heat exchange and cardiovascular strain during work eliciting an equivalent time‐weighted average heat production. On four occasions, six young (27 years [SD 4]) males performed 90‐min of semi‐recumbent cycling at a time‐weighted average rate of heat production (~200 W·m−2) in dry heat (40°C, ~20% relative humidity) elicited via cycling at a constant external work rate of 40 W·m−2 (CON) or variable intensity cycling performed in 10‐min cycles: 1) 5 min at an external work rate of 15 W·m−2 followed by 5‐min at 60 W·m−2 (VAR‐LOW); 2) 6 min at 15 W·m−2 followed by 4 min at 70 W·m−2 (VAR‐MOD); and 3) 7 min at 15 W·m−2 followed by 3 min at 80 W·m−2 (VAR‐HIGH). Heat production was measured via indirect calorimetry while whole‐body evaporative heat loss and dry heat gain were evaluated with direct calorimetry. Body heat storage was calculated as the temporal‐summation of heat production and net heat exchange (evaporative heat loss minus dry heat gain). Calorimetric data were expressed as a time‐weighted average (W·m−2) except for heat storage, which was presented as a cumulative value (kJ). Rectal temperature and heart rate were monitored continuously and reported as an average of the final 10‐min of exercise (i.e., final cycle) as well as a peak 30‐s average during this period. Across conditions, there were no differences in evaporative heat loss (241 [20] W·m−2, P=0.20), dry heat gain (59 [13] W·m−2, P=0.52), net heat exchange (281 [10] W·m−2, P=0.54) or body heat storage (224 [55] kJ, P=0.87). Likewise, rectal temperature was not influenced by varying work intensity (average: 37.79 [0.23]°C; peak: 37.84 [0.22]°C; both P≥0.65). However, compared to CON (115 [8] beats·min−1), average heart rate over the final 10 min of exercise was elevated in VAR‐LOW (124 [12] beats·min−1; P=0.01), VAR‐MOD (122 [10] beats·min−1; P=0.04) and VAR‐HIGH (124 [11] beats·min−1; P=0.01), but similar between the variable work conditions (all P≥0.17). Similarly, peak heart rate was elevated in each of the variable work conditions compared to CON (120 [10] beats·min−1; all P&lt;0.01) and in VAR‐HIGH (154 [11] beats·min−1) compared to both VAR‐LOW (140 [12] beats·min−1; P&lt;0.01) and VAR‐MOD (143 [12] beats·min−1; P=0.01). These preliminary data suggest that time‐dependent alterations in work intensity eliciting equivalent time‐weighted rates of metabolic heat production do not influence whole‐body heat exchange or thermal strain, but exacerbate cardiovascular strain compared to constant‐intensity work.Support or Funding InformationThis project was funded by the Government of Ontario and the Natural Sciences and Engineering Research Council of Canada.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Postexercise whole-body sweating increases during muscle metaboreceptor activation in young men.

We assessed the effect of metaboreceptor activation on whole-body evaporative heat loss (WB-EHL) in 12 men (aged 24 ± 4 years) in the early-to-late stages of a 60-min exercise recovery in the heat. Metaboreceptor activation induced by 1-min isometric-handgrip (IHG) exercise followed by 5-min forearm ischemia to trap metabolites increased WB-EHL by 25%-31% and 26%-34% during the ischemic period relative to IHG-only and control (natural recovery only), respectively, throughout recovery. We show that metaboreceptor activation enhances WB-EHL in recovery.

Are All Heat Loads Created Equal?

We evaluated physiological responses during exercise at a fixed evaporative requirement for heat balance (Ereq) but varying combinations of metabolic and environmental heat load. Nine healthy, physically active males (age: 46 ± 8 yr) performed four experimental sessions consisting of 75 min of semirecumbent cycling at various ambient temperatures. Whole-body dry heat loss (direct calorimetry) was monitored continuously as was heat production (indirect calorimetry), which was adjusted to achieve an Ereq of 400 W. The resultant metabolic heat productions and ambient temperatures for the sessions were as follows: (i) 440 W and 30°C (440 [30]), (ii) 388 W and 35°C (388 [35]), (iii) 317 W and 40°C (317 [40]), and (iv) 258 W and 45°C (258 [45]). Whole-body evaporative heat loss was determined via direct calorimetry. Esophageal (Tes) and mean skin (Tsk) temperatures as well as HR were monitored continuously. Mean body temperature (Tb) was calculated from Tes and Tsk. Physiological strain index (PSI) was determined from Tes and HR. End-exercise evaporative heat loss and Tb were similar between conditions (both P ≥ 0.48). Tes was greater in 440 [30] (37.67°C ± 0.04°C) and 388 [35] (37.58°C ± 0.07°C) relative to both 317 [40] (37.35°C ± 0.06°C) and 258 [45] (37.20°C ± 0.07°C; all P ≤ 0.05). Further, Tsk was different between each condition (440 [30], 33.85°C ± 0.16°C; 388 [35], 34.53°C ± 0.08°C; 317 [40], 35.67°C ± 0.07°C; and 258 [45], 36.54°C ± 0.08°C; all P < 0.01). In 440 [30], HR was elevated by about 13 and 18 bpm relative to 317 [40] and 258 [45], respectively (both P < 0.01). Finally, PSI was greater in both 440 [30] and 388 [35] compared with 317 [40] and 258 [45] (all P ≤ 0.04). Exercise at a fixed Ereq resulted in similar evaporative heat loss and Tb. However, the Tes, Tsk, HR, and PSI responses varied depending on the relative contribution of metabolic and environmental heat load.

Postexercise Activation of Muscle Metaboreceptors Modulates Whole-Body Evaporative Heat Loss

Postexercise Activation of Muscle Metaboreceptors Modulates Whole-Body Evaporative Heat Loss

Whole-body heat exchange during heat acclimation and its decay.

The purpose of this study was to quantify how much whole-body heat loss increases during heat acclimation and the decay in these improvements after heat acclimation. Ten males underwent a 14-d heat acclimation protocol that consisted of 90 min of cycling in the heat (40°C, 20% relative humidity) at approximately 50% of maximum oxygen consumption. Before (day 0), during (day 7), and at the end (day 14) of the heat acclimation protocol as well as 7 and 14 d after heat acclimation (days 21 and 28), whole-body heat exchange (evaporative and dry) was measured using direct calorimetry during three bouts of 30-min exercise at 300 (Ex1), 350 (Ex2), and 400 W·m (Ex3), each separated by 10 and 20 min of recovery, respectively, at 35°C and 16% relative humidity. Concurrent measurements of metabolic heat production (indirect calorimetry) allowed for the direct calculation of change in body heat content (ΔHb). After accounting for an increase in net dry heat gain, increases in whole-body evaporative heat loss were evident for Ex2 and Ex3 on day 7 (Ex2, 4.9 ± 5.6%; Ex3, 9.0 ± 6.0%; both P ≤ 0.05) and all heat loads on day 14 (Ex1, 7.6 ± 8.3%; Ex2, 7.7 ± 5.5%; Ex3, 11.2 ± 4.6%; all P ≤ 0.05) relative to day 0 (Ex1, 494 ± 27 W; Ex2, 583 ± 21 W; Ex3, 622 ± 36 W). As a result, a lower cumulative ΔHb was measured on day 7 (-18 ± 8%, P ≤ 0.001) and day 14 (-26 ± 10%, P ≤ 0.001) compared with that measured on day 0 (1062 ± 123 kJ). Most of these improvements were retained after 2 wk of nonexposure to the heat. This is the first study to quantify how much 14 d of heat acclimation can increase whole-body evaporative heat loss, which can improve by as much as approximately 11%.

Local Sweat Rates do not Accurately Represent Whole-Body Evaporative Heat Loss following 14-day Dry-Heat Acclimation

Local Sweat Rates do not Accurately Represent Whole-Body Evaporative Heat Loss following 14-day Dry-Heat Acclimation

Age-Related Decrements in Heat Dissipation during Physical Activity Occur as Early as the Age of 40

Older adults typically experience greater levels of thermal strain during physical efforts in the heat compared to young individuals. While this may be related to an age-dependent reduction in whole-body sweating, no study has clearly delineated at what age this occurs. In the present study, we report direct measurements of human heat dissipation during physical activity in the heat in males ranging in age from 20–70 years. Eighty-five males performed four 15-min bouts of cycling separated by 15-min rest periods, in a calorimeter regulated to 35°C and 20% relative humidity. Direct calorimetry was used to measure total heat loss (whole-body evaporative heat loss and dry heat exchange). We also used indirect calorimetry as a continuous measure of metabolic heat production. Body heat storage was calculated as the temporal summation of heat production and total heat loss over the experimental session. Whole-body sweat rate (WBSR) was calculated from measurements of evaporative heat loss. Males were divided into five age categories for the analysis of WBSR and body heat storage: 20–31 years (n = 18), 40–44 years (n = 15), 45–49 years (n = 15), 50–55 years (n = 21) and 56–70 years (n = 16). Relative to young males, WBSR was reduced in males aged 56–70 during each exercise (all P<0.05), in males aged 50–55 during the second (P = 0.031) and third exercises (P = 0.028) and in males aged 45–49 during the final exercise bout (P = 0.046). Although not significantly different, 40–44 years old males also had a lower rate of heat loss compared to younger males. Over the sum of two hours, the change in body heat content was greater in males 40–70 years compared to young males (all P<0.05). Our findings suggest that middle-aged and older adults have impairments in heat dissipation when doing physical activity in the heat, thus possibly increasing their risk of heat-related illness under such conditions.

Do Older Females Store More Heat than Younger Females during Exercise in the Heat?

Aging is associated with a reduction in the body's capacity to dissipate heat. To date, few studies have examined age-related changes in thermoregulatory function during short exercise periods in the heat in older females. This study aimed to investigate the effects of age on whole-body heat loss during intermittent exercise in the heat in young and older females. Direct and indirect calorimetry was used to measure whole-body evaporative heat loss (EHL), change in body heat content, and metabolic heat production. Eleven young (Y) (mean ± SD age = 24 ± 4 yr) and 13 older (O) (51 ± 8 yr) females matched for body surface area (Y, 1.72 ± 0.15; O, 1.75 ± 0.12 m²) and fitness (V(˙)O(2max)) (Y, 36.7 ± 6.8 mL O₂·kg⁻¹·min⁻¹; O, 33.8 ± 8.0 mL O₂·kg⁻¹·min⁻¹) performed four bouts of 15-min cycling (Ex1, Ex2, Ex3, and Ex4) at a constant rate of heat production (300 W) at 35°C and 20% relative humidity. Each exercise bout was separated by 15 min of rest. EHL was reduced in O compared with Y during Ex1 (O, 199 ± 6 W; Y, 240 ± 9 W; P = 0.001), Ex2 (O, 238 ± 4 W; Y, 261 ± 9 W, P = 0.023), and Ex3 (O, 249 ± 4 W; Y, 274 ± 11 W; P = 0.040). EHL was not different between groups during Ex4 or during the recovery periods. Older females had a greater change in body heat content compared with young females (O, 270 ± 20 kJ; Y, 166 ± 20 kJ; P = 0.001). These findings suggest that older females have a lower capacity for whole-body EHL compared with younger females during short intermittent exercise in the heat performed at a fixed rate of metabolic heat production.

Do older adults experience greater thermal strain during heat waves?

Heat waves are the cause of many preventable deaths around the world, especially among older adults and in countries with more temperate climates. In the present study, we examined the effects of age on whole-body heat loss and heat storage during passive exposure to environmental conditions representative of the upper temperature extremes experienced in Canada. Direct and indirect calorimetry measured whole-body evaporative heat loss and dry heat exchange, as well as the change in body heat content. Twelve younger (21 ± 3 years) and 12 older (65 ± 5 years) adults with similar body weight (younger: 72.0 ± 4.4 kg; older: 80.1 ± 4.2 kg) and body surface area (younger: 1.8 ± 0.1 m(2); older: 2.0 ± 0.1 m(2)) rested for 2 h in a hot-dry [36.5 °C, 20% relative humidity (RH)] or hot-humid (36.5 °C, 60% RH) environment. In both conditions, evaporative heat loss was not significantly different between groups (dry: p = 0.758; humid: p = 0.814). However, the rate of dry heat gain was significantly greater (by approx. 10 W) for older adults relative to younger adults during the hot-dry (p = 0.032) and hot-humid exposure (p = 0.019). Consequently, the cumulative change in body heat content after 2 h of rest was significantly greater in older adults in the hot-dry (older: 212 ± 25 kJ; younger: 131 ± 27 kJ, p = 0.018) as well as the hot-humid condition (older: 426 ± 37 kJ; younger: 317 ± 45 kJ, p = 0.037). These findings demonstrate that older individuals store more heat during short exposures to dry and humid heat, suggesting that they may experience increased levels of thermal strain in such conditions than people of younger age.

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

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 (Ereq) and % to whole-body sweat rate (WBSR) during exercise. We hypothesised that WBSR would be determined by Ereq 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 Ereq (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 Ereq (P < 0.001). In contrast, % correlated negatively with the residual variation in WBSR not explained by Ereq, and marginally increased (∼2%) the amount of total variability in WBSR described by Ereq 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 Ereq, not by % . Future studies should therefore use an experimental protocol which ensures a fixed Ereq when examining absolute WBSR between individuals, irrespective of potential differences in relative exercise intensity.

Quantification of Head Sweating in Desert and Tropic Conditions

Protective helmets can interfere with air flow to the head and limit the capacity for evaporative heat loss. The degree to which ventilation under a helmet can increase evaporative heat loss depends on sweating rate over the surface area covered. PURPOSE: To quantify head sweating rate (msw) during rest and exercise in desert (D; 45°C, 20% RH) and tropic (T; 35°C, 70%RH) conditions (matched at 31.6°C WBGT). METHODS: Six males completed 20 min each of rest and exercise at three metabolic rates (337 W; 492 W; 624 W) in D and T, which were counterbalanced. Heads were shaved before each trial, and surface area (485±42 cm2) was measured by 3D scanner. For measurement of msw, dry air was passed over the head through a sealed helmet. Forearm msw was measured with a capsule (15.9 cm2) ventilated with ambient air. Whole body sweat loss was calculated from change in nude body weight plus fluid intake. RESULTS: Head msw increased (p=0.007) with metabolic rate, and overall was lower (p=0.048) in D (0.3±0.2 mg·cm-2·min-1 at rest to 0.8±0.4 mg·cm-2·min-1 at 624 W), compared to T (0.4±0.2 to 1.2±0.7 mg·cm-2·min-1). Forearm msw also increased (p<0.001) with metabolic rate, but was higher (p=0.013) in D (0.5±0.3 to 1.5±0.7 mg·cm-2·min-1) than T (0.1±0.1 to 1.1±0.4 mg·cm-2·min-1, respectively). Esophageal temperature did not differ between trials (D: 36.8±0.1 to 37.7±0.3°C; T: 36.8±0.2 to 38.0±0.2°C). Mean skin temperature (Tsk) was initially higher (p=0.01) in D (36.3±0.3°C) than T (35.1±0.8°C), but at 624 W was similar between trials (D: 36.7±1.0°C; T: 36.5±0.5°C). The increase in heart rate (HR) with exercise was lower (p=0.022) during D (80±14 to 135±15 b·m-1) than T (81±17 to 150±19 b·m-1). Whole-body sweat loss was higher (p=0.03) in D (563±123 g·m-2·h-1) than T (470±171 g· m-2·h-1). For comparison, peak head sweat loss was 504±243 g·m-2·h-1 in D, and 696±447 g·m-2·h-1 In T. CONCLUSIONS: The higher head msw in T despite similar core and skin temperatures at 624 W suggests an improved sweat capacity at the head in compensation for suppressed whole-body evaporative heat loss. The results from this study suggest that improved ventilation through a helmet could increase heat loss by as much as 24 W. The opinions or assertions contained herein should not to be construed as official or reflecting the views of the Army or the Department of Defense.

Sex-related differences in evaporative heat loss: the importance of metabolic heat production

We evaluated the hypothesis that different rates of metabolic heat production between sexes, during exercise at the same percentage of maximum oxygen consumption [VO2 max] give proportional differences in evaporative heat loss. Seven males and seven females, exercised at 41.3 +/- 2.7% VO2 Max for 60-min at 40 degrees C and 30% relative humidity. Whole-body direct air calorimetry measured rate of whole-body evaporative heat loss (H e) while metabolic heat production (M - W) was measured by indirect calorimetry. M -W was greater in males (243 +/- 18 W m(-2)) relative to females (201 +/- 4 W m(-2)) (P <or= 0.05) throughout exercise. This was paralleled by a greater H e at end-exercise in males (207 +/- 51 W m(-2)) relative to females (180 +/- 3 W m(-2)) (P <or= 0.05). Differences in metabolic heat production between sexes during exercise at a fixed percentage of VO2 max give differences in evaporative heat loss. To compare thermoregulatory function between sexes, differences in metabolic heat production must therefore be accounted for.