Whole-body Heat Loss Research Articles

Heat Acclimation By Warm-water Immersion Enhances Whole-body Heat Loss In Older Adults

Heat Acclimation By Warm-water Immersion Enhances Whole-body Heat Loss In Older Adults

Hormonal intrauterine devices and heat exchange during exercise.

Synthetic progestins in oral contraceptives are thought to blunt heat dissipation by reducing skin blood flow and sweating. However, whether progestin-releasing intrauterine devices (IUDs) modulate heat loss during exercise-heat stress is unknown. We used direct calorimetry to measure whole-body total (dry + evaporative) heat loss in young, physically active women (mean (SD); aged 24(4)years, 39.3(5.3) ml/kg/min) with (IUD; n=19) and without (Control; n=17) IUDs in the follicular and luteal phases of the menstrual cycle during light- and moderate-intensity exercise at fixed rates of heat production (∼175 and ∼275W/m2 ) in 30°C, ∼21% relative humidity. Between-group and -phase differences were evaluated using traditional hypothesis testing and statistical equivalence testing within pre-determined bounds (±11W/m2 ; difference required to elicit a ±0.3°C difference in core temperature over 1h) in each exercise bout. Whole-body total heat loss was statistically equivalent between groups within ±11Wm-2 (IUD-Control [90% CIs]; Light: -2 [-8, 5] W/m2 , P=0.007; Moderate: 0 [-6, 6] W/m2 , P=0.002), as were dry and evaporative heat loss (P ≤ 0.023), except for evaporative heat loss during moderate-intensity exercise (equivalence: P=0.063, difference: P=0.647). Whole-body total and evaporative heat loss were not different between phases (P≥0.267), but dry heat loss was 3 [95% CIs: 1, 5] W/m2 greater in the luteal phase (P≤0.022). Despite this, all whole-body heat loss outcomes were equivalent between phases (P≤0.003). These findings expand our understanding of the factors that modulate heat exchange in women and provide valuable mechanistic insight of the role of endogenous and exogenous female sex hormones in thermoregulation. KEY POINTS: Progestin released by hormonal intrauterine devices (IUDs) may negatively impact heat dissipation during exercise by blunting skin blood flow and sweating. However, the influence of IUDs on thermoregulation has not previously been assessed. We used direct calorimetry to show that IUD users and non-users display statistically equivalent whole-body dry and evaporative heat loss, body heat storage and oesophageal temperature during moderate- and high-intensity exercise in a warm, dry environment, indicating that IUDs do not appear to compromise exercise thermoregulation. However, within IUD users and non-users, dry heat loss was increased and body heat storage and oesophageal temperature were reduced in the luteal compared to the follicular phase of the menstrual cycle, though these effects were small and unlikely to be practically meaningful. Together, these findings expand our understanding of the factors that modulate heat exchange in women and have important practical implications for the design of future studies of exercise thermoregulation.

Sleep Deprivation Does Not Influence Whole-body Heat Loss During Exercise-Heat Stress In Young Men

Sleep Deprivation Does Not Influence Whole-body Heat Loss During Exercise-Heat Stress In Young Men

Association between haemoglobin A1c and whole-body heat loss during exercise-heat stress in physically active men with type 2 diabetes.

What is the central question of this study? Is the impairment in heat dissipation during exercise observed in men with type 2 diabetes related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A1c )? What is the main finding and its importance? No association was found between haemoglobin A1c (range: 5.1-9.1%) and whole-body heat loss in men with type 2 diabetes during exercise in the heat. However, individuals with elevated haemoglobin A1c exhibited higher body core temperature and heart rate responses. Thus, while haemoglobin A1c is not associated with heat loss per se, it may still have important implications for physiological strain during exercise. Type 2 diabetes is associated with a reduced capacity to dissipate heat. It is unknown whether this impairment is related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A1c ) is unknown. We evaluated the association between haemoglobin A1c and whole-body heat loss (via direct calorimetry), body core temperature, and heart rate in 26 physically active men with type 2 diabetes (43-73 years; HbA1c 5.1-9.1%) during exercise at increasing rates of metabolic heat production (∼150, 200, 250Wm-2 ) in the heat (40°C, ∼17% relative humidity). Haemoglobin A1c was not associated with whole-body heat loss (P=0.617), nor the increase in core temperature from pre-exercise (P=0.347). However, absolute core temperature and heart rate were elevated ∼0.2°C (P=0.014) and ∼6beatsmin-1 (P=0.049), respectively, with every percentage point increase in haemoglobin A1c . Thus, while haemoglobin A1c does not appear to modify diabetes-related reductions in capacity for heat dissipation, it may still have important implications for physiological strain during exercise-heat stress.

The serum irisin response to prolonged physical activity in temperate and hot environments in older men with hypertension or type 2 diabetes

Current labor demographics are changing, with the number of older adults increasingly engaged in physically demanding occupations expected to continually rise, which are often performed in the heat. Given an age-related decline in whole-body heat loss, older adults are at an elevated risk of developing heat injuries that may be exacerbated by hypertension (HTN) and type 2 diabetes (T2D). Elevated irisin production may play a role in mitigating the excess oxidative stress and acute inflammation associated with physically demanding work in the heat. However, the effects of HTN and T2D on this response remain unclear. Therefore, we evaluated serum irisin before and after 3-h of moderate intensity exercise (metabolic rate: 200 W/m2) and at the end of 60-min of post-exercise recovery in a temperate (wet-bulb globe temperature (WBGT) 16 °C) and high-heat stress (WBGT 32 °C) environment in 12 healthy older men (mean ± SD; 59 ± 4 years), 10 men with HTN (60 ± 4 years), and 9 men with T2D (60 ± 5 years). Core temperature (Tco) was measured continuously. In the heat, total exercise duration was significantly lower in older men with HTN and T2D (both, p ≤ 0.049). Despite Tco not being different between groups, Tco was higher in the hot compared to the temperate condition for all groups (p < 0.001). Similarly, serum irisin concentrations did not differ between groups under either condition but were elevated relative to the temperate condition during post-exercise and end-recovery in the heat (+93.9 pg/mL SEM 26 and + 70.5 pg/mL SEM 38 respectively; both p ≤ 0.014). Thus, our findings indicate similar irisin responses in HTN and T2D compared to healthy, age-matched controls, despite reduced exercise tolerance during prolonged exercise in the heat. Therefore, older workers with HTN and T2D may exhibit greater cellular stress during prolonged exercise in the heat, underlying greater vulnerability to heat-induced cellular injury.

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.

Impact of uncomplicated controlled hypertension on thermoregulation during exercise-heat stress.

Exercise is promoted for management of hypertension and as a general healthy behavior, but environmental conditions are seldom considered in these recommendations. Hypertension may affect skin blood flow and sweating, two of the primary mechanisms which prevent continued elevations in core temperature by facilitating whole-body heat loss during exercise-heat stress. We show that during incremental exercise-heat stress (in hot-dry conditions), controlled and uncomplicated hypertension is unlikely to exert a meaningful effect on whole-body heat loss in individuals who are already physically active.

The Relation between Age and Sex on Whole-Body Heat Loss during Exercise-Heat Stress.

Increasing age is associated with decrements in sweat rate that compromise whole-body total heat loss (evaporative + dry heat exchange) in both men and women during moderate-to-vigorous exercise in dry heat. Similarly, young women also display reductions in sweating (that lower evaporative heat loss) relative to young men in such conditions. Nevertheless, it remained unclear whether these effects act synergistically to exacerbate the age-related decline in whole-body total heat loss in women relative to men. We therefore assessed the interrelation between age and sex on whole-body total heat loss during light, moderate, and vigorous exercise in dry heat. To achieve this, we used direct and indirect calorimetry to assess whole-body total heat loss and metabolic heat production (respectively) in 46 men and 34 women age between 18 and 70 yr. Participants performed three, 30-min bouts of cycling at metabolic heat productions of 150 (light), 200 (moderate), and 250 (vigorous) W·m, each separated by 15-min recovery in dry heat (40°C, ~15% relative humidity). Whole-body total heat loss was ~5% lower in women relative to men during moderate and vigorous exercise (both, P < 0.01), irrespective of age. Total heat loss declined with age during moderate and vigorous exercise in both men and women (all, P < 0.050), although the rate of that decline (~4% per decade) was similar between men and women across all exercise bouts (all, P > 0.050). We show that, when assessed in dry heat, whole-body total heat loss is lower in women relative to men, irrespective of age. Furthermore, total heat loss declines with increasing age in both men and women during moderate-to-vigorous exercise, albeit the rate of that decline is not appreciably modified by sex.

Ageing attenuates the effect of extracellular hyperosmolality on whole-body heat exchange during exercise-heat stress.

In humans, hypohydration attenuates sweat secretion and attenuates whole-body heat loss, probably to mitigate further fluid losses and thereby support blood pressure regulation. Recently, however, we demonstrated that the hypohydration-mediated reduction in net whole-body heat exchange (evaporative heat loss-dry heat gain) was blunted in middle-aged compared to younger men during moderate exercise in dry heat; albeit, the underpinning mechanisms could not be determined. Here we evaluated the hypothesis that those findings stemmed from a diminished influence of extracellular hyperosmolality on net whole-body heat exchange in middle-aged-to-older compared to young men. Consistent with that hypothesis, extracellular hyperosmolality induced by an intravenous infusion of hypertonic saline (3% NaCl) reduced net heat exchange and augmented rectal temperature to a greater extent in the young compared to middle-aged-to-older men. Thus, age-related differences in the influence of hypohydration on thermoregulatory function appear to be due to blunted sensitivity to hyperosmolality with ageing. We recently demonstrated that sweating-induced hypohydration attenuated whole-body heat dissipation to a greater extent in young compared to middle-aged men during exercise-heat stress. Here, we evaluated whether this divergent response stemmed from an attenuated influence of extracellular hyperosmolality on heat exchange with ageing. To achieve this, ten young (mean(SD): 25(5)years) and ten middle-aged-to-older (61(5)years) men completed two trials involving a 90-min intravenous infusion of isosmotic saline (0.9% NaCl; ISO) or hyperosmotic saline (3.0% NaCl; HYP) followed by 60min of cycling at a fixed metabolic heat production of 250W/m2 (∼50% peak aerobic power) in dry heat (40°C, ∼17% relative humidity). Whole-body net heat exchange (evaporative heat loss-dry heat gain) was measured via direct calorimetry. Rectal temperature was monitored continuously. Heat exchange was attenuated in HYP compared to ISO in the young (233(20) vs. 251(17)W/m2 ; P=0.002) but not older group (229(16) vs. 227(20)W/m2 ; P=0.621). Further, heat exchange was lower in the middle-aged-to-older vs. young men in ISO (P=0.034) but not in HYP (P=0.623). Similarly, end-exercise rectal temperature was greater in HYP relative to ISO in the young (38.3(0.4)°C vs. 37.9(0.3)°C; P=0.015) but not the middle-aged-to-older men (38.3(0.3)°C vs. 38.2(0.2)°C; P=0.652). Compared to the young, rectal temperature was greater in the middle-aged-to-older during ISO (P=0.035) whereas no between-group difference was observed in HYP (P=0.746). Our findings indicate that ageing blunts the effect of extracellular hyperosmolality on thermoregulatory function during exercise-heat stress.

Whole-body heat exchange in women during constant- and variable-intensity work in the heat.

Time-weighted averaging is used in occupational heat stress guidelines to estimate the metabolic demands of variable-intensity work. However, compared to constant-intensity work of the same time-weighted average metabolic rate, variable-intensity work may cause decrements in total heat loss (dry + evaporative heat loss) that exacerbate heat storage in women. We therefore used direct calorimetry to assess whole-body total heat loss and heat storage (metabolic heat production minus total heat loss) in women and men during constant- and variable-intensity work of equal average intensity. Ten women [mean (SD); 31 (11) years] and fourteen men [30 (8) years] completed two trials involving 90-min of constant- and variable-intensity work (cycling) eliciting an average metabolic heat production of ~ 200W/m2 in dry-heat (40°C, ~ 15% relative humidity). External work was fixed at ~ 40W/m2 for constant-intensity work, and alternated between ~ 15 and ~ 60W/m2 (5-min each) for variable-intensity work. When expressed as a time-weighted average over each work period, total heat loss did not differ between men and women (mean difference [95% CI]; 4W/m2 [-11, 20]; p = 0.572) or between constant- and variable-intensity work (1W/m2 [-3, 5]; p = 0.642). Consequently, heat storage did not differ significantly between men and women (-4W/m2 [-17, 8]; p = 0.468) or between constant- and variable-intensity work (0W/m2 [-3, 3]; p = 0.834). Neither whole-body heat loss nor heat storage was modulated by the partitioning of work intensity, indicating that time-weighted averaging is appropriate for estimating metabolic demand to assess occupational heat stress in women.

The Relationship Between Whole-body Heat Exchange And Thermoregulatory Responses During Exercise In The Heat In A Dehydrated State

Whole-body heat loss has been extensively examined during exercise in hot environments. Interestingly, the relationship between predicted heat balance parameters and thermoregulatory strain have been limited to individuals in an euhydrated state. As dehydration has been shown to exacerbate thermal and cardiovascular strain, it is imperative to examine whether dehydration modulates heat exchange during exercise in the heat and its relationship to physiological responses to exercise in the heat. PURPOSE: to examine whether predicted heat balance parameters explain variance in core temperature (Tcore) and heart rate (HR) responses to exercise in the heat following 24 h of fluid restriction. METHODS: 8 participants (4 males and 4 females; age:19±2yrs; VO2max:47.4±6.5ml/kg/min; body mass:61.5±9.4kg; height:166.7±7.5cm) performed a 30-min bout of treadmill running and walking in the heat (32°C, 55% relative humidity) under two conditions: 1) 24 h of fluid restriction (DEH) and 2) euhydration (EUH). Participants exercised at 11W/kg for 15 min followed by 7W/kg for 15 min, which were derived via indirect calorimetry. Tcore was continuously assessed through rectal thermometry and HR was measured at the end of each intensity. Metabolic heat production (M-W), dry heat loss (DHL), and evaporative requirement (Ereq) were calculated for both exercise intensities. Pearson correlation coefficients were utilized to examine the relationship between M-W, DHL, Ereq and Tcore, change in Tcore (Tcore), HR, and change in HR (HR) under DEH and EUH. Linear regressions were performed for all significant correlations. Statistical significance was set at p ≤ 0.05. RESULTS: M-W and DHL were not correlated with Tcore, Tcore, HR, or HR in DEH or EUH at 11W/kg. At 7W/kg under DEH, DHL was negatively correlated with Tcore (R2=0.54, p=0.05) and HR (R2=0.67, p=0.024). All other heat balance parameters were not correlated with physiological responses at 7W/kg in DEH or EUH. CONCLUSION: In this preliminary dataset, Ereq and M-W did not explain variance in physiological responses during DEH. DHL under DEH explained 67% of the variance in post HR and 54% of the variance in Tcore at 7W/kg. Therefore, DHL may contribute to the variation in physiological strain during low intensity exercise in the heat under DEH.

Exercise Heat Stress in Patients With and Without Type 2 Diabetes

This physiology study assesses whole-body heat loss in physically active middle-aged and older men with vs without type 2 diabetes after aerobic cycling to evaluate whether type 2 diabetes impairs heat loss and by what mechanism.

Intermittent sequential pneumatic compression does not enhance whole-body heat loss in elderly adults during extreme heat exposure.

Lower-limb intermittent sequential pneumatic compression (ISPC) improves circulation and vascular function in elderly adults. We evaluated the hypothesis that ISPC would also augment whole-body heat loss (WBHL) in elderly adults (aged 69 ± 4 years) resting in extreme heat (40 °C). While ISPC increased mean arterial pressure (91 ± 9 mm Hg) relative to no-ISPC (83 ± 5 mm Hg; P = 0.013) at the end of the exposure, no influence on WBHL was observed (81 ± 7 and 86 ± 11 W for ISPC and no-ISPC, respectively, P = 0.310). Novelty When assessed in elderly adults during an extreme heat exposure, intermittent sequential pneumatic compression augmented mean arterial pressure but did not enhance whole-body heat loss.

Interactive effects of age and hydration state on human thermoregulatory function during exercise in hot-dry conditions.

Ageing and hypohydration independently attenuate heat dissipation during exercise; however, the interactive effects of these factors remain unclear. We assessed the hypothesis that ageing suppresses hypohydration-induced reductions in whole-body heat loss during exercise in the heat. On two occasions, eight young (mean [SD]: 24 [4]years) and eight middle-aged (59 [5]years) men performed 30-minute bouts of light (heat production of 175Wm-2 ) and moderate (275Wm-2 ) cycling (separated by 15-minute rest) in the heat (40°C, 15% relative humidity) when euhydrated and hypohydrated (~4% reduction in body mass). Heat production and whole-body net heat exchange (evaporative heat loss+dry heat gain) were measured via indirect and direct calorimetry (respectively) and heat storage was calculated via their temporal summation. Net heat exchange was reduced, while heat storage was elevated, in the middle-aged men during moderate exercise when euhydrated (both P≤0.01). In the young, evaporative heat loss was attenuated in the hypohydrated vs euhydrated condition during light (199±6 vs 211±10Wm-2 ; P≤0.01) and moderate (287±15 vs 307±13Wm-2 ; P≤0.01) exercise, but was similar in the middle-aged men, averaging 223±6 and 299±15Wm-2 , respectively, across conditions (both P≥0.32). Heat storage was thereby exacerbated by hypohydration in the young (both P<0.01) but not the middle-aged (both P≥0.32) during both exercise bouts and, as a result, was similar between groups when hypohydrated (both P≥0.50). Hypohydration attenuates heat loss via sweating in young but not middle-aged men, indicating that ageing impairs one's ability to mitigate further sweat-induced fluid loss during hypohydration.

The Hexoskin physiological monitoring shirt does not impair whole-body heat loss during exercise in hot-dry conditions.

Using direct calorimetry, we determined if the Hexoskin shirt (Carré Technologies Inc., Que., Canada), a wearable device for monitoring physiological strain, would compromise whole-body heat loss and exacerbate body heat storage during moderate-intensity activity in hot-dry conditions. The shirt did not impair heat dissipation and resulted in similar body heat storage when worn alone relative to a semi-nude condition (214 vs. 211 kJ) or when worn underneath a work uniform compared with a cotton undershirt (307 vs. 318 kJ).

Heat Loss Is Impaired in Older Men on the Day after Prolonged Work in the Heat.

Prolonged work in the heat may exacerbate the rise in core temperature on the next work day, especially in older workers who display impairments in whole-body heat loss that increase body heat storage and core temperature relative to young adults during heat stress. We therefore evaluated whether whole-body heat loss in older adults was impaired on the day after prolonged work in the heat. Whole-body heat exchange and heat storage were assessed in nine older (53-64 yr) men during three 30-min bouts of semirecumbent cycling at fixed rates of metabolic heat production (150 [Ex1], 200 [Ex2], 250 W·m [Ex3]), each separated by 15-min recovery, in hot-dry conditions (40°C, 20% relative humidity), immediately before (day 1), and on the day after (day 2) a prolonged, work simulation (~7.5 h) involving moderate-intensity intermittent exercise in hot-dry conditions (38°C, 34% relative humidity). Total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was quantified as the temporal summation of heat production and loss. Total heat loss (mean ± SD) during Ex1 did not differ between days 1 and 2 (151 ± 15 and 147 ± 14 W·m, respectively; P = 0.27), but was attenuated on day 2 during Ex2 (181 ± 15 W·m) and Ex3 (218 ± 16 W·m) relative to day 1 (192 ± 14 and 230 ± 19 W·m, respectively; both P < 0.01). Consequently, body heat storage throughout the protocol on day 2 (276 ± 114 kJ) was 31% greater than on day 1 (191 ± 87 kJ; P < 0.01). Prolonged work in the heat causes next-day impairments in whole-body heat loss, which exacerbate heat storage and may elevate the risk of heat injury on the following day in older workers.

Do Graduated Compression Garments Enhance Whole‐body Heat Loss During an Extreme Heat Exposure in Older Adults?

Graduated compression garments worn on the lower limbs are commonly used in clinical settings to enhance blood circulation and have been shown to elicit elevations in whole‐body heat loss relative to a control condition (no compression) in young men during post‐exercise recovery in the heat (Am J Physiol 2010;299:R119). To date, it remains unknown whether these garments can enhance heat dissipation in heat vulnerable older adults. Relative to their younger counterparts, older adults demonstrate an impaired ability to increase whole‐body heat loss during an extreme heat exposure. Consequently, these individuals store more heat, leading to potentially dangerous increases in body temperature (Temperature (Austin) 2017;4:79). The use of compression garments may help alleviate the level of thermal strain in these individuals. We therefore sought to determine whether lower limb graduated compression garments would enhance whole‐body heat dissipation and reduce body heat storage relative to a control condition (i.e., no compression) in older adults during passive heat exposure. To evaluate this possibility, we assessed changes in whole‐body heat exchange and heat storage in 6 older (mean ± SD: 69 ± 6 years) adults (4 men, 2 women) during a 2‐hour extreme heat exposure (40°C, ~10 %RH) on two separate occasions (separated by ≥48 h). In both trials, participants sat in a semi‐recumbent position wearing a graduated compression garment (SCD Express Sleeves; Kendall, Mansfield, MA) on each leg containing three bladders that either delivered intermittent graded compression (ankle: 45 mmHg, calf: 40 mmHg, thigh: 30 mmHg; Compression trial) or no compression (Control trial) throughout each exposure. Trials were administered in a balanced order. Wholebody heat loss and metabolic heat production were measured continuously by direct and indirect calorimetry, respectively. Body heat storage was quantified as the temporal summation of heat production and loss. Metabolic heat production (Compression: 98 ± 10 W vs. Control: 94 ± 13 W), evaporative heat loss (135 ± 20 W vs. 134 ± 23 W), dry heat exchange (−48 ± 19 W vs. −52 ± 12 W), and net (total) heat loss (89 ± 14 W vs. 82 ± 14 W), were similar between the Compression and Control trials (all P&gt;0.05). Consequently, cumulative body heat storage did not differ between conditions (Compression: 85 ± 11 kJ; Control: 85 ± 24 kJ, P=0.98). Our preliminary findings suggest that graduated compression garments do not enhance whole‐body heat loss in older adults during exposure to hot ambient conditions.Support or Funding InformationThe Canadian Institutes of Health Research (funds held by Dr. Glen Kenny).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Does a Prolonged Work Day in the Heat Impair Heat Loss on the Next Day in Young Men?

Heat strain is known to be exacerbated on the second of consecutive work days. We therefore evaluated whether prolonged work in the heat would impair whole-body heat loss capacity on the next day. To evaluate this possibility, we assessed changes in whole-body heat exchange and heat storage in eight young (26 ± 4 yr) men during heat stress tests performed on the same day before (day 1) and on the day after (day 2) a prolonged work simulation. Each heat stress test involved three, 30-min bouts of semirecumbent cycling at fixed rates of metabolic heat production (200 W·m (Ex1), 250 W·m (Ex2), and 300 W·m (Ex3)), each separated by 15-min recovery, under hot, dry conditions (40°C, 20% relative humidity). The work simulation (7.5 h) involved three moderate-intensity intermittent work bouts (2 h), each separated by 30-min rest breaks, under similarly hot, dry conditions (38°C, 34% relative humidity). Total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was quantified as the temporal summation of heat production and loss. Total heat loss did not differ between days 1 and 2 (P = 0.66) and averaged (mean ± 95% confidence interval) 185 ± 7 W (Ex1), 233 ± 7 W (Ex2), and 261 ± 5 W (Ex3) across test days. Consequently, the change in body heat storage was also similar between days 1 and 2 (P = 0.32), averaging 133 ± 15 kJ (Ex1), 99 ± 16 kJ (Ex2), and 184 ± 15 kJ (Ex3) across test days. When assessed under controlled laboratory conditions in young men, prolonged work in the heat does not seem to impair whole-body heat loss or exacerbate heat storage on the following day.

Work Rate during Self-paced Exercise is not Mediated by the Rate of Heat Storage.

To date, there have been mixed findings on whether greater anticipatory reductions in self-paced exercise intensity in the heat are mediated by early differences in rate of body heat storage. The disparity may be due to an inability to accurately measure minute-to-minute changes in whole-body heat loss. Thus, we evaluated whether early differences in rate of heat storage can mediate exercise intensity during self-paced cycling at a fixed rate of perceived exertion (RPE of 16; hard-to-very-hard work effort) in COOL (15°C), NORMAL (25°C), and HOT (35°C) ambient conditions. On separate days, nine endurance-trained cyclists exercised in COOL, NORMAL, and HOT conditions at a fixed RPE until work rate (measured after first 5 min of exercise) decreased to 70% of starting values. Whole-body heat loss and metabolic heat production were measured by direct and indirect calorimetry, respectively. Total exercise time was shorter in HOT (57 ± 20 min) relative to both NORMAL (72 ± 23 min, P = 0.004) and COOL (70 ± 26 min, P = 0.045). Starting work rate was lower in HOT (153 ± 31 W) compared with NORMAL (166 ± 27 W, P = 0.024) and COOL (170 ± 33 W, P = 0.037). Rate of heat storage was similar between conditions during the first 4 min of exercise (all P > 0.05). Thereafter, rate of heat storage was lower in HOT relative to NORMAL and COOL until 30 min of exercise (last common time-point between conditions; all P < 0.05). Further, rate of heat storage was significantly higher in COOL compared with NORMAL at 15 min (P = 0.026) and 20 min (P = 0.020) of exercise. No differences were measured at end exercise. We show that rate of heat storage does not mediate exercise intensity during self-paced exercise at a fixed RPE in cool to hot ambient conditions.

Aging Impairs Whole-Body Heat Loss in Women under Both Dry and Humid Heat Stress.

This study was designed to determine whether age-related impairments in whole-body heat loss, which are known to exist in dry heat, also occur in humid heat in women. To evaluate this possibility, 10 young (25 ± 4 yr) and 10 older (51 ± 7 yr) women matched for body surface area (young, 1.69 ± 0.11; older, 1.76 ± 0.14 m, P = 0.21) and peak oxygen consumption (V˙O2peak) (young, 38.6 ± 4.6; older, 34.8 ± 6.6 mL·kg·min, P = 0.15) performed four 15-min bouts of cycling at a fixed metabolic heat production rate (300 W; equivalent to ~45% V˙O2peak), each separated by a 15-min recovery, in dry (35°C, 20% relative humidity) and humid heat (35°C, 60% relative humidity). Total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was measured as the temporal summation of heat production and loss. Total heat loss was lower in humid conditions compared with dry conditions during all exercise bouts in both groups (all P < 0.05), resulting in 49% and 39% greater body heat storage in young and older women, respectively (both P < 0.01). Total heat loss was also lower in older women compared with young women during exercise bouts 1, 2 and 3 in dry heat (all P < 0.05) and bouts 1 and 2 in humid heat (both P < 0.05). Consequently, body heat storage was 29% and 16% greater in older women compared with young women in dry and humid conditions, respectively (both P < 0.05). Increasing ambient humidity reduces heat loss capacity in young and older women. However, older women display impaired heat loss relative to young women in both dry and humid heat, and may therefore be at greater risk of heat-related injury during light-to-moderate activity.