Passive Heat Acclimation Research Articles

The Effect of Post-Exercise Warm Water Immersion on Anaerobic Power Performance in College Students

Background/Objectives: The purpose of this study was to investigate the effect of a short-term passive post-exercise heat acclimation (HA) protocol on anaerobic performance in college students. Methods: In total, 28 participants (age 22.3 ± 2.1 years, height 168 ± 9.1 cm, mass 71.5 ± 16.6 kg) volunteered and were randomly divided into either an intervention (INT) or control (CON) group. Both groups followed a consecutive two-day protocol, consisting of a 25 min submaximal cycle @ 50–60% heart rate reserve (HRR), followed by a 25 min hot water immersion (INT) or nothing (CON). All participants performed a 30 s Wingate Anaerobic Test (WAnT) on four separate occasions—familiarization, pre-intervention, acutely post-intervention, and 24 h post intervention. The WAnT measured their peak power (W), relative peak power (W/kg), mean power (W), fatigue index (%), and total work (kJ). Results: From the results of the Factorial ANOVA, it was found that there were no significant differences, with no main effect for group (INT vs. CON) or time (Trials 1–4) and no significant interaction across all measured variables (p > 0.05). Conclusions: A two-day post-exercise passive HA protocol utilizing hot water immersion had no significant effect on the measurement of anaerobic performance in college students.

The Importance of Maintaining and Improving a Healthy Gut Microbiota in Athletes as a Preventive Strategy to Improve Heat Tolerance and Acclimatization.

Exposure to passive heat (acclimation) and exercise under hot conditions (acclimatization), known as heat acclimation (HA), are methods that athletes include in their routines to promote faster recovery and enhance physiological adaptations and performance under hot conditions. Despite the potential positive effects of HA on health and physical performance in the heat, these stimuli can negatively affect gut health, impairing its functionality and contributing to gut dysbiosis. Blood redistribution to active muscles and peripheral vascularization exist during exercise and HA stimulus, promoting intestinal ischemia. Gastrointestinal ischemia can impair intestinal permeability and aggravate systemic endotoxemia in athletes during exercise. Systemic endotoxemia elevates the immune system as an inflammatory responses in athletes, impairing their adaptive capacity to exercise and their HA tolerance. Better gut microbiota health could benefit exercise performance and heat tolerance in athletes. This article suggests that: (1) the intestinal modifications induced by heat stress (HS), leading to dysbiosis and altered intestinal permeability in athletes, can decrease health, and (2) a previously acquired microbial dysbiosis and/or leaky gut condition in the athlete can negatively exacerbate the systemic effects of HA. Maintaining or improving the healthy gut microbiota in athletes can positively regulate the intestinal permeability, reduce endotoxemic levels, and control the systemic inflammatory response. In conclusion, strategies based on positive daily habits (nutrition, probiotics, hydration, chronoregulation, etc.) and preventing microbial dysbiosis can minimize the potentially undesired effects of applying HA, favoring thermotolerance and performance enhancement in athletes.

Responses of Healthy Young and Older Adults to Short-Term Passive Heat Acclimation

Context: Aging is accompanied by a reduced capacity to dissipate heat. Heat acclimation has been shown to improve heat loss capacity in young healthy adults, yet few studies have investigated the capacity of older adults to adapt to repeated bouts of heat exposure, particularly when using a passive heat acclimation protocol. Furthermore, it remains unclear whether the magnitude of adaptations is affected by age. Objective and hypothesis: To compare physiological adaptations following 7 consecutive days of passive heat exposure between healthy young and older adults. We hypothesized that heat acclimation would enhance heat loss capacity in both groups but to a higher extent in young than in older adults. Methods: Fourteen healthy young (5F/9M, 27 ± 5 years) and 9 older adults (6F/3M, 66 ± 5 years) were exposed to passive heat stress with a water-perfused suit before (pre) and after (post) 7 consecutive days of immersion in 40°C water (~30 min to reach a rectal temperature of 38.5°C, followed by 60 min ≥38.5°C). During the pre/post heat stress test, mean body temperature was computed as 0.8 × esophageal temperature + 0.2 × mean skin temperature. Local sweat rate (LSR) and cutaneous vascular conductance (CVC) were measured at the forearm level. The mean body temperature onset thresholds of LSR and CVC were determined by segmented linear regression. During acclimation visits, rectal temperature and change in body mass were measured. Data are presented as means ± standard deviation or mean differences [95% confidence interval]. Results: Immersion duration (young: 90 ± 10, older: 92 ± 9 min, p=0.53), water temperature (young: 40.3 ± 0.3, older: 40.0 ± 0.5°C, p=0.09) and mean rectal temperature (young: 38.9 ± 0.1, older: 38.8 ± 0.1°C, p=0.07) during heat acclimation did not differ significantly between groups. In response to heat acclimation, rectal temperature decreased similarly ( p=0.36) in young (-0.33°C [-0.50, -0.16]) and older adults (-0.46°C [-0.68, -0.25]) from day 1 to day 7 whereas sweat rate increased to a higher extent ( p=0.05) in young (+511 mL/h [285, 738]) compared with older adults (+232 mL/h [112, 352]). Heat acclimation similarly reduced the mean body temperature onset threshold for CVC in young (-0.18°C [-0.32, -0.05]) and older adults (-0.19°C [-0.36, -0.02]) ( p=0.91), as well as, the mean body temperature onset threshold for LSR (young: -0.13°C [-0.25, -0.01], older: -0.15°C [-0.30, 0.00], p=0.84) during the pre/post heat stress test. Conclusion: These results suggest that passive heat acclimation improves heat loss capacity in young and older adults. However, age may blunt the increase in whole-body sweat rate that accompanies passive heat acclimation, but not other classical markers of human heat acclimation. Natural Sciences and Engineering Research Council of Canada. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Heating Up to Keep Cool: Benefits and Persistence of a Practical Heat Acclimation Protocol in Elite Female Olympic Team-Sport Athletes

Although recommendations for effective heat acclimation (HA) strategies for many circumstances exist, best-practice HA protocols specific to elite female team-sport athletes are yet to be established. Therefore, the authors aimed to investigate the effectiveness and retention of a passive HA protocol integrated in a female Olympic rugby sevens team training program. Twelve elite female rugby sevens athletes undertook 10 days of passive HA across 2 training weeks. Tympanic temperature (TTymp), sweat loss, heart rate, and repeated 6-second cycling sprint performance were assessed using a sport-specific heat stress test Pre-HA, after 3days (Mid-HA), after 10days (Post-HA), and 15 days post-HA (Decay). Compared with Pre-HA, submaximal TTymp was lower Mid-HA and Post-HA (both by -0.2 [0.7] °C; d ≥ 0.71), while resting TTymp was lower Post-HA (by -0.3 [0.2] °C; d = 0.81). There were no differences in TTymp at Decay compared with Pre-HA, nor were there any differences in heart rate or sweat loss at any time points. Mean peak 6-second power output improved Mid-HA and Post-HA (76 [36]W; 75 [34]W, respectively; d ≥ 0.45) compared with Pre-HA. The observed performance improvement persisted at Decay by 65 (45) W (d = 0.41). Ten days of passive HA can elicit some thermoregulatory and performance benefits when integrated into a training program in elite female team-sport athletes. However, such a protocol does not provide a sufficient thermal impulse for thermoregulatory adaptations to be retained after 15 days with no further heat stimulus.

Heart Rate and Body Temperature Evolution in an Interval Program of Passive Heat Acclimation at High Temperatures (100 ± 2 °C) in a Sauna.

Heat exposure provokes stress on the human body. If it remains constant, it leads to adaptations such as heat acclimation. This study aims to observe the evolution of heart rate (HR), core temperature (Tcore), and skin temperature (Tskin) in an intervallic program of exposure to extreme heat. Twenty-nine healthy male volunteers were divided into a control group (CG; n = 14) and an experimental group (EG; n = 15). EG experienced nine sessions (S) of intervallic exposure to high temperatures (100 ± 2 °C), whereas CG was exposed to ambient temperatures (22 ± 2 °C). HR, Tskin, and Tcore were monitored in S1, 4, 5, 8, and 9. An important increase in HR occurred in the S4 compared to the rest (p < 0.05) in EG. A lower HR was discovered in S8 and S9 compared to S4 and in S9 in relation to S1 (p < 0.05) in EG. EG experiences a gradual decrease in Tcore and Tskin, which was detected throughout the assessments, although it was only significant in the S8 and S9 (p < 0.05). Interval exposure to heat at 100 ± 2 °C elicits stress on the human organism, fundamentally increasing Tcore, Tskin, and FC. This recurring stress in the full program caused a drop in the thermoregulatory response as an adaptation or acclimation to heat.

Impact of passive heat stress and passive heat acclimation on circulating extracellular vesicles: An exploratory analysis.

What is the central question of this study? How does passive heat stress and subsequent heat acclimation affect the circulating concentration of extracellular vesicles? What is the main finding and its importance? Passive heat stress increased the circulating concentration of total and platelet extracellular vesicles. Seven days of hot water immersion did not modify the change in circulating concentrations of extracellular vesicles during passive heat stress. This retrospective exploratory analysis aimed to improve our understanding of the effect of passive heat stress and subsequent heat acclimation on the circulating concentration of extracellular vesicles (EVs). Healthy young adults (four females and six males, 25±4 years of age, 1.72±0.08m in height and weighing 71.6±9.0kg) were heated with a water-perfused suit before and after seven consecutive days of hot water immersion. Pre-acclimation, participants were heated until oesophageal temperature increased to ∼1.4°C above baseline values. Post-acclimation, participants were heated until oesophageal temperature reached the same absolute value as the pre-acclimation visit (∼38.2°C). Venous blood samples were obtained before and at the end of passive heating to quantify plasma concentrations of EVs from all cell types (CSFE+ ), all cell types except erythrocytes (CSFE+ MHCI+ ), platelets (CSFE+ MHCI+ CD41+ ), endothelial cells (CSFE+ MHCI+ CD62e+ ), red blood cells (CSFE+ CD235a+ ) and leucocytes (CSFE+ MHCI+ CD45+ ) via flow cytometry. Passive heat stress increased the concentration of CFSE+ EVs (46,150,000/ml [3,620,784, 88,679,216], P=0.036), CFSE+ MHCI+ EVs (28,787,500/ml [9,851,127, 47,723,873], P=0.021) and CSFE+ MHCI+ CD41+ EVs (28,343,500/ml [9,637,432, 47,049,568], P=0.008). The concentration of CSFE+ MHCI+ CD62e+ EVs (94,230/ml [-55,099, 243,559], P=0.187), CSFE+ CD235a+ EVs (-1,414/ml [-15,709, 12,882], P=0.403) or CSFE+ MHCI+ CD45+ EVs (-192,915/ml [-690,166, 304,336], P=0.828) did not differ during heat stress. The change in circulating EVs during passive heat stress did not differ after heat acclimation (thermal state × acclimation interactions, all P≥0.180). These results demonstrate that passive heat stress increases the circulating concentration of total and platelet EVs and that passive heat acclimation does not alter this increase.

Post-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military context

To mitigate the effects of heat during operations in hot environments, military personnel will likely benefit from heat acclimation (HA) conducted prior to deployment. Using post-exercise, passive heating, 25 participants completed a 5 d HA regime in sauna (70 °C, 18% RH) or hot-water immersion (HWI) (40 °C) for ≤40 min, preceded and followed by a heat stress test (1-h walking at 5 km.h−1 in 33 °C, 77% RH in military uniform (20 kg) before an incremental ramp to exhaustion). Fifteen completed both regimes in a randomised, cross-over manner. While performance did not significantly improve (+14%, [-1, 29], p = .079), beneficial adaptations were observed for mean exercising core temperature (-0.2 °C, [-0.2, −0.2], p <.001), skin temperature (-0.2 °C, [-0.2, −0.2], p = 035) and heart rate (-8 bpm, [-6, −10], p<.001) in both conditions. Post-exercise, passive HA of either modality may benefit military units operating in the heat. Practitioner summary: Strategies are required to prevent health and performance impairments during military operations upon arrival in hot environments. Using a randomised, cross-over design, participants completed five-day passive, post-exercise heat acclimation using sauna or hot-water immersion. Both regimes elicited beneficial albeit modest heat adaptations. Abbreviations: HA: heat acclimation; HST: heat stress test; HWI: hot-water immersion; RH: relative humidity

Passive heat acclimation does not modulate processing speed and executive functions during cognitive tasks performed at fixed levels of thermal strain.

This study evaluated if passive controlled hyperthermia heat acclimation modulates cognitive performance during passive heat stress. Eight healthy adults (25± 4years) underwent 7 consecutive days of hot water immersion (core temperature ≥38.6°C) and a 7-day time-control period. On days 1 and 7 of heat acclimation, participants performed a digital Stroop test at baseline, when core temperature reached 38.6°C, and after 60minutes at a core temperature ≥38.6°C to evaluate reaction time during tasks targeting processing speed (reading and counting) and executive functions (inhibition and switching). On days 1 and 7 of the time-control intervention, participants performed the Stroop test with equivalent amounts of time separating each task as for heat acclimation. During day 1 of heat acclimation, reaction time was quicker during the reading (-44ms [-71 to -17], P< 0.01) and counting (-39ms [-76 to -2], P= 0.04) tasks when the rectal temperature reached 38.6°C, but after a further 60minutes of heat exposure, reaction time only remained quicker during the reading task (-56ms [-83 to -29], P< 0.01). Changes in reaction time during heat exposure were unaffected by subsequent heat acclimation (interaction, all P≥ 0.09). In conclusion, 7 days of heat acclimation does not modulate processing speed and executive functions during passive heat exposure. Novelty: Whether heat acclimation improves cognitive performance during heat exposure remains unclear. We tested the hypothesis that heat acclimation modulates reaction time during cognitive tasks performed at matched levels of thermal strain. Despite the classical signs of heat acclimation, reaction time during heat exposure is unaffected by heat acclimation.

3-Week passive acclimation to extreme environmental heat (100± 3°C) in dry sauna increases physical and physiological performance among young semi-professional football players.

This manuscript aims to evaluate the influence of a novel passive heat acclimation program among human participants in the physical performance, as well as in several physiological parameters. 36 male football players were acclimated using a dry sauna bath to extreme hot (100±3°C), performing a total of nine sauna sessions with a weekly frequency of three sessions. The players were randomly into the sauna group (SG; n=18; age: 20.69±2.09 years) and the control group (CG; n=18; age: 20.23±1.98 years). All participants performed maximal effort test until exhaustion as well as hamstring flexibility test before and after the acclimation program. Anthropometric, respiratory, circulatory, hematological and physiological variables were evaluated at the beginning and at the end of the survey. Statistical analysis consisted of a Mann-Whitney U test to determine differences between groups at the beginning and at the end of the survey and a Wilcoxon test for paired samples to compare the differences for each group separately. Additionally, size effects of the pre-post acclimation changes were calculated. After the acclimation program SG participants experienced a diminution in body weight (p<0.01), body mass index (p<0.01), body fat (p<0.05) and fat percentage (p<0.05) decreased. Hamstring flexibility (p<0.05) and work capacity (p<0.05) increased. External basal temperature decreased (p<0.05) as well as post-exercise systolic and diastolic blood pressures (p<0.05). Finally, maximal oxygen uptake (ml Kg-1 min-1) (p<0.05), maximal minute ventilation (p<0.05) and maximal breath frequency (p<0.05) increased at the end of the intervention. There were no significant changes in the CG in any variable. Favorable adaptations have been observed in this survey, suggesting a beneficial effect of extreme heat acclimation on physical performance. Several of the observed responses seem interesting for sport performance and health promotion as well. However, this is a novel, extreme protocol which requires further research.

Three weeks of passive and intervallic heat at high temperatures (100±2 °C) in a sauna improve acclimation to external heat (42±2 °C) in untrained males

Currently, the effect of passive heat acclimation on aerobic performance is still controversial. Therefore, this study aimed to observe the effect of passive and intervallic exposure to high temperatures (100±2°C) in untrained males. Forty healthy untrained men participated in this investigation. They were randomised into a Control Group (CG; n=18) and an Experimental Group (EG; n=22). Both groups performed maximum incremental tests until exhaustion in normothermia (GXT1; 22±2°C), and 48h afterwards, in hyperthermia (GXT2; 42±2°C). The EG performed 9 sessions of intervallic exposure to heat (100±2°C) over 3 weeks. Subsequently, both groups performed two maximal incremental trials in normothermia (GXT3; 22±2°C) and 48h later, in hyperthermia (GXT4; 42±2°C). In each test, the maximal ergospirometric parameters and the aerobic (VT1), anaerobic (VT2) and recovery ventilatory thresholds were recorded. The Wilcoxon Test was used for intra-group comparisons and the Mann-Whitney U for inter-group comparisons. There were improvements in absolute VO2max (p=0.049), W (p=0.005) and O2pulse (p=0.006) in hyperthermia. In VT1 there was an increase in W (p=0.046), in VO2 in absolute (p=0.025) and relative (p=0.013) values, O2pulse (p=0.006) and VE (p=0.028) in hyperthermia. While W increased in hyperthermia (p=0.022) at VT2. The results suggest that passive and intervallic acclimation at high temperatures improves performance in hyperthermia. This protocol could be implemented in athletes when they have to compete in hot environments.

Effectiveness of skin-heating using a water-perfused suit as passive and post-exercise heat acclimation strategies.

The purpose of the present study was to evaluate the effectiveness of passive and post-exercise heat acclimation strategies through directly heating the skin with a water-perfused suit. Nineteen young males participated in the heat acclimation (HA) protocols for 10 days, which were conducted at an air temperature of 33oC with 60%RH. The exercise-only condition (N=6) conducted 1-h treadmill walking (6km·h-1) followed by 1-h rest. The post-exercise passive-heating condition (N=6) wore the suit (inflow water temperature 44.2oC) for 1-h after 1-h walking. The passive-heating condition (N=7) donned the suit for 2h. Heat tolerance tests (leg immersion in 42oC water for 60min) were conducted before and after the training to evaluate changes due to the 10-day intervention. Reflecting that suit-wearing for 10 days as both passive and post-exercise HA strategies can effectively induce adaptive changes, significant interaction effects appeared in: increase or decrease in mean skin temperature (P<0.05) and elevation in whole-body sweat rate (P<0.05). Reduction in rectal temperature (P<0.05) and blood pressure (P<0.05) were found most prominently in the passive-heating condition. These results indicate that this new method of heat acclimation training, donning a skin-heating water-perfused suit, can generate thermoregulatory benefits. The passive HA intervention could be applied to individuals for whom doing exercise regularly are not feasible.

Cardiac function during heat stress: impact of short-term passive heat acclimation.

A lower heart rate (HR) during heat exposure is a classic marker of heat acclimation (HA), although it remains unclear whether this adaptation occurs secondary to reduced thermal strain and/or improvements in cardiac function. We evaluated the hypothesis that short-term passive HA reduces HR and improves cardiac function during passive heating. Echocardiography was performed under thermoneutral and hyperthermic conditions in 10 healthy adults (9 men/1 woman, 29 ± 6 yr old), pre and post 7 days of controlled hyperthermia. HR (P = 0.61), stroke volume (P = 0.99), and cardiac output (P = 0.99), were similar on days 1 and 7 of HA. Core (pre: 38.17 ± 0.42, post: 38.15 ± 0.27°C, P = 0.95) and mean skin (pre: 38.24 ± 0.41, post: 38.51 ± 0.29°C, P = 0.17) temperatures were similar during hyperthermic echocardiographic assessments. Cardiac systolic function was unaffected by HA (P ≥ 0.10). HA attenuated the decrease in end-diastolic volume (pre: -18 ± 18, post: -12 ± 19 mL, P = 0.05), accentuated the greater atrial contribution to diastolic filling (pre: +11 ± 5, post: +14 ± 5%, P = 0.02), and attenuated the increase in left atrial reservoir strain rate (pre: +1.5 ± 1.2, post: +0.8 ± 0.8 1/s, P = 0.02) during heating. Nonetheless, there were no differences in HR (pre: 106 ± 12, post: 104 ± 12 beats/min, P = 0.50), stroke volume (pre: 65 ± 15, post: 68 ± 13 mL, P = 0.55), or cardiac output (pre: 6.9 ± 2.0, post: 7.1 ± 1.7 L/min, P = 0.70) during passive heating. Short-term controlled hyperthermia HA results in limited adaptations of cardiac function during passive heating.NEW & NOTEWORTHY A lower heart rate during heat exposure is a classic marker of heat acclimation (HA). It remains unknown if improved cardiac function contributes to this response. A 7-day passive HA protocol did not alter cardiac systolic function during passive heating, whereas it improved some indexes of diastolic function in young adults. Nonetheless, heart rate during heating was unaffected by HA. These results suggest that passive HA induces limited adaptations in cardiac function during passive heating.

Impact of passive heat acclimation on markers of kidney function during heat stress.

What is the central question of this study? Does passive heat acclimation alter glomerular filtration rate and urine-concentrating ability in response to passive heat stress? What is the main finding and its importance? Glomerular filtration rate remained unchanged after passive heat stress, and heat acclimation did not alter this response. However, heat acclimation mitigated the reduction in urine-concentrating ability and reduced the incidence of albuminuria in young healthy adults after passive heat stress. Collectively, these results suggest that passive heat acclimation might improve structural integrity and reduce glomerular permeability during passive heat stress. Little is known about the effect of heat acclimation on kidney function during heat stress. The purpose of this study was to determine the impact of passive heat stress and subsequent passive heat acclimation on markers of kidney function. Twelve healthy adults (seven men and five women; 26±5 years of age; 72.7±8.6kg; 172.4±7.5cm) underwent passive heat stress before and after a 7day controlled hyperthermia heat acclimation protocol. The impact of passive heat exposure on urine and serum markers of kidney function was evaluated before and after heat acclimation. Glomerular filtration rate, determined from creatinine clearance, was unchanged with passive heat stress before (pre, 133±41mlmin-1 ; post, 127±51mlmin-1 ; P=0.99) and after (pre, 129±46mlmin-1 ; post, 130±36mlmin-1 ; P=0.99) heat acclimation. The urine-to-serum osmolality ratio was reduced after passive heating (P<0.01), but heat acclimation did not alter this response. In comparison to baseline, free water clearance was greater after passive heating before (pre, -0.86±0.67mlmin-1 ; post, 0.40±1.01mlmin-1 ; P<0.01) but not after (pre, -0.16±0.57mlmin-1 ; post, 0.76±1.2mlmin-1 ; P=0.11) heat acclimation. Furthermore, passive heating increased the fractional excretion rate of potassium (P<0.03) but not sodium (P=0.13) or chloride (P=0.20). Lastly, heat acclimation reduced the fractional incidence of albuminuria after passive heating (before, 58±51%; after, 8±29%; P=0.03). Collectively, these results demonstrate that passive heat stress does not alter the glomerular filtration rate. However, heat acclimation might improve urine-concentrating ability and filtration within the glomerulus.

Passive exposure to heat improves glucose metabolism in overweight humans.

AimHeat exposure has been indicated to positively affect glucose metabolism. An involvement of heat shock protein 72 (HSP72) in the enhancement of insulin sensitivity upon heat exposure has been previously suggested. Here, we performed an intervention study exploring the effect of passive heat acclimation (PHA) on glucose metabolism and intracellular (a) HSP72 concentrations in overweight humans.MethodsEleven non‐diabetic overweight (BMI 27‐35 kg/m2) participants underwent 10 consecutive days of PHA (4‐6 h/day, 34.4 ± 0.2°C, 22.8 ± 2.7%RH). Before and after PHA, whole‐body insulin sensitivity was assessed using a one‐step hyperinsulinaemic‐euglycaemic clamp, skeletal muscle biopsies were taken to measure intracellular iHSP72, energy expenditure and substrate oxidation were measured using indirect calorimetry and blood samples were drawn to assess markers of metabolic health. Thermophysiological adaptations were measured during a temperature ramp protocol before and after PHA.ResultsDespite a lack of change in iHSP72, 10 days of PHA reduced basal (9.7 ± 1.4 pre‐ vs 8.4 ± 2.1 μmol · kg–1 · min–1 post‐PHA, P = .038) and insulin‐stimulated (2.1 ± 0.9 pre‐ vs 1.5 ± 0.8 μmol · kg–1 · min–1 post‐PHA, P = .005) endogenous glucose production (EGP) and increased insulin suppression of EGP (78.5 ± 9.7% pre‐ vs 83.0 ± 7.9% post‐PHA, P = .028). Consistently, fasting plasma glucose (6.0 ± 0.5 pre‐ vs 5.8 ± 0.4 mmol/L post‐PHA, P = .013) and insulin concentrations (97 ± 55 pre‐ vs 84 ± 49 pmol/L post‐PHA, P = .026) decreased significantly. Moreover, fat oxidation increased, and free fatty acids as well as cholesterol concentrations and mean arterial pressure decreased after PHA.ConclusionOur results show that PHA for 10 days improves glucose metabolism and enhances fat metabolism, without changes in iHSP72. Further exploration of the therapeutic role of heat in cardio‐metabolic disorders should be considered.

Adaptive changes in physiological and perceptual responses during 10-day heat acclimation training using a water-perfused suit

BackgroundWhile active heat acclimation strategies have been robustly explored, not many studies highlighted passive heat acclimation strategies. Particularly, little evidence demonstrated advantages of utilizing a water-perfused suit as a passive heating strategy. This study aimed to explore heat adaptive changes in physiological and perceptual responses during 10-day heat acclimation training using a water-perfused suit.MethodsNineteen young males were divided into three experimental groups: exercise condition (N = 6, HAEXE, 1-h exercise at 6 km h−1 followed by 1-h rest in a sitting position), exercise and passive heating condition (N = 6, HAEXE+SUIT, 1-h exercise at 6 km h−1 followed 1-h passive heating in a sitting position), and passive heating condition (N = 7, HASUIT, 2-h passive heating in a sitting position). All heating programs were conducted for 10 consecutive days in a climatic chamber maintained at 33 °C with 60% relative humidity. The passive heating was conducted using a newly developed water-perfused suit with 44 °C water.ResultsGreater whole-body sweat rate and alleviated perceptual strain were found in HASUIT and HAEXE+SUIT after 5 and/or 10 days (P < 0.05) but not in the exercise-only condition (HAEXE). Lower rectal temperature and heart rate were found in all conditions after the training (P < 0.05). Heat adaptive changes appeared earlier in HASUIT except for sweat responses.ConclusionsFor heat acclimation in hot humid environments, passive and post-exercise heat acclimation training using the suit (water inflow temperature 44 °C) were more effective than the mild exercise (1-h walking at 6 km h−1). This form of passive heating (HASUIT) may be an especially effective strategy for the elderly and the disabled who are not able to exercise in hot environments.

Heart Rate Variability during Heat Exposure is not Affected by Short‐term Passive Heat Acclimation in Young Healthy Participants

INTRODUCTIONHeat acclimation attenuates the increase in heart rate during heat exposure. This heat acclimation‐induced bradycardia has been associated with increased stroke volume due to blood volume expansion. However, exercise‐induced heat acclimation also modulates cardiac parasympathetic activity, as measured through heart rate variability (HRV). Therefore, it is possible that modulation of cardiac autonomic activity could contribute to the effect of heat acclimation on heart rate during heat exposure. The purpose of this study was to determine if HRV during heat exposure is modified by short‐term passive heat acclimation.METHODSFifteen healthy adults (27 ± 5 years, 5 females/10 males) underwent heat exposure in a water‐perfused suit to increase esophageal temperature (Tes) by 1.2°C, before and after a 7‐day controlled hyperthermia protocol. During heat exposure, R‐R interval was measured continuously with a 5‐lead electrocardiogram and used to calculate heart rate (HR), mean normal‐to‐normal intervals (NN), standard deviation of NN intervals (SDNN), root mean square of successive NN interval differences (RMSSD), low‐frequency (LF) and high‐frequency spectral power (HF) and the LF‐to‐HF power ratio (LF/HF). The acclimation protocol consisted of hot water immersion to elevate and subsequently maintain rectal temperature ≥38.6°C for 60 minutes once a day for 7 consecutive days. Variables were compared between the pre and post acclimation visit at baseline and at increases in Tes of 0.6°C and 1.2°C.RESULTSHeat acclimation lowered resting Tes (36.81 ± 0.29 to 36.53 ± 0.32°C, P&lt;0.001) and, consequently absolute Tes during heat exposure. When analyzed as a function of the change in Tes from baseline, no difference was observed for HR (P=0.072) at baseline (59 ± 10 vs. 57 ± 9 bpm), +0.6°C (88 ± 12 vs. 86 ± 11 bpm), and +1.2°C (103 ± 13 to 99 ± 12 bpm). Similarly, NN (baseline: 1049 ± 172 vs. 1081 ± 162 ms, +0.6°C: 691 ± 104 vs. 705 ± 100 ms, +1.2°C: 595 ± 74 vs. 600 ± 96 ms, P=0.295), SDNN (baseline: 84 ± 38 vs. 90 ± 37 ms, +0.6°C: 43 ± 15 vs. 45 ± 15 ms, +1.2°C: 28 ± 9 vs. 31 ± 8 ms, P=0.294) and RMSSD (baseline: 83 ± 49 vs. 93 ± 57 ms, +0.6°C: 19 ± 9 vs. 19 ± 14 ms, +1.2°C: 9 ± 4 vs. 9 ± 3 ms, P=0.336) did not differ after heat acclimation. No effect of heat acclimation was also observed for LF (P=0.339), HF (P=0.509) and LF/HF ratio (P=0.449).CONCLUSIONSThese results suggest that short‐term, passive and isothermic heat acclimation does not modulate cardiac autonomic activity during passive heat exposure in healthy young adults.Support or Funding InformationNatural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation and Montreal Heart Institute Foundation.

Oxygen Consumption and Substrate Oxidation Rates Following Passive Acclimation to a Hot Environment

IntroductionAcclimation to a hot environment via prolonged or repeated, daily exposure to a heat stress results in adaptations of the human thermoregulatory responses. In this study, it was investigated whether metabolic responses to passive heat acclimation (HA) adapted similarly to thermoregulatory heat loss responses including following a 10‐day, 2 h·day−1 passive heat acclimation (HA) to 50°C and 20 % RH. This study was approved by the Simon Fraser University Office of Research Ethics.PurposeIt was hypothesized that, similar to that seen during active heat acclimation with exercise, there would be an elevation of oxygen consumption following a passive heat acclimation. It was also hypothesized that following heat acclimation there would be increased lipid oxidation relative to the pre‐acclimation state.MethodsNine male participants performed two immersions in a 40°C bath separated by a 10‐day passive heat acclimation. Heat acclimation was performed in a climatic chamber controlled at 50°C and 20 % RH where participants remained seated at rest for 2 hr·day−1 with their rectal temperature maintained between 38.5 – 39.0°C. A two‐way repeated measures ANOVA with factors of Acclimation State (Pre and Post) and delta TES (0 to 1.4°C above rest) was employed with post hoc analysis completed with two‐tailed, paired t‐tests. The significance level was p&lt;0.05.ResultsHA was confirmed by a significant decrease in resting esophageal temperature (TES) from 37.60 ± 0.19 (mean ± SD) to 37.31 ± 0.11°C (p=0.001). During the 40°C immersions significant decreases in the TES thresholds for temple cutaneous blood velocity from 37.34 ± 0.12 to 37.14 ± 0.16°C (p=0.008) and for forehead eccrine sweating from 37.61 ± 0.21 to 37.24 ± 0.24°C (p=0.007) also confirmed heat acclimation. HA did not influence oxygen consumption from 0.21 ± 0.07 to 0.22 ± 0.07 L/min (p=0.15). Carbohydrate oxidation rate increased from 0.21 ± 0.21 to 0.28 ± 0.26 g·min−1 (p=0.002). Lipid oxidation rate decreased from 1.16 ± 0.22 to 1.09 ± 0.24 g·min−1 (p&lt;0.001).ConclusionFollowing a passive heat acclimation the evidence reports no change in oxygen consumption, and contrary to that hypothesized, there was a decrease in lipid oxidation rate and an increase in carbohydrate oxidation rate.Support or Funding Information Funding SourceThis project was supported by Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.

Thermal Hyperpnea in Humans Changes Following Passive Heat Acclimation

IntroductionExercise ventilation was shown to increase after a 10‐day, 2 h·day−1 passive heat acclimation (HA) to 50°C and 20 % RH and this study assessed if thermal hyperpnoea for those at rest show a similar adaptation.PurposeIt was hypothesized that similar to that seen for eccrine sweating and cutaneous blood flow, the core temperature thresholds for human pulmonary ventilation would decrease after heat acclimation. It was also hypothesized that pulmonary ventilation would increase following heat acclimation as it does for an active heat acclimation.MethodsSix male participants performed two 40°C immersions separated by a 10‐day passive heat acclimation. Heat acclimation included 10 consecutive days with participants’ rectal temperatures maintained between 38.5–39.0°C. The heat acclimation was performed in a climatic chamber at 50°C and 20 % RH where participants remained seated at rest for 2 hr·day−1. A repeated measures ANOVA was employed with factors of Acclimation State (Pre &amp; Post) and delta TES (0 to 1.4°C from rest). The p value was set at 0.05. An ANOVA with oxygen consumption as the covariate was employed to assess for heat acclimation‐induced changes in pulmonary ventilation. This study was approved by the SFU Office of Research Ethics.ResultsAcclimation was confirmed by a decrease in resting esophageal temperature (TES) from 37.70 ± 0.19 (mean ± SD) to 37.31 ± 0.11°C (p=0.001), as well as by significant decreases in the TES thresholds for temple cutaneous blood velocity from 37.30 ± 0.12 to 37.07 ± 0.13°C (p=0.014) and for forehead eccrine sweating from 37.60 ± 0.25 to 37.25 ± 0.22°C (p=0.03). The pulmonary ventilation thresholds were 38.28 ± 0.28 before and 37.84 ± 0.44°C after heat acclimation (p=0.13). After accounting for the covariate of oxygen consumption, a significant main effect of Acclimation State (F=10.05, p=0.025) plus an acclimation state x delta TES interaction for pulmonary ventilation (F=3.6, p=0.008) were explained by a significantly greater pulmonary ventilation following acclimation at delta TES of 0.6, 0.8, 1.0°C (0.018 &lt;p &lt; 0.05).ConclusionFollowing a passive heat acclimation the resting TES and TES thresholds for human thermoregulatory responses were significantly lower and pulmonary ventilation was significantly increased during passive heating as indicative of an increased thermal hyperpnea.Support or Funding Information Funding SourceThis project was supported by Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.

Mixed Active and Passive, Heart Rate-Controlled Heat Acclimation Is Effective for Paralympic and Able-Bodied Triathletes.

Purpose: The aims of this study are to explore the effectiveness of mixed active and passive heat acclimation (HA), controlling the relative intensity of exercise by heart rate (HR) in paratriathletes (PARA), and to determine the adaptation differences to able-bodied (AB) triathletes.Methods: Seven elite paratriathletes and 13 AB triathletes undertook an 8-day HA intervention consisting of five HR-controlled sessions and three passive heat exposures (35°C, 63% relative humidity). On the first and last days of HA, heat stress tests were conducted, whereby thermoregulatory changes were recorded during at a fixed, submaximal workload. The AB group undertook 20 km cycling time trials pre- and post-HA with performance compared to an AB, non-acclimated control group.Results: During the heat stress test, HA lowered core temperature (PARA: 0.27 ± 0.32°C; AB: 0.28 ± 0.34°C), blood lactate concentration (PARA: 0.23 ± 0.15 mmol l−1; AB: 0.38 ± 0.31 mmol l−1) with concomitant plasma volume expansion (PARA: 12.7 ± 10.6%; AB: 6.2 ± 7.7%; p ≤ 0.047). In the AB group, a lower skin temperature (0.19 ± 0.44°C) and HR (5 ± 6 bpm) with a greater sweat rate (0.17 ± 0.25 L h−1) were evident post-HA (p ≤ 0.045), but this was not present for the PARA group (p ≥ 0.177). The AB group improved their performance by an extent greater than the smallest worthwhile change based on the normal variation present with no HA (4.5 vs. 3.7%).Conclusions: Paratriathletes are capable of displaying partial HA, albeit not to same extent as AB triathletes. The HA protocol was effective at stimulating thermoregulatory adaptations with performance changes noted in AB triathletes.

The Influence of Heat Acclimation and Hypohydration on Post-Weight-Loss Exercise Performance.

To examine the influence of fluid intake on heat acclimation and the subsequent effects on exercise performance following acute hypohydration. Participants were randomly assigned to 1 of 2 groups, either able to consume water ad libitum (n = 10; age 23 [3]y, height 1.81 [0.09]m, body mass 87 [13]kg; HAW) or not allowed fluid (n = 10; age 26 [5]y, height 1.76 [0.05]m, body mass 79 [10]kg; HANW) throughout 12 × 1.5-h passive heat-acclimation sessions. Experimental trials were completed on 2 occasions before (2 baseline trials) and 1 following the heat-acclimation sessions. These sessions involved 3h of passive heating (45°C, 38% relative humidity) to induce hypohydration followed by 3h of ad libitum food and fluid intake after which participants performed a repeat sled-push test to assess physical performance. Urine and blood samples were collected before, immediately, and 3h following hypohydration to assess hydration status. Mood was also assessed at the same time points. No meaningful differences in physiological or performance variables were observed between HANW and HAW at any time point. Using pooled data, mean sprint speed was significantly (P < .001) faster following heat acclimation (4.6 [0.7]s compared with 5.1 [0.8]s). Furthermore, heat acclimation appeared to improve mood following hypohydration. Results suggest that passive heat-acclimation protocols may be effective at improving short-duration repeat-effort performance following acute hypohydration.