Passive exposure to heat improves glucose metabolism in overweight humans

A double-edged sword: risks and benefits of heat for human health

Extreme heat events will become more frequent and intense across the globe. In this science and society article we summarize how heat affects our body and discuss the associated health threats, but also the potential health benefits of heat exposure. Moreover, we provide practical suggestions for sustainable and health-oriented strategies to cope with heat.

Influences of Indoor Air Temperatures on Empathy and Positive Affect

The consequences of climate change are already visible, and yet, its effect on psychosocial factors, including the expression of empathy, affect, and social disconnection, is widely unknown. Outdoor conditions are expected to influence indoor conditions. Therefore, the aim of this study was to investigate the effect of indoor air temperature during work hours on empathy, positive and negative affect, and social disconnection. Participants (N = 31) were exposed, in a cross-over design, to two thermal conditions in a simulated office environment. Questions on empathy and social disconnection were administered before and after the exposure to each condition, while affect was measured throughout the day. Subjective thermal sensation and objective measures of mean skin temperature were considered. The results indicated a significant difference in empathy (F(1, 24) = 5.37, p = 0.03, with an η2 = 0.126) between conditions. Participants reported increases in empathy after exposure to the warm condition compared to the cool condition, in which reductions in empathy were reported. Although the same pattern was observed for positive affect, the difference was smaller and the results were not significant. Thermal sensation had a significant effect on changes in empathy too (F(1, 54) = 7.015, p = 0.01, with an R2 = 0.115), while mean skin temperature had no effect on empathy (F(1, 6) = 0.53, p = 0.89, with an R2 = 0.81). No effects were observed for positive and negative affect and social disconnection. Longitudinal studies are needed to support these findings.

Impact of ambient temperature, diurnal temperature range on hyperventilation syndrome emergency admission: a time-series analysis in Beijing, China

OBJECTIVES

To assess the association between ambient temperature and diurnal temperature range (DTR) on emergency admissions for hyperventilation syndrome (HVS).

DESIGN

Distributed lag non-linear model design was used with a lag time to 5 days.

SETTING

Emergency admission data used were from the Beijing Red Cross Emergency Centre (2017-2018).

PARTICIPANTS AND EXPOSURE

Cases were those with emergency visits to the Beijing Emergency Center during the period 2017-2018 and who were given the primary outcome indicator defined as HVS according to the International Classification of Diseases, 10th edition code F45.303. Ambient temperature and DTR were used as exposure factors with adjustments for relative humidity, wind speed, precipitation, seasonality long-term trend and day of the week.

MAIN OUTCOME MEASURE

We used the minimum emergency visits temperature as a reference to indicate the relative risk with 95% CI of exposure-response for the risk of HVS visits at different temperatures.

RESULTS

A u-shape was described between ambient temperature and HVS visits, with a minimum risk at 12°C. Moderate heat (23°C) at lag (0-3) days, extreme heat at lag 0 days, had greatest relative risks on HVS visits, with 2.021 (95% CI 1.101 to 3.71) and 1.995 (95% CI 1.016 to 3.915), respectively. A stronger association between HVS visits and temperature was found in women and aged ≤44 years. Notably, the relationship between DTR and HVS visits appeared a reverse u-shaped. Low DTR (4°C) effect appeared at lag (0-1) days with 0.589 (95% CI 0.395 to 0.878), lasting until lag (0-3) days with 0.535 (95% CI 0.319 to 0.897) and was associated with a reduced risk of HVS visits in women and those aged ≤44 years.

CONCLUSIONS

Ambient temperature and DTR were associated with HVS visits, appearing a differentiation in gender and age groups. Timely prevention strategies during high temperatures and control mild changes in temperature might reduce the risk of HVS.

The effect of repeated hot water immersion on insulin sensitivity, heat shock protein 70, and inflammation in individuals with type 2 diabetes mellitus

Repeated hot water immersion (HWI) can improve glycemic control in healthy individuals but data are limited for individuals with type 2 diabetes mellitus (T2DM). The present study investigated whether repeated HWI improves insulin sensitivity and inflammatory status and reduces plasma ([extracellular heat shock protein 70]) [eHSP70] and resting metabolic rate (RMR). Fourteen individuals with T2DM participated in this pre- versus postintervention study, with outcome measures assessed in fasted (≥12 h) and postprandial (2-h post-75 g glucose ingestion) states. HWI consisted of 1 h in 40°C water (target rectal temperature 38.5°C-39°C) repeated 8-10 times within a 14-day period. Outcome measures included insulin sensitivity, plasma [glucose], [insulin], [eHSP70], inflammatory markers, RMR, and substrate utilization. The HWI intervention increased fasted insulin sensitivity (QUICKI; P = 0.03) and lowered fasted plasma [insulin] (P = 0.04), but fasting plasma [glucose] (P = 0.83), [eHSP70] (P = 0.08), [IL-6] (P = 0.55), [IL-10] (P = 0.59), postprandial insulin sensitivity (P = 0.19), plasma [glucose] (P = 0.40), and [insulin] (P = 0.47) were not different. RMR was reduced by 6.63% (P < 0.05), although carbohydrate (P = 0.43) and fat oxidation (P = 0.99) rates were unchanged. This study shows that 8-10 HWIs within a 14-day period improved fasting insulin sensitivity and plasma [insulin] in individuals with T2DM, but not when glucose tolerance is challenged. HWI also improves metabolic efficiency (i.e., reduced RMR). Together these results could be clinically important and have implications for metabolic health outcomes and well-being in individuals with T2DM.NEW & NOTEWORTHY This is the first study to investigate repeated HWI to raise deep body temperature on insulin sensitivity, inflammation, eHSP70, and substrate utilization in individuals with T2DM. The principal novel findings were improvements in fasting insulin sensitivity and fasting plasma [insulin] but no change in fasting plasma [glucose], postprandial insulin sensitivity, plasma [insulin], or [glucose]. There was also no change in eHSP70, inflammatory status, or substrate utilization but there were reductions in RMR and oxygen consumption.

Prevalence of insulin resistance in Chinese solar greenhouse and field workers: evidence from a solar greenhouse and field workers study

Evidence suggests that agricultural workers are at higher risk of insulin resistance (IR), but few studies have investigated IR in solar greenhouse workers, who are exposed to higher concentrations of agricultural risk factors than traditional agricultural workers. A prevalence study was conducted in a greenhouse vegetable farm in China. In total, 948 participants were enrolled in this study. Among them, 721 participants were allocated to the greenhouse worker group (G group), and 227 participants were assigned to the field worker group (F group). The TyG index, which is an indicator to evaluate prediabetes (IR), was calculated by the formula: TyG index = ln [fasting triglycerides (mg/dL) × fasting plasma glucose (mg/dL)/2]. To evaluate the associations of TyG index alternation with solar greenhouse and field work, multiple linear regression (MLR) and logistic regression models were performed. The TyG index in the G group (8.53 ± 0.56) was higher than that in the F group (8.44 ± 0.59) (p < 0.05). Solar greenhouse work was positively associated with an increased TyG index in both the multiple linear regression model [β = 0.207, (0.006, 0.408)] and the logistic regression model [OR = 1.469, (1.070, 2.016)]. IR was associated with the solar greenhouse work. However, the determination of agricultural hazard factors needs to be further strengthened to improve exposure assessment. Graphical Abstract.

Diabetes and climate change: current evidence and implications for people with diabetes, clinicians and policy stakeholders

Climate change will be a major challenge for the world's health systems in the coming decades. Elevated temperatures and increasing frequencies of heat waves, wildfires, heavy precipitation and other weather extremes can affect health in many ways, especially if chronic diseases are already present. Impaired responses to heat stress, including compromised vasodilation and sweating, diabetes-related comorbidities, insulin resistance and chronic low-grade inflammation make people with diabetes particularly vulnerable to environmental risk factors, such as extreme weather events and air pollution. Additionally, multiple pathogens show an increased rate of transmission under conditions of climate change and people with diabetes have an altered immune system, which increases the risk for a worse course of infectious diseases. In this review, we summarise recent studies on the impact of climate-change-associated risk for people with diabetes and discuss which individuals may be specifically prone to these risk conditions due to their clinical features. Knowledge of such high-risk groups will help to develop and implement tailored prevention and management strategies to mitigate the detrimental effect of climate change on the health of people with diabetes.

The impact of heat therapy on neuromuscular function and muscle atrophy in diabetic rats

Introduction: Diabetes Mellitus (DM) is the most common metabolic disease worldwide and is associated with many systemic complications. Muscle atrophy is one of the significant complications in DM patients, making routine tasks laborious as atrophy continues. It is known that heat stress stimulates heat shock proteins and other proteins that maintain muscle mass; however, it is not thoroughly studied in diabetic conditions. This study addressed whether heat therapy can attenuate muscle atrophy in STZ-induced diabetic rats and explored its mechanism of action on specific muscle proteins. Methods: Male Sprague Dawley rats were randomly divided into short-term (3 weeks) and long-term (6 weeks) experiments. In each experiment rats were divided into control, heat therapy, diabetic and diabetic + heat therapy groups. Rats in heat therapy groups were exposed to heat therapy for 30 min daily for three or six weeks in a temperature-controlled (42°C) chamber. Results: The attenuation of neuromuscular functions assessed by Rotarod, Kondziella's inverted screen, and extensor postural thrust tests showed that diabetic rats exposed to heat therapy performed significantly better than diabetic controls. Muscle cross sectional area data established that heat therapy reduced muscle atrophy by 34.3% within 3 weeks and 44.1% within 6 weeks in the diabetic groups. Further, heat therapy significantly decreased muscle atrophy markers (CD68, KLF, and MAFbx) and significantly elevated muscle hypertrophy markers (AKT, mTOR, and HSP70). Conclusions: This study shows the relevance and clinical significance of utilizing heat therapy as a viable treatment to attenuate muscle atrophy in diabetic patients.

Effect of HEAT therapy in patiEnts with type 2 Diabetes mellitus (HEATED): protocol for a randomised controlled trial

INTRODUCTION

The burden of type 2 diabetes mellitus (T2DM) is increasing worldwide. Heat therapy has been found effective in improving glycaemic control. However, to date, there is a lack of randomised controlled studies investigating the efficacy of heat therapy in T2DM. Therefore, we aim to investigate whether heat therapy with natural thermal mineral water can improve glycaemic control in patients with T2DM.

METHODS AND ANALYSIS

The HEAT therapy in patiEnts with type 2 Diabetes mellitus (HEATED) Study is a single-centre, two-arm randomised controlled trial being conducted at Harkány Thermal Rehabilitation Centre in Hungary. Patients with T2DM will be randomly assigned to group A (bath sessions in 38°C natural thermal mineral water) and group B (baths in thermoneutral water (30°C-32°C)). Both groups will complete a maximum of 5 weekly visits, averaging 50-60 visits over the 12-week study. Each session will last 30 min, with a physical check-up before the bath. At baseline, patients' T2DM status will be investigated thoroughly. Possible microvascular and macrovascular complications of T2DM will be assessed with physical and laboratory examinations. The short form-36 questionnaire will assess the quality of life. Patients will also be evaluated at weeks 4, 8 and 12. The primary endpoint will be the change of glycated haemoglobin from baseline to week 12. An estimated 65 patients will be enrolled per group, with a sample size re-estimation at the enrolment of 50% of the calculated sample size.

ETHICS AND DISSEMINATION

The study has been approved by the Scientific and Research Ethics Committee of the Hungarian Medical Research Council (818-2/2022/EÜIG). Written informed consent is required from all participants. We will disseminate our results to the medical community and will publish our results in peer-reviewed journals.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov, NCT05237219.

Interindividual variability of human thermoregulation: Toward personalized ergonomics of the indoor thermal environment

OBJECTIVE

The study was undertaken to show the magnitude of interindividual differences in energy expenditure (i.e., heat production) under normal living conditions with the aim of providing physiological evidence to support the advancement of a personalized thermal conditioning approach.

METHODS

Three sets of experimental protocols with six participants were conducted at neutral and mild cold temperatures. Energy expenditure, local skin temperatures, and core body temperature were measured continuously, while cognitive performance and thermal sensation were surveyed intermittently. The protocols were designed to study the effects of several normal day activities, low-level physical activity and eating a meal, on metabolic and physiological parameters.

RESULTS

Large interindividual differences among the subjects were demonstrated using non-normalized data by design. The resting metabolic rate difference was 58%, the percentage change in energy expenditure during standing compared to sitting was up to 31%, and the difference in mechanical work efficiency between the least and the most efficient individual was 39.1%. Energy expenditure increase due to the meal effect was 11.2% to 23.3% at neutral and 9.9% to 33.9% at mild cold temperatures across individuals.

CONCLUSIONS

Large interindividual differences in metabolic rate under typical everyday living and office activities suggest facilitating personalized thermal conditioning instead of providing uniform temperature. Therefore, it is necessary to find noninvasive markers that can be easily measured and used as surrogates for human heat production to individualize the climate control of buildings.

Intermediate- and long-term associations between air pollution and ambient temperature and glycated hemoglobin levels in women of child bearing age

BACKGROUND

Air pollution has been linked to obesity while higher ambient temperatures typically reduce metabolic demand in a compensatory manner. Both relationships may impact glucose metabolism, thus we examined the association between intermediate- and long-term exposure to fine particulate matter (PM2.5) and ambient temperature and glycated hemoglobin(HbA1c), a longer-term marker of glucose control.

METHODS

We assessed 3-month, 6-month, and 12-month average air pollution and ambient temperature at 1-km2 spatial resolution via satellite remote sensing models (2013-2019), and assessed HbA1c at four, six, and eight years postpartum in women enrolled in the Programming Research in Obesity, Growth, Environment and Social Stressors (PROGRESS) cohort based in Mexico City. PM2.5 and ambient temperature were matched to participants' addresses and confirmed by GPS tracker. Using linear mixed-effects models, we examined the association between 3-month, 6-month, and 12-month average PM2.5 and ambient temperature with repeated log-transformed HbA1c values. All models included a random intercept for each woman and were adjusted for calendar year, season, and individual-level confounders (age, marital status, smoking, alcohol consumption level, and education level).

RESULTS

We analyzed 1,265 HbA1c measurements of 484 women. Per 1 µg/m3 increase in 3-month and 6-month PM2.5, HbA1c levels increased by 0.28% (95% confidence interval (95 %CI): 0.14, 0.42%) and 0.28% (95 %CI: 0.04, 0.52%) respectively. No association was seen for 12-month average PM2.5. Per 1 °C increase in ambient temperature, HbA1c levels decreased by 0.63% (95 %CI: -1.06, -0.21%) and 0.61% (95 %CI: -1.08, -0.13%), while the 12-month average again is not associated with HbA1c.

CONCLUSIONS

Intermediate-term exposure to PM2.5 and ambient temperature are associated with opposing changes in HbA1c levels, in this region of high PM2.5 and moderate temperature fluctuation. These effects, measurable in mid-adult life, may portend future risk of type 2 diabetes and possible heart disease.

Endothelial HSP72 is not reduced in type 2 diabetes nor is it a key determinant of endothelial insulin sensitivity

Impaired endothelial insulin signaling and consequent blunting of insulin-induced vasodilation is a feature of type 2 diabetes (T2D) that contributes to vascular disease and glycemic dysregulation. However, the molecular mechanisms underlying endothelial insulin resistance remain poorly known. Herein, we tested the hypothesis that endothelial insulin resistance in T2D is attributed to reduced expression of heat shock protein 72(HSP72). HSP72 is a cytoprotective chaperone protein that can be upregulated with heating and is reported to promote insulin sensitivity in metabolically active tissues, in part via inhibition of JNK activity. Accordingly, we further hypothesized that, in T2D individuals, seven days of passive heat treatment via hot water immersion to waist-level would improve leg blood flow responses to an oral glucose load (i.e., endogenous insulin stimulation) via induction of endothelial HSP72. In contrast, we found that: 1) endothelial insulin resistance in T2D mice and humans was not associated with reduced HSP72 in aortas and venous endothelial cells, respectively; 2) after passive heat treatment, improved leg blood flow responses to an oral glucose load did not parallel with increased endothelial HSP72; 3) downregulation of HSP72 (via small-interfering RNA) or upregulation of HSP72 (via heating) in cultured endothelial cells did not impair or enhance insulin signaling, respectively, nor was JNK activity altered. Collectively, these findings do not support the hypothesis that reduced HSP72 is a key driver of endothelial insulin resistance in T2D but provide novel evidence that lower-body heating may be an effective strategy for improving leg blood flow responses to glucose ingestion-induced hyperinsulinemia.

Acute effect of passive heat exposure on markers of cardiometabolic function in adults with type 2 diabetes mellitus

AIM

Heat therapy is a promising strategy to improve cardiometabolic health. This study evaluated the acute physiological responses to hot water immersion in adults with type 2 diabetes mellitus (T2DM).

METHODS

On separate days in randomized order, 13 adults with T2DM (8 males/5 females, 62 ± 12 yrs, BMI: 30.1 ± 4.6 kg/m²) were immersed in thermoneutral (34°C, 90 minutes) or hot (41°C, core temperature ≥38.5°C for 60 minutes) water. Insulin sensitivity was quantified via the minimal oral model during an oral glucose tolerance test (OGTT) performed 60 minutes after immersion. Brachial artery flow-mediated dilation (FMD) and reactive hyperemia were evaluated before and 40 minutes after immersion. Blood samples were drawn to quantify protein concentrations and mRNA levels of HSP70 and 90, and circulating concentrations of cytokines.

RESULTS

Relative to thermoneutral water immersion, hot water immersion increased core temperature (+1.66°C [+1.47, +1.87], P<0.01), heart rate (+34 bpm [+24, +44], P<0.01), antegrade shear rate (+96 s^-1 [+57, +134], P<0.01), and IL-6 (+1.38 pg/mL [+0.31, +2.45], P=0.01). Hot water immersion did not exert an acute change in insulin sensitivity (-0.3 dl/kg/min/μU/ml [-0.9, +0.2], P=0.18), FMD (-1.0% [-3.6, +1.6], P=0.56), peak (+0.36 mL/min/mmHg [-0.71, +1.43], P=0.64) and total (+0.11 mL/min/mmHg x min [-0.46, +0.68], P=0.87) reactive hyperemia. There was also no change in eHSP70 (P=0.64), iHSP70 (P=0.06), eHSP90 (P=0.80), iHSP90 (P=0.51), IL1-RA (P=0.11), GLP-1 (P=0.59) and NF_kB (P=0.56) after hot water immersion.

CONCLUSION

The physiological responses elicited by hot water immersion do not acutely improve markers of cardiometabolic function in adults with T2DM.

Combining adaptive and heat balance models for thermal sensation prediction: A new approach towards a theory and data-driven adaptive thermal heat balance model

The adaptive thermal heat balance (ATHB) framework introduced a method to account for the three adaptive principals, namely physiological, behavioral, and psychological adaptation, individually within existing heat balance models. This work presents a more detailed theoretical framework together with a theory-driven empirical determination toward a new formulation of the ATHB_PMV . The empirical development followed a rigor statistical process known from machine learning approaches including training, validation, and test phase and makes use of a subset (N = 57 084 records) of the ASHRAE Global Thermal Comfort Database. Results show an increased predictive performance among a wide range of outdoor climates, building types, and cooling strategies of the buildings. Furthermore, individual findings question the common believe that psychological adaptation is highest in naturally ventilated buildings. The framework offers further opportunities to include a variety of context-related variables as well as personal characteristics into thermal prediction models, while keeping mathematical equations limited and enabling further advancements related to the understanding of influences on thermal perception.

Gender differences in thermal responses to temperature ramps in moderate environments

Some studies revealed that steady-uniform thermal environments are not the optimal environmental state to ensure thermal comfort, and temperature ramps offer potential advantages over traditional air conditioning methods. Moreover, when exposed to the same environmental conditions, gender differences in thermal responses were often observed, but the gender differences in the ramped conditions and causal relationships remain unclear. Therefore, an experimental research was conducted in a chamber by controlling the rates and directions to study the gender differences in thermal responses to temperature ramps. Three temperature ramps conditions (A: 26 °C-24 °C-26 °C; B: 26 °C-28 °C-26 °C; C: 26 °C-30 °C-26 °C) were investigated with 60 healthy participants (30 females and 30 males) recruited. The main conclusions indicated that women are more sensitive to temperature ramp-down environments than those of their male counterparts. Direction of temperature ramps had a significant effect on human responses in cool environments but no effect was observed in warm environments. Moreover, there was no significant differences in subjective responses between genders in a 2 °C ramp-up environment from 26 °C to 28 °C. Due to psychological differences, men have a wider range of temperature acceptability than women. Furthermore, the relationships between thermal sensation and thermal comfort, thermal sensation and thermal acceptability were also established, indicating that thermal sensation had significant impacts on other psychological responses. This paper has reference value for related researchers and designers to take temperature ramps and gender differences account in the design of indoor thermal environments, which benefits to improve thermal comfort, health and energy efficiency.

The effects of a novel personal comfort system on thermal comfort, physiology and perceived indoor environmental quality, and its health implications - Stimulating human thermoregulation without compromising thermal comfort

The classical textbook interpretation of thermal comfort is that it occurs when the thermoregulatory effort is minimized. However, stimulating human thermoregulatory systems may benefit health and increase body thermal resilience. To address this gap, we tested a novel personal comfort system (PCS) that targets only the extremities and the head, leaving the rest of the body exposed to a moderately drifting temperature (17-25°C). A randomized, cross-over study was conducted under controlled laboratory conditions, mimicking an office setting. Eighteen participants completed two scenarios, one with a PCS and another one without a PCS in 17-25°C ambient conditions. The results indicate that the PCS improved thermal comfort in 17-23°C and retained active thermoregulatory control. The torso skin temperature, underarm-finger temperature gradients, energy expenditure, substrate oxidations and physical activity were not affected by the PCS in most cases. Only slight changes in cardiovascular responses were observed between the two scenarios. Moreover, the PCS boosted pleasure and arousal. At 25°C, the PCS did not improve thermal comfort, but significantly improved air quality perceptions and mitigated eye strain. These findings suggest that human physiological thermoregulation can be stimulated without compromising thermal comfort by using a PCS that only targets the extremities in cold conditions.

Heat therapy shows benefit in patients with type 2 diabetes mellitus: a systematic review and meta-analysis

AIMS

Type-2 diabetes mellitus (T2DM) is a common health condition which prevalence increases with age. Besides lifestyle modifications, passive heating could be a promising intervention to improve glycemic control. This study aimed to assess the efficacy of passive heat therapy on glycemic and cardiovascular parameters, and body weight among patients with T2DM.

METHODS

A systematic review and meta-analysis were reported according to PRISMA Statement. We conducted a systematic search in three databases (MEDLINE, Embase, CENTRAL) from inception to 19 August 2021. We included interventional studies reporting on T2DM patients treated with heat therapy. The main outcomes were the changes in pre-and post-treatment cardiometabolic parameters (fasting plasma glucose, glycated plasma hemoglobin, and triglyceride). For these continuous variables, weighted mean differences (WMD) with 95% confidence intervals (CIs) were calculated. Study protocol number: CRD42020221500.

RESULTS

Five studies were included in the qualitative and quantitative synthesis, respectively. The results showed a not significant difference in the hemoglobin A1c [WMD -0.549%, 95% CI (-1.262, 0.164), p = 0.131], fasting glucose [WMD -0.290 mmol/l, 95% CI (-0.903, 0.324), p = 0.355]. Triglyceride [WMD 0.035 mmol/l, 95% CI (-0.130, 0.200), p = 0.677] levels were comparable regarding the pre-, and post intervention values.

CONCLUSION

Passive heating can be beneficial for patients with T2DM since the slight improvement in certain cardiometabolic parameters support that. However, further randomized controlled trials with longer intervention and follow-up periods are needed to confirm the beneficial effect of passive heat therapy.

The influence of a moderate temperature drift on thermal physiology and perception

Humans spend approximately 80-90% of their time indoors. In current practice, indoor temperatures in many buildings are controlled very tightly. However, allowing more variation in indoor temperature results in more energy-efficient buildings and could potentially improve human metabolic and cardiovascular health. Therefore, this study aimed to evaluate the effect of a drifting ambient temperature versus a fixed ambient temperature on thermal physiological parameters and subjective perception. A cross-over intervention design was conducted in 16 healthy men (age 26 ± 4 y; BMI 23.0 ± 1.7 kg/m²) between July 2018 and May 2019. All participants underwent two whole-day (8:30-17:00) experimental sessions, during which they were exposed to a drifting (17-25°C with a morning ramp of 2.58°C/h and afternoon ramp of -2.58°C/h) or constant ambient temperature (21°C) in randomized order. The experiments took place in respiratory chambers, which simulated a typical office environment and in which temperature conditions can be controlled accurately. Throughout the experimental sessions core and skin temperature, heart rate, blood pressure, energy expenditure as well as activity levels were measured. Subjective thermal perception, such as thermal comfort and sensation, was assessed by questionnaires every 30 min. Results reveal that energy expenditure was higher in the morning during the drifting session, which was accompanied by an increase in activity levels. Both drifting and fixed sessions were judged as comfortable although during the drift thermal comfort was lower in the morning and afternoon and higher during midday. The results indicate that a drifting ambient temperature can be applied in practice, and as such, can contribute to a healthier and more sustainable built environment. More research is needed to understand the role of a drifting temperature on the long term.

Associations of Relative Humidity and Lifestyles with Metabolic Syndrome among the Ecuadorian Adult Population: Ecuador National Health and Nutrition Survey (ENSANUT-ECU) 2012

The effects of the physical environment on metabolic syndrome (MetS) are still largely unexplained. This study aimed to analyze the associations of relative humidity of residence, lifestyles, and MetS among Ecuadorian adults. Data from 6024 people aged 20 to 60 years were obtained from an Ecuador national population-based health and nutrition survey (i.e., ENSANUT-ECU, 2012) and the mean annual relative humidity (%) from the Ecuador National Institute for Meteorology and Hydrology (2012). Odds ratio (OR) with 95% confidence intervals (CI) for MetS according to groups of relative humidity were calculated using multiple logistic regression. Living in high relative humidity (>80%) increased ORs of reduced high-density lipoprotein (HDL) cholesterol (1.25; 95 % CI, 1.06–1.56) and MetS (OR = 1.20; 95 % CI,1.01–1.42) in women. Furthermore, physically active men living in high relative humidity showed lower OR of elevated triglycerides (0.56; 95 % CI,0.37–0.85) while menopausal women living in high relative humidity showed increased ORs of MetS (5.42; 95 % CI, 1.92–15.27), elevated blood pressure (3.10; 95 % CI, 1.15–8.35), and increased waist circumference (OR = 1.34; 95 % CI, 1.09–1.63). Our results show that residence in high relative humidity and menopausal status increase ORs of MetS and its components in Ecuadorian women; however, physical activity significantly reduces the OR of elevated triglycerides in men. The obtained findings may help make public health policies regarding environmental humidity management, nutritional education, menopausal care, and physical activity promotion to prevent the onset of MetS among Ecuadorian adults.

The health benefits of passive heating and aerobic exercise: To what extent do the mechanisms overlap?

Exercise can induce numerous health benefits that can reduce the risk of chronic diseases and all-cause mortality, yet a significant percentage of the population do not meet minimal physical activity guidelines. Several recent studies have shown that passive heating can induce numerous health benefits, many of which are comparable with exercise, such as improvements to cardiorespiratory fitness, vascular health, glycemic control, and chronic low-grade inflammation. As such, passive heating is emerging as a promising therapy for populations who cannot perform sustained exercise or display poor exercise adherence. There appears to be some overlap between the cellular signaling responses that are regulated by temperature and the mechanisms that underpin beneficial adaptations to exercise, but detailed comparisons have not yet been made. Therefore, the purpose of this mini review is to assess the similarities and distinctions between adaptations to passive heating and exercise. Understanding the potential shared mechanisms of action between passive heating and exercise may help to direct future studies to implement passive heating more effectively and identify differences between passive heating and exercise-induced adaptations.

Passive exposure to heat improves glucose metabolism in overweight humans

AIM

Heat 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.

METHODS

Eleven 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.

RESULTS

Despite 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.

CONCLUSION

Our 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.