Heat stress and cardiovascular, hormonal, and heat shock proteins in humans

The effects and mechanisms of heat stress on mammalian oocyte and embryo development

The sum of nonspecific physiological responses exhibited by mammals in response to the disruption of thermal balance caused by high-temperature environments is referred to as heat stress (HS). HS affects the normal development of mammalian oocyte and embryos and leads to significant economic losses. Therefore, it is of great importance to gain a deep understanding of the mechanisms underlying the effects of HS on oocyte and embryonic development and to explore strategies for mitigating or preventing its detrimental impacts in the livestock industry. This article provides an overview of the negative effects of HS on mammalian oocyte growth, granulosa cell maturation and function, and embryonic development. It summarizes the mechanisms by which HS affects embryonic development, including generation of reactive oxygen species (ROS), endocrine disruption, the heat shock system, mitochondrial autophagy, and molecular-level alterations. Furthermore, it discusses various measures to ameliorate the effects of HS, such as antioxidant use, enhancement of mitochondrial function, gene editing, cultivating varieties possessing heat-resistant genes, and optimizing the animals'rearing environment. This article serves as a valuable reference for better understanding the relationship between HS and mammalian embryonic development as well as for improving the development of mammalian embryos and economic benefits under HS conditions in livestock production.

Plasma epinephrine and norepinephrine responses to extreme heat exposures in young and older adults

Hyperthermia is known as a hyperadrenergic state, yet there is a lack of data on the sympathetic responses to ambient heat stress in humans. Therefore, we investigated the plasma epinephrine and norepinephrine concentrations of healthy young and older adults exposed to 3 h of very hot and dry, as well as hot and humid, heat, both with accompanying activities of daily living. We hypothesized that older adults, compared with young adults, would have augmented increases in epinephrine and norepinephrine concentrations secondary to increased thermal strain. Young (n = 20) and older (n = 18) participants underwent two 3-h heat exposures on different days: very hot and dry [47°C and 15% relative humidity (RH)] and hot and humid (41°C and 40% RH). To mimic heat generation comparable to activities of daily living, participants performed seven 5-min bouts of light cycling (approximately 3 METS) dispersed throughout the heat exposure. We measured plasma concentrations of epinephrine and norepinephrine at baseline, end, and 2-h postheat exposure. There was a group-wide increase in epinephrine from baseline to the end of the heat exposure (Δ19 ± 27 pg/mL; P < 0.001) in the hot and humid condition, but not in the very hot and dry condition (Δ6 ± 19 pg/mL; P = 0.10). There were group-wide decreases in norepinephrine concentrations from baseline to the end of the heat exposure in both the very hot and dry (Δ-131 ± 169 pg/mL; P < 0.001) and the hot and humid (Δ-138 ± 157 pg/mL; P < 0.001) conditions, with both returning to near baseline at 2-h postexposure. These data suggest that ambient heating with accompanying bouts of light intermittent exercise may lead to decreases in circulating concentrations of norepinephrine.NEW & NOTEWORTHY Herein we present plasma epinephrine and norepinephrine concentrations to 3 h of very hot and dry, as well as hot and humid, heat exposures with accompanying activities of daily living in young and older participants. We found 1) increased plasma concentrations of epinephrine in young and older adults following the hot and humid, but not the very hot and dry exposures and 2) decreased concentrations of norepinephrine in both groups following exposure to both conditions.

The hormesis principle of neuroplasticity and neuroprotection

Animals live in habitats fraught with a range of environmental challenges to their bodies and brains. Accordingly, cells and organ systems have evolved stress-responsive signaling pathways that enable them to not only withstand environmental challenges but also to prepare for future challenges and function more efficiently. These phylogenetically conserved processes are the foundation of the hormesis principle, in which single or repeated exposures to low levels of environmental challenges improve cellular and organismal fitness and raise the probability of survival. Hormetic principles have been most intensively studied in physical exercise but apply to numerous other challenges known to improve human health (e.g., intermittent fasting, cognitive stimulation, and dietary phytochemicals). Here we review the physiological mechanisms underlying hormesis-based neuroplasticity and neuroprotection. Approaching natural resilience from the lens of hormesis may reveal novel methods for optimizing brain function and lowering the burden of neurological disorders.

Phenotypic, endocrinological, and metabolic effects of zearalenone exposure and additive effect of heat stress in prepubertal female pigs

Independently, both heat stress (HS) and zearalenone (ZEN) compromise female reproduction, thus the hypothesis that ZEN would affect phenotypic, endocrine, and metabolic parameters in pigs with a synergistic and/or additive impact of HS was investigated. Prepubertal gilts (n = 6-7) were assigned to: thermoneutral (TN) vehicle control (TC; n = 6); TN ZEN (40 μg/kg; TZ; n = 6); pair-fed (PF; n = 6) vehicle control (PC; n = 6); PF ZEN (40 μg/kg; PZ; n = 6); HS vehicle control (HC; n = 7); and HS ZEN (40 μg/kg; HZ; n = 7) and experienced either constant 21.0 ± 0.10 °C (TN and PF) or 35.0 ± 0.2 °C (12 h) and 32.2 ± 0.1 °C (12 h) to induce HS for 7 d. Elevated rectal temperature (P < 0.01) and respiration rate (P < 0.01) confirmed induction of HS. Rectal temperature was decreased (P = 0.03) by ZEN. Heat stress decreased (P < 0.01) feed intake, body weight, and average daily gain, with absence of a ZEN effect (P > 0.22). White blood cells, hematocrit, and lymphocytes decreased (P < 0.04) with HS. Prolactin increased (P < 0.01) in PC and PZ and increased in HZ females (P < 0.01). 17β-estradiol reduced (P < 0.01) in HC and increased in TZ females (P = 0.03). Serum metabolites were altered by both HS and ZEN. Neither HS nor ZEN impacted ovary weight, uterus weight, teat size or vulva area in TN and PF treatments, although ZEN increased vulva area (P = 0.02) in HS females. Thus, ZEN and HS, independently and additively, altered blood composition, impacted the serum endocrine and metabolic profile and increased vulva size in prepubertal females, potentially contributing to infertility.

Neuromuscular and gene signaling responses to passive whole-body heat stress in young adults

This study aimed to investigate whether acute passive heat stress 1) decreases muscle Maximal Voluntary Contraction (MVC); 2) increases peripheral muscle fatigue; 3) increases spinal cord excitability, and 4) increases key skeletal muscle gene signaling pathways in skeletal muscle. Examining the biological and physiological markers underlying passive heat stress will assist us in understanding the potential therapeutic benefits. MVCs, muscle fatigue, spinal cord excitability, and gene signaling were examined after control or whole body heat stress in an environmental chamber (heat; 82 °C, 10% humidity for 30 min). Heart Rate (HR), an indicator of stress response, was correlated to muscle fatigue in the heat group (R = 0.59; p < 0.05) but was not correlated to MVC, twitch potentiation, and H reflex suppression. Sixty-one genes were differentially expressed after heat (41 genes >1.5-fold induced; 20 < 0.667 fold repressed). A strong correlation emerged between the session type (control or heat) and principal components (PC1) (R = 0.82; p < 0.005). Cell Signal Transduction, Metabolism, Gene Expression and Transcription, Immune System, DNA Repair, and Metabolism of Proteins were pathway domains with the largest number of genes regulated after acute whole body heat stress. Acute whole-body heat stress may offer a physiological stimulus for people with a limited capacity to exercise.

Identification of Key Genes Associated with Heat Stress in Rats by Weighted Gene Co-Expression Network Analysis

Heat stress has been a big challenge for animal survival and health due to global warming. However, the molecular processes driving heat stress response were unclear. In this study, we exposed the control group rats (n = 5) at 22 °C and the other three heat stress groups (five rats in each group) at 42 °C lasting 30, 60, and 120 min, separately. We performed RNA sequencing in the adrenal glands and liver and detected the levels of hormones related to heat stress in the adrenal gland, liver, and blood tissues. Weighted gene co-expression network analysis (WGCNA) was also performed. Results showed that rectal temperature and adrenal corticosterone levels were significantly negatively related to genes in the black module, which was significantly enriched in thermogenesis and RNA metabolism. The genes in the green-yellow module were strongly positively associated with rectal temperature and dopamine, norepinephrine, epinephrine, and corticosterone levels in the adrenal glands and were enriched in transcriptional regulatory activities under stress. Finally, 17 and 13 key genes in the black and green-yellow modules were identified, respectively, and shared common patterns of changes. Methyltransferase 3 (Mettl3), poly(ADP-ribose) polymerase 2 (Parp2), and zinc finger protein 36-like 1 (Zfp36l1) occupied pivotal positions in the protein-protein interaction network and were involved in a number of heat stress-related processes. Therefore, Parp2, Mettl3, and Zfp36l1 could be considered candidate genes for heat stress regulation. Our findings shed new light on the molecular processes underpinning heat stress.

The effects of a single and a series of Finnish sauna sessions on the immune response and HSP-70 levels in trained and untrained men

BACKGROUND

The aim of the study was to investigate the effect of a Finnish sauna on the immune status parameters. The hypothesis was that hyperthermia would improve immune system's functioning by changing the proportion of lymphocyte subpopulations and would activate heat shock proteins. We assumed that the responses of trained and untrained subjects would be different.

MATERIAL AND METHODS

Healthy men (20-25 years old) were divided into groups: the trained (T; n = 10), and the untrained group (U; n = 10). All participants were subjected to 10 baths (each one consisted of: 3 × 15-minute exposure with cooled down for 2 min. Body composition, anthropometric measurements, VO₂ peak were measured before 1st sauna bath. Blood was collected before the 1st and 10th sauna bath, and 10 min after their completion to asses an acute and a chronic effect. Body mass, rectal temperature and heart rate (HR) were assessed in the same time points. The serum levels of cortisol, Il-6, HSP70 were measured with use of ELISA method, IgA, IgG and IgM by turbidimetry. White blood cells (WBC), leukocyte populations counts: neutrophils, lymphocytes, eosinophils, monocytes, and basophils were determined with use of flow cytometry as well as T-cell subpopulations.

RESULTS

No differences were found in the increase in rectal temperature, cortisol and immunoglobulins between groups. In response to the 1st sauna bath, a greater increase in HR was observed in the U group. After the last one, the HR value was lower in the T group. The impact of sauna baths on WBC, CD56+, CD3+, CD8+, IgA, IgG and IgM was different in trained and untrained subjects' responses. A positive correlation between the increase in cortisol concentrations and increase in internal temperatures after the 1st sauna was found in the T (r = 0.72) and U group (r = 0.77), between the increase in IL-6 and cortisol concentrations in the T group after the 1st treatment (r = 0.64), between the increase in IL-10 concentration and internal temperature (r = 0.75) and between the increase in IL-6 and IL-10 (r = 0.69) concentrations, also.

CONCLUSIONS

Sauna bathing can be a way to improve the immune response, but only when it is undertaken as a series of treatments.

Validating dietary assessment tools with energy expenditure measurement methods: Is this accurate?

Having an accurate dietary assessment tool is a necessity for most nutritional studies. As a result, many validation studies have been carried out to assess the validity of commonly used dietary assessment tools. Since based on the energy balance equation, among individuals with a stable weight, Energy Intake (EI) is equal to Energy Expenditure (EE) and there are precise methods for measurement of EE (e.g. doubly labeled water method), numerous studies have used this technique for validating dietary assessment tools. If there was a discrepancy between measured EI and EE, the researchers have concluded that self-reported dietary assessment tools are not valid or participants misreport their dietary intakes. However, the calculation of EI with common dietary assessment tools such as food frequency questionnaires (FFQs), 24-hour dietary recalls, or weighed food records, is based on fixed factors that were introduced by Atwater and the accuracy of these factors are under question. Moreover, the amount of energy absorption, and utilization from a diet, depends on various factors and there are considerable interindividual differences in this regard, for example in gut microbiota composition. As a result, the EI which is calculated using dietary assessment tools is likely not representative of real metabolizable energy which is equal to EE in individuals with stable weight, thus validating dietary assessment tools with EE measurement methods may not be accurate. We aim to address this issue briefly and propose a feasible elucidation, albeit not a complete solution.

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 heat stress on prolactin-mediated ovarian JAK-STAT signaling in postpubertal gilts

Heat stress (HS) compromises almost every aspect of animal agriculture including reproduction. In pigs, this infecundity is referred to as seasonal infertility (SI), a phenotype including ovarian dysfunction. In multiple species, HS-induced hyperprolactinemia has been described; hence, our study objectives were to characterize and compare HS effects on circulating prolactin (PRL) and ovarian Janus kinase/signal transducer and activator of transcription (JAK-STAT) signaling during the follicular (FOL) or luteal (LUT) phases of the estrous cycle in postpubertal gilts. Gilts were estrus synchronized using altrenogest and environmental treatments began immediately after altrenogest withdrawal. For the FOL study: postpubertal gilts were allocated to constant thermoneutral (TN; n = 6; 20 ± 1.2 °C) or cyclical HS (n = 6; 25 to 32 ± 1.2 °C) conditions for 5 d. In the LUT study: postpubertal gilts were assigned to either TN (n = 7; 20 ± 2.6 °C) or cyclical HS (n = 7; 32 to 35 ± 2.6 °C) conditions from 2 to 12 days postestrus (dpe). Blood was collected by jugular venipuncture for PRL quantification on day 5 in the FOL and on day 0 and day 12 in the LUT gilts. Ovaries and corpora lutea (CL) were obtained from euthanized FOL and LUT gilts on day 5 and day 12, respectively. Western blotting was performed to quantify prolactin receptor (PRLR) and JAK/STAT pathway protein abundance. In the FOL phase, no difference (P = 0.20) in circulating PRL between thermal groups was observed. There was no effect (P ≥ 0.34) of HS on PRLR, signal transducer and activator of transcription 3 (STAT3), signal transducer and activator of transcription 5α (STAT5α), and phosphorylated signal transducer and activator of transcription α/β tyrosine 694/699 (pSTAT5α/β^Tyr694/699) abundance and Janus kinase 2 (JAK2), phosphorylated janus kinase 2 tyrosine 1007/1008 (pJAK2^Tyr1007/1008), STAT1, phosphorylated signal transducer and activator of transcription 1 tyrosine 701 (pSTAT1^Tyr701), phosphorylated signal transducer and activator of transcription 1 serine 727 (pSTAT1^Ser727), and phosphorylated signal transducer and activator of transcription 3 tyrosine 705 (pSTAT3^Tyr705) were undetectable in FOL gilt ovaries. Ovarian pSTAT5α/β^Tyr694/699 abundance tended to moderately increase (4%; P = 0.07) in FOL gilts by HS. In the LUT phase, circulating PRL increased progressively from 2 to 12 dpe, but no thermal treatment-induced difference (P = 0.37) was noted. There was no effect (P ≥ 0.16) of HS on CL abundance of PRLR, pJAK2^Tyr1007/1008, JAK2, STAT1, pSTAT1^Tyr701, pSTAT1^Ser727, pSTAT3^Tyr705, STAT5α, or pSTAT5α/β^Tyr694/699. In LUT phase, CL STAT3 abundance was increased (11%; P < 0.03) by HS. There was no impact of HS (P ≥ 0.76) on levels of pJAK2^Tyr1007/1008 and pSTAT5α/β^Tyr694/699 in LUT gilts; however, the CL pSTAT3^Tyr705:STAT3 ratio tended to be decreased (P = 0.10) due to HS. These results indicate an HS-induced estrous cycle-stage-dependent effect on the ovarian JAK/STAT pathway, establishing a potential role for this signaling pathway as a potential contributor to SI.

Validating dietary assessment tools with energy expenditure measurement methods: Is this accurate?

Abstract. Having an accurate dietary assessment tool is a necessity for most nutritional studies. As a result, many validation studies have been carried out to assess the validity of commonly used dietary assessment tools. Since based on the energy balance equation, among individuals with a stable weight, Energy Intake (EI) is equal to Energy Expenditure (EE) and there are precise methods for measurement of EE (e.g. doubly labeled water method), numerous studies have used this technique for validating dietary assessment tools. If there was a discrepancy between measured EI and EE, the researchers have concluded that self-reported dietary assessment tools are not valid or participants misreport their dietary intakes. However, the calculation of EI with common dietary assessment tools such as food frequency questionnaires (FFQs), 24-hour dietary recalls, or weighed food records, is based on fixed factors that were introduced by Atwater and the accuracy of these factors are under question. Moreover, the amount of energy absorption, and utilization from a diet, depends on various factors and there are considerable interindividual differences in this regard, for example in gut microbiota composition. As a result, the EI which is calculated using dietary assessment tools is likely not representative of real metabolizable energy which is equal to EE in individuals with stable weight, thus validating dietary assessment tools with EE measurement methods may not be accurate. We aim to address this issue briefly and propose a feasible elucidation, albeit not a complete solution.

Theoretical effectiveness of steam inhalation against SARS-CoV-2 infection: updates on clinical trials, mechanism of actions, and traditional approaches

Steam inhalation therapy can be a contemporary approach for COVID-19 affected patients of all age groups to manage respiratory conditions, though it presently lacks the scientific backing to establish itself as a befitting practice. The age of COVID-19 has facilitated this traditional home remedy to resurface among the general mass as a helpful approach for the prevention and adjuvant treatment of the disease. In this review, the means of SARS-CoV-2 infection and impact of the parameters, namely steam inhalation and heat on such infection has been delineated via enumerating the effect of the parameters in the human body and against SARS-CoV-2. The literature search was conducted using PubMed, Web of Science, Scopus, ScienceDirect, Wiley Online Library, Google Scholar, and CNKI Scholar databases. The keywords used in the survey include 'Steam inhalation', 'SARS-CoV-2', 'COVID-19', 'Clinical study', 'Mechanism of action', 'Traditional uses', 'Phytochemistry' and 'Adverse effects'. Clinical studies concerning steam inhalation by COVID-19 patients have been comprehended to demarcate the scientific obscurity of the practice. The safety profile of the procedure has also been outlined emphasizing evading measures against COVID-19 and other related disease states. To recapitulate, application of the steam inhalation with herbal concoctions and phytochemicals having folkloric prevalence as an inhalable remedy against respiratory illnesses has been explored in this review work to focus on a new aspect in the COVID-19 treatment paradigm using steam and progress of further research hither.

Whole-body repeated hyperthermia increases irisin and brain-derived neurotrophic factor: A randomized controlled trial

CONTEXT

Hyperthermia is known to be beneficial to patients affected by various diseases. Irisin is a key regulators of fat metabolism known to be released as response to cold. Brain Derived Neurotrophic Factor (BDNF) is a marker of neuroplasticity usually increased as response to acute exposure to human body stressors.

OBJECTIVE

Effect of a repeated hyperthermia exposure programme on changes in circulating irisin and serum BDNF in healthy humans.

DESIGN

Setting, Participants: Randomized, single-blind, cross-over trial in healthy humans conducted at Sechenov University Physiology Laboratory from April 2019. The treatment period was 2 weeks (wash-out 3 weeks). Researchers analysing serum biomarkers and questionnaires data were blinded to participants allocation. Participants were 20 healthy male (age 21.5 ± 2.1 years).

INTERVENTION

Hyperthermia exposure programme (WBPH) versus sham exposure (SHAM) to hyperthermia (10 sessions in two weeks).

MAIN OUTCOME MEASURE

Changes in irisin and BDNF before and after short hyperthermia exposure.

RESULTS

Twenty participants were analyzed. Irisin increased significantly in group WBPH only: 6.3 μg/ml (mean with SD = 1.6) compared to 5.4 μg/ml (SD = 1.7) in SHAM group; This value was also higher than baseline (5.0 mean with SD = 1.1) in WBPH. After 10 sessions mean change in BDNF was higher in WBPH group vs SHAM: BDNF was 28,263 (SD = 4213) pg/ml in WBPH group and 24,064 (SD = 5600) pg/ml in SHAM group. BDNF concentrations were significantly higher than baseline values in WBPH group only, 28,263 (SD 4213) vs 25,888 (SD 4316) pg/ml.

CONCLUSION

In healthy young humans a 2-week, ten sessions programme consisting of repeated exposure to hyperthermia resulted in a significantly higher increase of circulating Irisin and BDNF.

Overheating or overcooling: heat transfer in the spot to fight against the pandemic obesity

The prevalence of obesity has nearly doubled worldwide over the past three and a half decades, reaching pandemic status. Obesity is associated with decreased life expectancy and with an increased risk of metabolic, cardiovascular, nervous system diseases. Hence, understanding the mechanisms involved in the onset and development of obesity is mandatory to promote planned health actions to revert this scenario. In this review, common aspects of cold exposure, a process of heat generation, and exercise, a process of heat dissipation, will be discussed as two opposite mechanisms of obesity, which can be oversimplified as caloric conservation. A common road between heat generation and dissipation is the mobilization of Free Faty Acids (FFA) and Carbohydrates (CHO). An increase in energy expenditure (immediate effect) and molecular/metabolic adaptations (chronic effect) are responses that depend on SNS activity in both conditions of heat transfer. This cycle of using and removing FFA and CHO from blood either for heat or force generation disrupt the key concept of obesity: energy accumulation. Despite efforts in making the anti-obesity pill, maybe it is time to consider that the world's population is living at thermoneutrality since temperature-controlled places and the lack of exercise are favoring caloric accumulation.

Thermotherapy Effects on Healthy and Type 2 Diabetes Human Skeletal Muscle Myoblast Cell Lines

Diabetes mellitus is a chronic metabolic disease characterized by elevated blood glucose levels with associated disordered carbohydrate and lipid metabolism. Type 2 diabetes (T2D) specifically has been shown to cause a decrease in skeletal muscle mass due to oxidative stress. This study investigated a treatment option for T2D through thermotherapy on healthy (HSMM) and T2D (D-HSMM) human skeletal muscle cells. The goals were to determine the effects of thermotherapy, long-term (chronic) and short-term (acute), on HSMM and D-HSMM cell viabilities and oxidative stress. HSMM and D-HSMM cells were grown to confluency, harvested, and counted to determine density. Acute and chronic heat treatments were applied to both cell lines. The chronic treatment consisted of a 30-minute exposure to 40°C, three times a week for three weeks; the acute treatment was a one-time exposure. Oxidative stress assays and cell viabilities were tested 24 hours after heat treatments. Results indicated no significant effect on the cell viability of HSMM and D-HSMM cells. The acute treatment had a significant increase (p ≤ 0.05) of MDA concentration compared to the chronic treatment. The chronic treatment had a significant increase (p ≤ 0.05) in catalase activity compared to the acute treatment. The SOD activity had no significant change (p > 0.05) between the chronic and acute treatments. In conclusion, acute thermotherapy may not be beneficial for skeletal muscle cells due to the observed increase in oxidative stress, especially in the D-HSMM cells.

Sauna use as a lifestyle practice to extend healthspan

Sauna use, sometimes referred to as "sauna bathing," is characterized by short-term passive exposure to high temperatures, typically ranging from 45 °C to 100 °C (113 °F to 212 °F), depending on modality. This exposure elicits mild hyperthermia, inducing a thermoregulatory response involving neuroendocrine, cardiovascular, and cytoprotective mechanisms that work in a synergistic fashion in an attempt to maintain homeostasis. Repeated sauna use acclimates the body to heat and optimizes the body's response to future exposures, likely due to the biological phenomenon known as hormesis. In recent decades, sauna bathing has emerged as a probable means to extend healthspan, based on compelling data from observational, interventional, and mechanistic studies. Of particular interest are the findings from large, prospective, population-based cohort studies of health outcomes among sauna users that identified strong dose-dependent links between sauna use and reduced morbidity and mortality. This review presents an overview of sauna practices; elucidates the body's physiological response to heat stress and the molecular mechanisms that drive the response; enumerates the myriad health benefits associated with sauna use; and describes sauna use concerns.

Effects of Twelve Sessions of High-Temperature Sauna Baths on Body Composition in Healthy Young Men

The health benefits of sauna baths are attracting ever-increasing interest. Therefore, the purpose of this study was to evaluate the effects of 12 high-temperature (100 °C) sauna baths on body composition of 23 healthy young men, divided into a control group (CG) and a sauna group (SG). Both groups were initially evaluated by dual-energy X-ray absorptiometry (DXA), after which the SG experienced 12 sessions of sauna baths at high temperatures (100 °C). Initial measurements were carried out after the sauna sessions and after two weeks of decay in both groups. The muscle mass of the right leg (pre vs. decay: 9.50 (5.59) vs. 10.52 (5.15); p < 0.05; Δ 1.07%), bone mineral density (pre vs. post: 1.221 (0.35) vs. 1.315 (0.45); p < 0.05; Δ 7.7%) and bone mineral content (pre vs. post: 0.470 (0.21) vs. 0.499 (0.22); p < 0.05; Δ 6.17%) of the left leg increased in the SG after the sauna baths. It seems that exposure to heat at high temperatures could produce improvements in bone and muscle mass.

The Cardiometabolic Health Benefits of Sauna Exposure in Individuals with High-Stress Occupations. A Mechanistic Review

Components of the metabolic syndrome (i.e., hypertension, insulin resistance, obesity, atherosclerosis) are a leading cause of death in the United States and result in low-grade chronic inflammation, excessive oxidative stress, and the eventual development of cardiometabolic diseases (CMD). High-stress occupations (HSO: firefighters, police, military personnel, first responders, etc.) increase the risk of developing CMD because they expose individuals to chronic and multiple stressors (i.e., sleep deprivation, poor nutrition habits, lack of physical activity, psychological stress). Interestingly, heat exposure and, more specifically, sauna bathing have been shown to improve multiple markers of CMD, potentially acting as hormetic stressors, at the cellular level and in the whole organism. Therefore, sauna bathing might be a practical and alternative intervention for disease prevention for individuals with HSO. The purpose of this review is to detail the mechanisms and pathways involved in the response to both acute and chronic sauna bathing and collectively present sauna bathing as a potential treatment, in addition to current standard of care, for mitigating CMD to both clinicians and individuals serving in HSO.

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.

Reaction Kinetics Modeling of eHsp70 Induced by Norepinephrine in Response to Exercise Stress

Exercise elicits a systemic adaptation reaction, involving both neuroendocrine and cellular/paracrine stress responses, exemplified by the sympathoadrenergic activity and the release of cellular Hsp70 into the circulation. Regular sports training is known to result in increased fitness. In this study, we characterized the plasma norepinephrine and Hsp70 levels and modeled their relationship in response to exercise stress by bicycle ergometer in 12 trained judoka athletes and in 10 healthy controls. Resting norepinephrine was similar in both groups, whereas Hsp70 was significantly higher in controls compared to athletes. Intense exercise load induced both norepinephrine and Hsp70 elevation. However, both norepinephrine and Hsp70 were significantly lower in athletes compared to the control group. A reaction kinetic model was developed that provided a quantitative description of norepinephrine-facilitated extracellular Hsp70 release, congruent with the experimental data. Our study indicates that exercise-induced norepinephrine and extracellular Hsp70 may be coordinated responses to physiological stress, which are robustly affected by regular sports activity.

Heat stress-induced deficits in growth, metabolic efficiency, and cardiovascular function coincided with chronic systemic inflammation and hypercatecholaminemia in ractopamine-supplemented feedlot lambs

Heat stress hinders growth and well-being in livestock, an effect that is perhaps exacerbated by the β1 agonist ractopamine. Heat stress deficits are mediated in part by reduced feed intake, but other mechanisms involved are less understood. Our objective was to determine the direct impact of heat stress on growth and well-being in ractopamine-supplemented feedlot lambs. Commercial wethers were fed under heat stress (40 °C) for 30 d, and controls (18 °C) were pair-fed. In a 2 × 2 factorial, lambs were also given a daily gavage of 0 or 60 mg ractopamine. Growth, metabolic, cardiovascular, and stress indicators were assessed throughout the study. At necropsy, 9th to 12th rib sections (four-rib), internal organs, and feet were assessed, and sartorius muscles were collected for ex vivo glucose metabolic studies. Heat stress increased (P < 0.05) rectal temperatures and respiration rates throughout the study and reduced (P < 0.05) weight gain and feed efficiency over the first week, ultrasonic loin-eye area and loin depth near the end of the study, and four-rib weight at necropsy. Fat content of the four-rib and loin were also reduced (P < 0.05) by heat stress. Ractopamine increased (P < 0.05) loin weight and fat content and partially moderated the impact of heat stress on rectal temperature and four-rib weight. Heat stress reduced (P < 0.05) spleen weight, increased (P < 0.05) adrenal and lung weights, and was associated with hoof wall overgrowth but not organ lesions. Ractopamine did not affect any measured indicators of well-being. Heat stress reduced (P < 0.05) blood urea nitrogen and increased (P < 0.05) circulating monocytes, granulocytes, and total white blood cells as well as epinephrine, TNFα, cholesterol, and triglycerides. Cortisol and insulin were not affected. Heat stress reduced (P < 0.05) blood pressure and heart rates in all lambs and increased (P < 0.05) left ventricular wall thickness in unsupplemented but not ractopamine-supplemented lambs. No cardiac arrhythmias were observed. Muscle glucose uptake did not differ among groups, but insulin-stimulated glucose oxidation was reduced (P < 0.05) in muscle from heat-stressed lambs. These findings demonstrate that heat stress impairs growth, metabolism, and well-being even when the impact of feed intake is eliminated by pair-feeding and that systemic inflammation and hypercatecholaminemia likely contribute to these deficits. Moreover, ractopamine improved muscle growth indicators without worsening the effects of heat stress.

Effect of warm environment on the skin blood flow response to food intake

BACKGROUND AND OBJECTIVE

Warm exposure places high demands on thermoregulation mechanisms, which depend on the effectiveness of the microvascular function. The associations between the microcirculation and metabolism in warm environments have received little attention. The purpose of this study was to explore skin blood flow (SkBF) in response to food intake in a warm environment compared to control.

METHODS

Thirty-two healthy, acclimated-to-warm-environment and physically active participants were recruited (20 females and 12 males). They participated in two sessions (warm environment: 31 °C and control: 20 °C, presented in randomized order). SkBF was measured before and after standardized food intake through the acquisition of perfusion signals by laser Doppler flowmetry (Periflux System 5000), following a local heating protocol.

RESULTS

SkBF was affected by the environmental temperature, showing an increase in the warm environment compared to control (all p < .001). SkBF was significantly affected by food intake (all p < .007), being reduced after meals. In the men's group, SkBF was reduced in both environmental temperatures after meals. In women, meals affected SkBF at 20 °C but not in the warm environment.

CONCLUSION

These results may indicate a competition between thermo- and glyco-regulation in a warm environment to the detriment of glucose homeostasis in women.

Acute Myocardial Infarction among Hospitalizations for Heat Stroke in the United States

Background: This study aimed to assess the risk factors and impact of acute myocardial infarction on in-hospital treatments, complications, outcomes, and resource utilization in hospitalized patients for heat stroke in the United States. Methods: Hospitalized patients with a principal diagnosis of heat stroke were identified in the National Inpatient Sample dataset from the years 2003 to 2014. Acute myocardial infarction was identified using the hospital International Classification of Diseases, Ninth Revision (ICD-9), diagnosis of 410.xx. Clinical characteristics, in-hospital treatment, complications, outcomes, and resource utilization between patients with and without acute myocardial infarction were compared. Results: A total of 3372 heat stroke patients were included in the analysis. Of these, acute myocardial infarction occurred in 225 (7%) admissions. Acute myocardial infarction occurred more commonly in obese female patients with a history of chronic kidney disease, but less often in male patients aged <20 years with a history of hypothyroidism. The need for mechanical ventilation, blood transfusion, and renal replacement therapy were higher in patients with acute myocardial infarction. Acute myocardial infarction was associated with rhabdomyolysis, metabolic acidosis, sepsis, gastrointestinal bleeding, ventricular arrhythmia or cardiac arrest, renal failure, respiratory failure, circulatory failure, liver failure, neurological failure, and hematologic failure. Patients with acute myocardial infarction had 5.2-times greater odds of in-hospital mortality than those without myocardial infarction. The length of hospital stay and hospitalization cost were also higher when an acute myocardial infarction occurred while hospitalized. Conclusion: Acute myocardial infarction was associated with worse outcomes and higher economic burden among patients hospitalized for heat stroke. Obesity and chronic kidney disease were associated with increased risk of acute myocardial infarction, while young male patients and hypothyroidism were associated with decreased risk.

Turning up the heat on COVID-19: heat as a therapeutic intervention

Enveloped viruses such as SAR-CoV-2 are sensitive to heat and are destroyed by temperatures tolerable to humans. All mammals use fever to deal with infections and heat has been used throughout human history in the form of hot springs, saunas, hammams, steam-rooms, sweat-lodges, steam inhalations, hot mud and poultices to prevent and treat respiratory infections and enhance health and wellbeing. This paper reviews the evidence for using heat to treat and prevent viral infections and discusses potential cellular, physiological and psychological mechanisms of action. In the initial phase of infection, heat applied to the upper airways can support the immune system's first line of defence by supporting muco-ciliary clearance and inhibiting or deactivating virions where they first lodge. This may be further enhanced by the inhalation of steam containing essential oils with anti-viral, mucolytic and anxiolytic properties. Heat applied to the whole body can further support the immune system's second line of defence by mimicking fever and activating innate and acquired immune defences and building physiological resilience. Heat-based treatments also offer psychological benefits and enhanced mental wellness by focusing attention on positive action, enhancing relaxation and sleep, inducing 'forced-mindfulness', and invoking the power of positive thinking and 'remembered wellness'. Heat is a cheap, convenient and widely accessible therapeutic modality and while no clinical protocols exist for using heat to treat COVID-19, protocols that draw from traditional practices and consider contraindications, adverse effects and infection control measures could be developed and implemented rapidly and inexpensively on a wide scale. While there are significant challenges in implementing heat-based therapies during the current pandemic, these therapies present an opportunity to integrate natural medicine, conventional medicine and traditional wellness practices, and support the wellbeing of both patients and medical staff, while building community resilience and reducing the likelihood and impact of future pandemics.

Turning up the heat on COVID-19: heat as a therapeutic intervention

Enveloped viruses such as SAR-CoV-2 are sensitive to temperature and are destroyed by temperatures tolerable to humans. All mammals use fever to deal with infections and heat has been used throughout human history in the form of hot springs, saunas, hammams, steam-rooms, sweat-lodges, steam inhalations, hot mud and poultices to prevent and treat respiratory infections and enhance health and wellbeing. This paper reviews the evidence for using heat to treat and prevent viral infections and discusses potential cellular, physiological and psychological mechanisms of action. In the initial phase of infection, heat applied to the upper airways can support the immune system's first line of defence by supporting muco-ciliary clearance and inhibiting or deactivating virions in the place where they first lodge. This may be further enhanced by the inhalation of steam containing essential oils with anti-viral, mucolytic and anxiolytic properties. Heat applied to the whole body can further support the immune system's second line of defence by mimicking fever and activating innate and acquired immune defences and building physiological resilience. Heat-based treatments also offer psychological benefits by directing focus on positive action, enhancing relaxation and sleep, inducing 'forced-mindfulness', and invoking the power of positive thinking and remembered wellness. Heat is a cheap, convenient and widely accessible therapeutic modality and while no clinical protocols exist for using heat to treat COVID-19, protocols that draw from traditional practices and consider contraindications, adverse effects and infection control measures could be developed and implemented rapidly and inexpensively on a wide scale. While there are significant challenges in implementing heat-based therapies during the current pandemic, these therapies present an opportunity to integrate natural medicine, conventional medicine and traditional wellness practices, and support the wellbeing of both patients and medical staff, while building community resilience and reducing the likelihood and impact of future pandemics.

Habitual tub bathing and risks of incident coronary heart disease and stroke

Objective

Tub bathing is considered to have a preventive effect against cardiovascular disease (CVD) by improving haemodynamic function. However, no prospective studies have investigated the long-term effects of tub bathing with regard to CVD risk.

Methods

A total of 30 076 participants aged 40–59 years with no history of CVD or cancer were followed up from 1990 to 2009. Participants were classified by bathing frequency: zero to two times/week, three to four times/week and almost every day. The HRs of incident CVD were estimated using Cox proportional hazards models after adjusting for traditional CVD risk factors and selected dietary factors.

Results

During 538 373 person-years of follow-up, we documented a total of 2097 incident cases of CVD, comprising 328 coronary heart diseases (CHDs) (275 myocardial infarctions and 53 sudden cardiac deaths) and 1769 strokes (991 cerebral infarctions, 510 intracerebral haemorrhages, 255 subarachnoid haemorrhages and 13 unclassified strokes). The multivariable HRs (95% CIs) for almost daily or every day versus zero to two times/week were 0.72 (0.62 to 0.84, trend p<0.001) for total CVD; 0.65 (0.45 to 0.94, trend p=0.065) for CHD; 0.74 (0.62 to 0.87, trend p=0.005) for total stroke; 0.77 (0.62 to 0.97, trend p=0.467) for cerebral infarction; and 0.54 (0.40 to 0.73, trend p<0.001) for intracerebral haemorrhage. No associations were observed between tub bathing frequency and risk of sudden cardiac death or subarachnoid haemorrhage.

Conclusion

The frequency of tub bathing was inversely associated with the risk of CVD among middle-aged Japanese.

Hand heating lowers postprandial blood glucose concentrations: A double-blind randomized controlled crossover trial

OBJECTIVES

Examine effect of single hand heating with and without negative pressure on fasting blood glucose (FBG) and postprandial blood glucose (PBG).

DESIGN

Double-blind randomized controlled trial with crossover design.

SUBJECTS

FBG experiment: 17 healthy subjects (4 males). PBG experiment: 13 healthy subjects (1 males).

INTERVENTIONS

Devices included one providing heat only, one heat and negative pressure, and one acting as a sham. For the FBG experiment the devices were used for 30 min. For the PBG experiment the devices were used for one hour during an oral glucose tolerance test (OGTT).

OUTCOME MEASURES

Blood glucose measurements were used to determine change in FBG, peak PBG, area under the curve (AUC), and incremental AUC (iAUC).

RESULTS

Temperature: Change in tympanic temperature was ≤ 0.15 °C for all trials. FBG: There was no effect on FBG. PBG: Compared to the sham device the heat plus vacuum and heat only device lowered peak blood glucose by 16(31)mg/dL, p = 0.092 and 18(28)mg/dL, p = 0.039, respectively. AUC and iAUC: Compared to the sham device, the heat plus vacuum device and heat only device lowered the AUC by 5.1(15.0)%, p = 0.234 and 7.9(11.1)%, p = 0.024 respectively and iAUC by 17.2(43.4)%, p = 0.178 and 20.5(34.5)%, p = 0.054, respectively.

CONCLUSIONS

Heating a single hand lowers postprandial blood glucose in healthy subjects.

Repeated hyperthermia exposure increases circulating Brain Derived Neurotrophic Factor levels which is associated with improved quality of life, and reduced anxiety: A randomized controlled trial

CONTEXT

Hyperthermia is known to be beneficial to patients affected by various diseases. Brain Derived Neurotrophic Factor (BDNF) is a marker of neuroplasticity usually increased as response to acute exposure to human body stressors. Little is known about BDNF changes after repeated exposure to hyperthermia.

OBJECTIVE

To investigate the effect of a repeated hyperthermia exposure programme (HTC) on serum BDNF in healthy humans.

DESIGN, SETTING, PARTICIPANTS

Randomized, single-blind, controlled trial in healthy humans conducted at Sechenov University Physiology Laboratory between December 2016 and November 2018. The treatment period was 10 weeks. Researchers analysing serum BDNF and questionnaires data were blinded to participants allocation.

PARTICIPANTS

Were 34 healthy male (age 20.2 ± 1.6 years).

INTERVENTION

Repeated Hyperthermia exposure programme, HTC, versus Light Intermittent Exercise, LIE, programme as control (10 weeks).

MAIN OUTCOME MEASURE

Change in BDNF from baseline to final visit three days after treatment completion.

RESULTS

25 participants were analyzed. One participant withdrew before signing the informed consent and 8 participants (n = 3 in HTC and n = 5 in LIE) could not undertake the first assessment and were excluded. Mean change in BDNF was higher in HTC group vs LIE after both time points (after 12 and after 24 sessions). After 24 sessions BDNF was 30170 (SD 5268) pg/ml in HTC group a value that was significantly higher than 24104 (SD 2876) pg/ml measured in LIE group. BDNF concentrations were significantly higher than baseline values in HTC group only, 30170 (SD 5268) vs 26710 (SD 5437) pg/ml.

CONCLUSION

A 10-week programme consisting of repeated exposure to hyperthermia resulted in a significantly higher increase of circulating BDNF compared to a programme consisting of intermittent light intensity exercise.

Thermoregulation in the Aging Population and Practical Strategies to Overcome a Warmer Tomorrow

As global temperatures continue to rise, improving thermal tolerance in the aged population is crucial to counteract age-associated impairments in thermoregulatory function. Impairments in reflex cutaneous vasodilation and sweating response can augment the vulnerability of older adults to heat-related injuries following exposure to heat stress. Mechanisms underlying a compromised cutaneous vasodilation are suggested to include reduced sympathetic neural drive, diminished cholinergic co-transmitter contribution, and altered second messenger signaling events. On the other hand, impairments in sweating response are ascribed to reduced sweat gland cholinergic sensitivity and altered cyclooxygenase and nitric oxide signaling. Several practical mitigation strategies such as exercise, passive heating, and behavioral adaptations are proposed as means to overcome heat stress and improve thermal tolerance in the aged. Aerobic exercise training is shown to be amongst the most effective ways to enhance thermoregulatory function. However, in elderly with limited exercise capability due to chronic diseases and mobility issues, passive heating can serve as a functional alternative as it has been shown to confer similar benefits to that of exercise training. Supplementary to exercise training and passive heating, behavioral adaptations can be applied to further enhance the heat-preparedness of the aged.

The Rationale for Photobiomodulation Therapy of Vaginal Tissue for Treatment of Genitourinary Syndrome of Menopause: An Analysis of Its Mechanism of Action, and Current Clinical Outcomes

Objective: Light, particularly in the visible to far-infrared spectrum, has been applied to the female genital tract with lasers and other devices for nearly 50 years. These have included procedures on both normal and neoplastic tissues, management of condylomata, endometriosis, and menometrorrhagia, and, more recently, a number of fractional laser devices have been applied for the management of genitourinary syndrome of menopause (GSM) and stress urinary incontinence (SUI), and to achieve so-called vaginal rejuvenation. Photobiomodulation therapy (PBMT) has been proposed as an alternative for use in managing GSM and SUI.

Methods: This article reviews the biological basis, symptoms, and management of GSM, and investigates the current status and rationale for the use of PBMT.

Results and conclusions: Based on the preliminary evidence available, PBMT is safe and appears to be efficacious in treating GSM.

A hot topic for health: Results of the Global Sauna Survey

OBJECTIVE

Sauna-bathing is an ancient tradition that is gaining popularity across the world as a wellness tool. There is a growing body of medical evidence supporting the role of saunas, or whole-body thermotherapy, as a form of treatment for a range of health issues. However, the demographics, motivations and experiences of current sauna bathers have not yet been explored on a global scale. This study is designed to explore these themes.

DESIGN

An online 71-item questionnaire compiling information on the individual characteristics, sauna-related habits and perceived health and wellness experiences of regular sauna bathers was conducted from October 2016 to October 2017. The validated 'SF-12' quality of life scoring tool was incorporated into the questionnaire to measure physical and mental indicators of well-being.

RESULTS

Of 572 logins recorded, 482 valid responses were generated. Both men (51.3%) and women (48.7%) were represented, and respondents were predominantly well-educated (81.8%), non-smoking (90.6%), regularly-exercising (78.8%) individuals of normal-to-overweight status (87.1%) who sauna-bathed approximately 1-2 times per week. The key reasons indicated by respondents for sauna-bathing included relaxation/stress reduction, pain relief and socializing. Nearly a third of respondents reported medical conditions and of this subset, those with back/musculoskeletal pain and mental issues cited the greatest improvements in their conditions with sauna-bathing. Of all respondents, 83.5% reported sleep benefits after sauna use. Analysis of well-being scores after stratifying respondents into three groups by sauna-bathing frequency (group I: <5 times per month; group II: 5-15 times per month; group III: >15 times per month) revealed group II respondents had slightly higher mental well-being scores (Kruskal-Wallis testing: H = 6.603 > ꭔ2 of 5.991, p = 0.0368, df = 2 with post hoc analysis using Mann-Whitney U test: p = 0.016) as compared to respondents who were sauna-bathing less frequently (group I). No respective differences were detected between the physical well-being scores of any of the three groups. Adverse reactions to sauna-bathing were recorded as mostly minor (93.1%), including primarily symptoms of dizziness, dehydration and headache. However, there were two reports (0.3%) of chest pain and eye irritation requiring hospitalization.

CONCLUSIONS

This cross-sectional study documents that sauna-bathing participants, particularly those from Finland, Australia and the United States, are motivated to use saunas predominantly for relaxation, reporting health benefits especially around mental well-being and sleep, with relatively few adverse effects. While these results reinforce some of the known health benefits of sauna bathing, they indicate that further research and better dissemination of existing evidence is needed to fully develop the sauna's potential as a therapeutic intervention.

Passive heating and glycaemic control in non-diabetic and diabetic individuals: A systematic review and meta-analysis

OBJECTIVE

Passive heating (PH) has begun to gain research attention as an alternative therapy for cardio-metabolic diseases. Whether PH improves glycaemic control in diabetic and non-diabetic individuals is unknown. This study aims to review and conduct a meta-analysis of published literature relating to PH and glycaemic control.

METHODS

Electronic data sources, PubMed, Embase and Web of Science from inception to July 2018 were searched for randomised controlled trials (RCT) studying the effect of PH on glycaemic control in diabetic or non-diabetic individuals. To measure the treatment effect, standardised mean differences (SMD) with 95% confidence intervals (CI) were calculated.

RESULTS

Fourteen articles were included in the meta-analysis. Following a glucose load, glucose concentration was greater during PH in non-diabetic (SMD 0.75, 95% CI 1.02 to 0.48, P < 0.001) and diabetic individuals (SMD 0.27, 95% CI 0.52 to 0.02, P = 0.030). In non-diabetic individuals, glycaemic control did not differ between PH and control only (SMD 0.11, 95% CI 0.44 to -0.22, P > 0.050) and a glucose challenge given within 24 hours post-heating (SMD 0.30, 95% CI 0.62 to -0.02, P > 0.050).

CONCLUSION

PH preceded by a glucose load results in acute glucose intolerance in non-diabetic and diabetic individuals. However, heating a non-diabetic individual without a glucose load appears not to affect glycaemic control. Likewise, a glucose challenge given within 24 hours of a single-bout of heating does not affect glucose tolerance in non-diabetic individuals. Despite the promise PH may hold, no short-term benefit to glucose tolerance is observed in non-diabetic individuals. More research is needed to elucidate whether this alternative therapy benefits diabetic individuals.

The association between heat stroke and subsequent cardiovascular diseases

Background

Recent studies have indicated that several critical illnesses are associated with an increased risk of cardiovascular diseases (CVDs). Nonetheless, studies of the association between heat-related illnesses (HRIs) and subsequent CVDs are still limited. We sought to evaluate whether heat stroke (HS) was associated with an increased CVD incidence.

Methods

The data from the nationwide, population-based, retrospective, cohort study described herein were obtained from the National Health Insurance Research Database in Taiwan. The outcome evaluated in this study was the cumulative incidence of CVDs, which was compared between patients with HS, patients with other HRIs and a control group during a 14-year follow-up period.

Results

Our analyses included 150 HS cases, 150 patients with other HRIs and 150 patients without HRIs. The HS patients had a significantly higher incidence of developing CVDs than the other HRI and control patients (32.67% vs. 23.33% vs. 16.67%, p = 0.005). Patients with HS had an increased incidence of acute myocardial infarction (AMI) compared with that of the controls (6% vs. 2.67%, p = 0.042) and an increased incidence of acute ischemic stroke (AIS) compared with those of the other HRI and control patients (12% vs. 6% vs. 4.67%, p = 0.038). An increased risk of chronic kidney disease (CKD) was also found in the patients with HS and other HRIs compared to that in the controls (17.33% vs. 14.67% vs. 6.67%, p = 0.016).

Conclusion

Prior HS was associated with an increased incidence of CVDs, particularly AMI and AIS, and an increased incidence of CKD.

Clinical Effects of Regular Dry Sauna Bathing: A Systematic Review

Introduction

Many health benefits are claimed by individuals and facilities promoting sauna bathing; however the medical evidence to support these claims is not well established. This paper aims to systematically review recent research on the effects of repeated dry sauna interventions on human health.

Methods

A systematic search was made of medical databases for studies reporting on the health effects of regular dry sauna bathing on humans from 2000 onwards. Risk of bias was assessed according to the Cochrane Collaboration guidelines.

Results

Forty clinical studies involving a total of 3855 participants met the inclusion criteria. Only 13 studies were randomized controlled trials and most studies were small (n < 40). Reported outcome measures were heterogeneous with most studies reporting beneficial health effects. Only one small study (n = 10) reported an adverse health outcome of disrupted male spermatogenesis, demonstrated to be reversible when ceasing sauna activity.

Conclusions

Regular dry sauna bathing has potential health benefits. More data of higher quality is needed on the frequency and extent of adverse side effects. Further study is also needed to determine the optimal frequency and duration of distinct types of sauna bathing for targeted health effects and the specific clinical populations who are most likely to benefit.

Effect of Morinda citrifolia (Noni)-Enriched Diet on Hepatic Heat Shock Protein and Lipid Metabolism-Related Genes in Heat Stressed Broiler Chickens

Heat stress (HS) has been reported to alter fat deposition in broilers, however the underlying molecular mechanisms are not well-defined. The objectives of the current study were, therefore: (1) to determine the effects of acute (2 h) and chronic (3 weeks) HS on the expression of key molecular signatures involved in hepatic lipogenic and lipolytic programs, and (2) to assess if diet supplementation with dried Noni medicinal plant (0.2% of the diet) modulates these effects. Broilers (480 males, 1 d) were randomly assigned to 12 environmental chambers, subjected to two environmental conditions (heat stress, HS, 35°C vs. thermoneutral condition, TN, 24°C) and fed two diets (control vs. Noni) in a 2 × 2 factorial design. Feed intake and body weights were recorded, and blood and liver samples were collected at 2 h and 3 weeks post-heat exposure. HS depressed feed intake, reduced body weight, and up regulated the hepatic expression of heat shock protein HSP60, HSP70, HSP90 as well as key lipogenic proteins (fatty acid synthase, FASN; acetyl co-A carboxylase alpha, ACCα and ATP citrate lyase, ACLY). HS down regulated the hepatic expression of lipoprotein lipase (LPL) and hepatic triacylglycerol lipase (LIPC), but up-regulated ATGL. Although it did not affect growth performance, Noni supplementation regulated the hepatic expression of lipogenic proteins in a time- and gene-specific manner. Prior to HS, Noni increased ACLY and FASN in the acute and chronic experimental conditions, respectively. During acute HS, Noni increased ACCα, but reduced FASN and ACLY expression. Under chronic HS, Noni up regulated ACCα and FASN but it down regulated ACLY. In vitro studies, using chicken hepatocyte cell lines, showed that HS down-regulated the expression of ACCα, FASN, and ACLY. Treatment with quercetin, one bioactive ingredient in Noni, up-regulated the expression of ACCα, FASN, and ACLY under TN conditions, but it appeared to down-regulate ACCα and increase ACLY levels under HS exposure. In conclusion, our findings indicate that HS induces hepatic lipogenesis in chickens and this effect is probably mediated via HSPs. The modulation of hepatic HSP expression suggest also that Noni might be involved in modulating the stress response in chicken liver.

Irisin and Fibronectin Type III Domain-Containing 5 Responses to Exercise in Different Environmental Conditions

Fibronectin type III domain-containing 5 (FNDC5) is a skeletal muscle membrane-bound precursor to the myokine irisin. Irisin is involved in stimulating adipose tissue to become more metabolically active in order to produce heat. The purpose of this study was to determine the effects of exercise in a hot (33 °C), cold (7 °C), and room temperature (RT, 20 °C) environment on the skeletal muscle gene expression of FNDC5 and the plasma concentrations of irisin. Twelve recreationally trained males completed three separate, 1 h cycling bouts at 60% of W_max in a hot, cold, and RT environment followed by three hours of recovery at room temperature. Blood samples were taken from the antecubital vein and muscle biopsies were taken from the vastus lateralis pre-, post-, and 3 h post-exercise. Plasma concentrations of irisin did not change from pre- (9.23 ± 2.68 pg·mL^-1) to post-exercise (9.6 ± 0.2 pg·mL^-1, p = 0.068), but did decrease from post-exercise to 3 h post-exercise (8.9 ± 0.5 pg·mL^-1, p = 0.047) regardless of temperature. However, when plasma volume shifts were considered, no differences were found in irisin (p = 0.086). There were no significant differences between trials for irisin plasma concentrations (p > 0.05). No significant differences in FNDC5 were observed between the hot, cold, or RT or pre-, post-, or 3 h post-exercise time points (p > 0.05). These data indicate that the temperature in which exercise takes place does not influence FNDC5 transcription or circulating irisin in a human model.

Effect of Heat Exposure on Cognition in Persons with Tetraplegia

Individuals with cervical spinal cord injury (SCI) have impaired thermoregulatory mechanisms attributed to interruption of motor, sensory, and autonomic neuropathways. To determine the effects of heat exposure on core body temperature (Tcore) and cognitive performance in persons with tetraplegia, 8 individuals with chronic tetraplegia (C3-C7, American Spinal Cord Injury Association Impairment Scale A-B) and 9 able-bodied controls were acclimated to 27°C at baseline (BL) before being exposed to 35°C for up to 120 min (Heat Challenge). Rectal temperature (Tcore), distal skin temperatures (Tsk_avg), sweat rate (QS_avg), microvascular skin perfusion (LDF_avg), and plasma norepinephrine (NE) were measured. Cognitive performance was assessed using Stroop Color and Word and Wechsler Adult Intelligence Scale-Fourth Edition Digit Span tests at BL and at the end of Heat Challenge. After Heat Challenge, Tcore increased 0.78 ± 0.18°C (p < 0.001) in tetraplegics after an average of 118 ± 5 min. Tcore did not change in controls after 120 min. The increase in QS_avg was larger in controls than in tetraplegics (946 ± 672% vs. 51 ± 12%; p = 0.007, respectively). LDF_avg increased only in controls (109 ± 93%; p = 0.008). Tsk_avg appeared to increase less in tetraplegics than in controls. Plasma NE levels remained lower in tetraplegics compared to controls after Heat Challenge (86 ± 64 vs. 297 ± 84 pg/mL, respectively; p < 0.001). Stroop Color, Interference, and WAIS-IV Sequence scores increased only in tetraplegics (19.4 ± 17.2%; p < 0.05, 8.3 ± 5.9%; p < 0.05, 29.1 ± 27.4%; p < 0.05, respectively). Dysfunctional thermoregulatory mechanisms in the tetraplegic group allowed Tcore to rise from subnormal levels to normothermia during heat exposure. Normothermia was associated with improvements in attention, working memory, and executive function.

The effect of passive heating on heat shock protein 70 and interleukin-6: A possible treatment tool for metabolic diseases?

Increasing physical activity remains the most widely publicized way of improving health and wellbeing. However, in populations that benefit most from exercise (EX), adherence is often poor and alternatives to EX are important to bring about health improvements. Recent work suggests a role for passive heating (PH) and heat shock proteins (HSP) in improving cardio-metabolic health. The aim of this study was to investigate the expression of HSP70 and interleukin-6 in response to either EX or PH and the subsequent effect on glucose control. Fourteen males volunteered and were categorized lean (BMI 23.5 ± 2.2 kg·m^-2) or overweight (29.2 ± 2.7 kg·m^-2) and completed 60 minutes of either moderate cycling at a fixed rate of metabolic heat production (EX) or warm water immersion in 40°C water (PH). Extracellular HSP70 increased from baseline in both conditions with no differences between PH (0.98 ± 1.1 ng·mL^-1) or EX (0.84 ± 1.0 ng·mL^-1, p = 0.814). IL-6 increased following both conditions with a two-fold increase after PH and four-fold after EX. Energy expenditure increased by 61.0 ± 14.4 kcal·h^-1 (79%) after PH. Peak glucose concentration after a meal immediately following PH was reduced when compared with EX (6.3 ± 1.4 mmol·L^-1 versus 6.8 ± 1.2 mmol·L^-1; p < 0.05). There was no difference in 24-hour glucose area under the curve (AUC) between conditions. These data indicate the potential for thermal therapy as an alternative treatment and management strategy for those at risk of developing metabolic disease where adherence, or ability to EX, may be compromised.

The influence of occupational heat exposure on cognitive performance and blood level of stress hormones: a field study report

INTRODUCTION

This article aimed to investigate the effect of heat stress on cognitive performance and the blood concentration of stress hormones among workers of a foundry plant.

METHODS

Seventy workers within the exposed (35 people) and unexposed (35 people) groups were studied. The wet bulb globe temperature (WBGT) index was measured for heat stress assessment. The cognitive performance tests were conducted using the Stroop color word test (SCWT) before and during working hours. For the assessment of the serum level of cortisol and the plasma level of adrenaline and noradrenaline, blood samples were taken during working hours from both groups.

RESULTS

Only for SCWT III was there a significant relationship between heat stress and test duration, error rate and reaction time. The laboratory test results revealed significantly higher concentrations of cortisol, adrenaline and noradrenaline in the exposed subjects than in the unexposed group. There existed a positive correlation between cortisol, adrenaline, noradrenaline and WBGT index and also test duration and reaction time of SCWT III, and number of errors of SCWT I, SCWT II and SCWT III during work.

CONCLUSION

Heat stress can lead to an increase in the blood level of stress hormones, resulting in cognitive performance impairment.

Allicin protects traumatic spinal cord injury through regulating the HSP70/Akt/iNOS pathway in mice

Allicin is a major component of garlic, extracted as an oily liquid. The present study was designed to investigate the beneficial effects of allicin on traumatic spinal cord injury (TSCI) in mice, and whether the effects are mediated via regulation of the heat shock protein 70 (HSP70), v-akt murine thymoma viral oncogene homolog 1 (Akt) and inducible nitric oxide synthase (iNOS) pathways. Adult BALB/c mice (30–40 g) received a laminectomy at the T9 vertebral level as a model of TSCI. In the present study, treatment of the TSCI mice with allicin significantly increased their Basso, Beattie and Bresnahan (BBB) scores (P<0.01) and reduced the spinal cord water content (P<0.01). This protective effect was associated with the inhibition of oxidative stress and inflammatory responses in TSCI mice. Western blot analysis demonstrated that allicin increased the protein levels of HSP70, increased the phosphorylation of Akt and reduced the iNOS protein expression levels in TSCI mice. Additionally, treatment with allicin significantly reduced the levels of ROS and enhanced the NADH levels in TSCI mice. Collectively, these data demonstrate that the effects of allicin on TSCI are mediated via regulation of the HSP70, Akt and iNOS pathways in mice.

Distinct Skeletal Muscle Gene Regulation from Active Contraction, Passive Vibration, and Whole Body Heat Stress in Humans

Skeletal muscle exercise regulates several important metabolic genes in humans. We know little about the effects of environmental stress (heat) and mechanical stress (vibration) on skeletal muscle. Passive mechanical stress or systemic heat stress are often used in combination with many active exercise programs. We designed a method to deliver a vibration stress and systemic heat stress to compare the effects with active skeletal muscle contraction. Purpose: The purpose of this study is to examine whether active mechanical stress (muscle contraction), passive mechanical stress (vibration), or systemic whole body heat stress regulates key gene signatures associated with muscle metabolism, hypertrophy/atrophy, and inflammation/repair. Methods: Eleven subjects, six able-bodied and five with chronic spinal cord injury (SCI) participated in the study. The six able-bodied subjects sat in a heat stress chamber for 30 minutes. Five subjects with SCI received a single dose of limb-segment vibration or a dose of repetitive electrically induced muscle contractions. Three hours after the completion of each stress, we performed a muscle biopsy (vastus lateralis or soleus) to analyze mRNA gene expression. Results: We discovered repetitive active muscle contractions up regulated metabolic transcription factors NR4A3 (12.45 fold), PGC-1α (5.46 fold), and ABRA (5.98 fold); and repressed MSTN (0.56 fold). Heat stress repressed PGC-1α (0.74 fold change; p < 0.05); while vibration induced FOXK2 (2.36 fold change; p < 0.05). Vibration similarly caused a down regulation of MSTN (0.74 fold change; p < 0.05), but to a lesser extent than active muscle contraction. Vibration induced FOXK2 (p < 0.05) while heat stress repressed PGC-1α (0.74 fold) and ANKRD1 genes (0.51 fold; p < 0.05). Conclusion: These findings support a distinct gene regulation in response to heat stress, vibration, and muscle contractions. Understanding these responses may assist in developing regenerative rehabilitation interventions to improve muscle cell development, growth, and repair.

Heat therapy promotes the expression of angiogenic regulators in human skeletal muscle

Heat therapy has been shown to promote capillary growth in skeletal muscle and in the heart in several animal models, but the effects of this therapy on angiogenic signaling in humans are unknown. We evaluated the acute effect of lower body heating (LBH) and unilateral thigh heating (TH) on the expression of angiogenic regulators and heat shock proteins (HSPs) in healthy young individuals. Exposure to LBH (n = 18) increased core temperature (Tc) from 36.9 ± 0.1 to 37.4 ± 0.1°C (P < 0.01) and average leg skin temperature (Tleg) from 33.1 ± 0.1 to 39.6 ± 0.1°C (P < 0.01), but did not alter the levels of circulating angiogenic cytokines and bone marrow-derived proangiogenic cells (CD34(+)CD133(+)). In skeletal muscle, the change in mRNA expression from baseline of vascular endothelial growth factor (VEGF), angiopoietin 2 (ANGPT2), chemokines CCL2 and CX3CL1, platelet factor-4 (PF4), and several members of the HSP family was higher 30 min after the intervention in the individuals exposed to LBH (n = 11) compared with the control group (n = 12). LBH also reduced the expression of transcription factor FOXO1 (P = 0.03). Exposure to TH (n = 14) increased Tleg from 32.8 ± 0.2 to 40.3 ± 0.1°C (P < 0.05) but Tc remained unaltered (36.8 ± 0.1°C at baseline and 36.9 ± 0.1°C at 90 min). This intervention upregulated the expression of VEGF, ANGPT1, ANGPT2, CCL2, and HSPs in skeletal muscle but did not affect the levels of CX3CL1, FOXO-1, and PF4. These findings suggest that both LBH and TH increase the expression of factors associated with capillary growth in human skeletal muscle.

Substantive hemodynamic and thermal strain upon completing lower-limb hot-water immersion; comparisons with treadmill running

Exercise induces arterial flow patterns that promote functional and structural adaptations, improving functional capacity and reducing cardiovascular risk. While heat is produced by exercise, local and whole-body passive heating have recently been shown to generate favorable flow profiles and associated vascular adaptations in the upper limb. Flow responses to acute heating in the lower limbs have not yet been assessed, or directly compared to exercise, and other cardiovascular effects of lower-limb heating have not been fully characterized. Lower-limb heating by hot-water immersion (30 min at 42°C, to the waist) was compared to matched-duration treadmill running (65-75% age-predicted heart rate maximum) in 10 healthy, young adult volunteers. Superficial femoral artery shear rate assessed immediately upon completion was increased to a greater extent following immersion (mean ± SD: immersion +252 ± 137% vs. exercise +155 ± 69%, interaction: p = 0.032), while superficial femoral artery flow-mediated dilation was unchanged in either intervention. Immersion increased heart rate to a lower peak than during exercise (immersion +38 ± 3 beats·min^-1 vs. exercise +87 ± 3 beats·min^-1, interaction: p < 0.001), whereas only immersion reduced mean arterial pressure after exposure (−8 ± 3 mmHg, p = 0.012). Core temperature increased twice as much during immersion as exercise (+1.3 ± 0.4°C vs. +0.6 ± 0.4°C, p < 0.001). These data indicate that acute lower-limb hot-water immersion has potential to induce favorable shear stress patterns and cardiovascular responses within vessels prone to atherosclerosis. Whether repetition of lower-limb heating has long-term beneficial effects in such vasculature remains unexplored.

Inflammatory cytokines and plasma redox status responses in hypertensive subjects after heat exposure

Hypertension is characterized by a pro-inflammatory status, including redox imbalance and increased levels of pro-inflammatory cytokines, which may be exacerbated after heat exposure. However, the effects of heat exposure, specifically in individuals with inflammatory chronic diseases such as hypertension, are complex and not well understood. This study compared the effects of heat exposure on plasma cytokine levels and redox status parameters in 8 hypertensive (H) and 8 normotensive (N) subjects (age: 46.5±1.3 and 45.6±1.4 years old, body mass index: 25.8±0.8 and 25.6±0.6 kg/m2, mean arterial pressure: 98.0±2.8 and 86.0±2.3 mmHg, respectively). They remained at rest in a sitting position for 10 min in a thermoneutral environment (22°C) followed by 30 min in a heated environmental chamber (38°C and 60% relative humidity). Blood samples were collected before and after heat exposure. Plasma cytokine levels were measured using sandwich ELISA kits. Plasma redox status was determined by thiobarbituric acid reactive substances (TBARS) levels and ferric reducing ability of plasma (FRAP). Hypertensive subjects showed higher plasma levels of IL-10 at baseline (P<0.05), although levels of this cytokine were similar between groups after heat exposure. Moreover, after heat exposure, hypertensive individuals showed higher plasma levels of soluble TNF receptor (sTNFR1) and lower TBARS (P<0.01) and FRAP (P<0.05) levels. Controlled hypertensive subjects, who use angiotensin-converting-enzyme inhibitor (ACE inhibitors), present an anti-inflammatory status and balanced redox status. Nevertheless, exposure to a heat stress condition seems to cause an imbalance in the redox status and an unregulated inflammatory response.

Whole body heat stress increases motor cortical excitability and skill acquisition in humans

OBJECTIVE

Vigorous systemic exercise stimulates a cascade of molecular and cellular processes that enhance central nervous system (CNS) plasticity and performance. The influence of heat stress on CNS performance and learning is novel. We designed two experiments to determine whether passive heat stress (1) facilitated motor cortex excitability and (2) improved motor task acquisition compared to no heat stress.

METHODS

Motor evoked potentials (MEPs) from the first dorsal interosseus (FDI) were collected before and after 30 min of heat stress at 73 °C. A second cohort of subjects performed a motor learning task using the FDI either following heat or the no heat condition.

RESULTS

Heat stress increased heart rate to 65% of age-predicted maximum. After heat, mean resting MEP amplitude increased 48% (p<0.05). MEP stimulus-response amplitudes did not differ according to stimulus intensity. In the second experiment, heat stress caused a significant decrease in absolute and variable error (p<0.05) during a novel movement task using the FDI.

CONCLUSIONS

Passive environmental heat stress (1) increases motor cortical excitability, and (2) enhances performance in a motor skill acquisition task.

SIGNIFICANCE

Controlled heat stress may prime the CNS to enhance motor skill acquisition during rehabilitation.