Association Between Exercise-Induced Hyperthermia and Intestinal Permeability: A Systematic Review

Markers of enterocyte damage, microbial translocation, and systemic inflammation following 9 h of heat exposure in young and older adults

Heat stress induced damage to the gastrointestinal barrier can induce local and systemic inflammatory reactions implicated in heat-stroke. Gastrointestinal barrier damage has been shown to be greater in older relative to young adults following hyperthermia. However, comparisons between young and older adults have been limited to brief exposures (3 h), which may not reflect the duration of heat stress experienced during heat waves. We therefore evaluated markers of intestinal epithelial damage (log transformed intestinal fatty acid binding protein, IFABPLOG), microbial translocation (soluble cluster of differentiation 14, sCD14LOG), and systemic inflammation (tumour necrosis factor alpha, TNF-αLOG; interleukin 6, IL-6LOG; C-reactive protein, CRP) in 19 young (interquartile range: 21-27 years; 10 females) and 37 older (68-73 years; 10 females) adults before and after 9 h of rest in 40 °C (9% relative humidity). The magnitude of the increase in IFABPLOG was 0.38 log pg/mL (95% CI, 0.10, 0.65 log pg/mL) greater in the older relative to young cohort (P = 0.049) after 9 h heat exposure. At baseline both IL-6LOG and CRP concentrations were higher in the older (IL-6: 2.67 (1.5) log pg/mL, CRP: 0.28 (1.5) mg/mL) relative to the young (IL-6: 1.59 log pg/mL, SD 1.2; CRP: 0.11 mg/mL, SD 1.7) group (both P ≤ 0.001). The change in IL-6 and CRP was similar between groups following 9 h heat exposure (IL-6: P = 0.053; CRP: P = 0.241). Neither sCD14LOG and TNF-αLOG were different between groups at baseline nor altered after 9 h heat exposure. Our data indicate that age may modify intestinal epithelial injury following 9 h of passive heat exposure.

Modulation of gut microbiota on intestinal permeability: A novel strategy for treating gastrointestinal related diseases

Accumulating evidence emphasizes the critical reciprocity between gut microbiota and intestinal barrier function in maintaining the gastrointestinal homeostasis. Given the fundamental role caused by intestinal permeability, which has been scrutinized as a measurable potential indicator of perturbed barrier function in clinical researches, it seems not surprising that recent decades have been marked by augmented efforts to determine the interaction between intestinal microbes and permeability of the individual. However, despite the significant progress in characterizing intestinal permeability and the commensal bacteria in the intestine, the mechanisms involved are still far from being thoroughly revealed. In the present review, based on multiomic methods, high-throughput sequencing and molecular biology techniques, the impacts of gut microbiota on intestinal permeability as well as their complex interaction networks are systematically summarized. Furthermore, the diseases related to intestinal permeability and main causes of changes in intestinal permeability are briefly introduced. The purpose of this review is to provide a novel prospection to elucidate the correlation between intestinal microbiota and permeability, and to explore a promising solution for diagnosis and treatment of gastrointestinal related diseases.

Intestinal barrier permeability: the influence of gut microbiota, nutrition, and exercise

The intestinal wall is a selectively permeable barrier between the content of the intestinal lumen and the internal environment of the body. Disturbances of intestinal wall permeability can potentially lead to unwanted activation of the enteric immune system due to excessive contact with gut microbiota and its components, and the development of endotoxemia, when the level of bacterial lipopolysaccharides increases in the blood, causing chronic low-intensity inflammation. In this review, the following aspects are covered: the structure of the intestinal wall barrier; the influence of the gut microbiota on the permeability of the intestinal wall via the regulation of functioning of tight junction proteins, synthesis/degradation of mucus and antioxidant effects; the molecular mechanisms of activation of the pro-inflammatory response caused by bacterial invasion through the TLR4-induced TIRAP/MyD88 and TRAM/TRIF signaling cascades; the influence of nutrition on intestinal permeability, and the influence of exercise with an emphasis on exercise-induced heat stress and hypoxia. Overall, this review provides some insight into how to prevent excessive intestinal barrier permeability and the associated inflammatory processes involved in many if not most pathologies. Some diets and physical exercise are supposed to be non-pharmacological approaches to maintain the integrity of intestinal barrier function and provide its efficient operation. However, at an early age, the increased intestinal permeability has a hormetic effect and contributes to the development of the immune system.

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

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

Intrinsic and Extrinsic Contributors to the Cardiac Benefits of Exercise

Among its many cardiovascular benefits, exercise training improves heart function and protects the heart against age-related decline, pathological stress, and injury. Here, we focus on cardiac benefits with an emphasis on more recent updates to our understanding. While the cardiomyocyte continues to play a central role as both a target and effector of exercise's benefits, there is a growing recognition of the important roles of other, noncardiomyocyte lineages and pathways, including some that lie outside the heart itself. We review what is known about mediators of exercise's benefits-both those intrinsic to the heart (at the level of cardiomyocytes, fibroblasts, or vascular cells) and those that are systemic (including metabolism, inflammation, the microbiome, and aging)-highlighting what is known about the molecular mechanisms responsible.

Changes in surrogate markers of intestinal epithelial injury and microbial translocation in young and older men during prolonged occupational heat stress in temperate and hot conditions

PURPOSE

Exertional heat stress can cause damage to the intestinal epithelium and disrupt gastrointestinal barrier integrity, leading to microbial translocation (MT) linked to the development of heat stroke. This study aimed to assess age-related differences in markers of intestinal epithelial injury and MT following non-heat stress and high-heat stress exercise in healthy young and older men.

METHODS

Markers of intestinal epithelial injury (intestinal fatty acid-binding protein-'IFABP') and MT (soluble cluster of differentiation 14-'sCD14'; and lipopolysaccharide-binding protein-'LBP') were assessed in healthy young (18-30 y, n = 13) and older (50-70 y) men (n = 12). Blood samples were collected before, after 180 min of moderate-intensity (metabolic rate: 200 W/m2) walking and following 60 min recovery in either a non-heat stress [temperate: 21.9 °C, 35% relative humidity (RH)] or high-heat stress (hot: 41.4 °C, 35% RH) environment.

RESULTS

There were no differences in IFABP and sCD14 between the young and older groups in the temperate condition, while LBP was greater in the older group (+ 0.66 ug/mL; + 0.08 to + 1.24 ug/mL). In the hot condition, the older group experienced greater increases in IFABP compared to the young group (+ 712 pg/mL/hr; + 269 to + 1154 pg/mL/hr). However, there were no clear between-group differences for sCD14 (+ 0.24 ug/mL/hr, - 0.22 to + 0.70 ug/mL/hr) or LBP (+ 0.86 ug/mL/hr, - 0.73 to + 2.46 ug/mL/hr).

CONCLUSION

While older men may experience greater intestinal epithelial injury following exercise in the heat; this did not lead to a greater magnitude of microbial translocation relative to their younger counterparts.

What if gastrointestinal complications in endurance athletes were gut injuries in response to a high consumption of ultra-processed foods? Please take care of your bugs if you want to improve endurance performance: a narrative review

To improve performance and recovery faster, athletes are advised to eat more often than usual and consume higher doses of simple carbohydrates, during and after exercise. Sports energetic supplements contain food additives, such as artificial sweeteners, emulsifiers, acidity regulators, preservatives, and salts, which could be harmful to the gut microbiota and impair the intestinal barrier function. The intestinal barrier plays a critical function in bidirectionally regulation of the selective transfer of nutrients, water, and electrolytes, while preventing at the same time, the entrance of harmful substances (selective permeability). The gut microbiota helps to the host to regulate intestinal homeostasis through metabolic, protective, and immune functions. Globally, the gut health is essential to maintain systemic homeostasis in athletes, and to ensure proper digestion, metabolization, and substrate absorption. Gastrointestinal complaints are an important cause of underperformance and dropout during endurance events. These complications are directly related to the loss of gut equilibrium, mainly linked to microbiota dysbiosis and leaky gut. In summary, athletes must be cautious with the elevated intake of ultra-processed foods and specifically those contained on sports nutrition supplements. This review points out the specific nutritional interventions that should be implemented and/or discontinued depending on individual gut functionality.

Gastrointestinal manifestations of critical ill heatstroke patients and their associations with outcomes: A multicentre, retrospective, observational study

BACKGROUND

Extreme heat exposure is a growing health problem, and the effects of heat on the gastrointestinal (GI) tract is unknown. This study aimed to assess the incidence of GI symptoms associated with heatstroke and its impact on outcomes.

AIM

To assess the incidence of GI symptoms associated with heatstroke and its impact on outcomes.

METHODS

Patients admitted to the intensive care unit (ICU) due to heatstroke were included from 83 centres. Patient history, laboratory results, and clinically relevant outcomes were recorded at ICU admission and daily until up to day 15, ICU discharge, or death. GI symptoms, including nausea/vomiting, diarrhoea, flatulence, and bloody stools, were recorded. The characteristics of patients with heatstroke concomitant with GI symptoms were described. Multivariable regression analyses were performed to determine significant predictors of GI symptoms.

RESULTS

A total of 713 patients were included in the final analysis, of whom 132 (18.5%) patients had at least one GI symptom during their ICU stay, while 26 (3.6%) suffered from more than one symptom. Patients with GI symptoms had a significantly higher ICU stay compared with those without. The mortality of patients who had two or more GI symptoms simultaneously was significantly higher than that in those with one GI symptom. Multivariable logistic regression analysis revealed that older patients with a lower GCS score on admission were more likely to experience GI symptoms.

CONCLUSION

The GI manifestations of heatstroke are common and appear to impact clinically relevant hospitalization outcomes.

Gut-muscle-brain axis: Molecular mechanisms in neurodegenerative disorders and potential therapeutic efficacy of probiotic supplementation coupled with exercise

Increased longevity is often associated with age-related conditions. The most common neurodegenerative disorders in the older population are Alzheimer's disease (AD) and Parkinson's disease (PD), associated with progressive neuronal loss leading to functional and cognitive impairments. Although symptomatic treatments are available, there is currently no cure for these conditions. Gut dysbiosis has been involved in the pathogenesis of AD and PD, thus interventions targeting the "gut-brain axis" could potentially prevent or delay these pathologies. Recent evidence suggests that the skeletal muscle and the gut microbiota can affect each other via the "gut-muscle axis". Importantly, cognitive functions in AD and PD patients significantly benefit from physical activity. In this review, we aim to provide a comprehensive picture of the crosstalk between the brain, the skeletal muscle and the gut microbiota, introducing the concept of "gut-muscle-brain axis". Moreover, we discuss human and animal studies exploring the modulatory role of exercise and probiotics on cognition in AD and PD. Collectively, the findings presented here support the potential benefits of physical activity and probiotic supplementation in AD and PD. Further studies will be needed to develop targeted and multimodal strategies, including lifestyle changes, to prevent or delay the course of these pathologies.

Psychosocial stress-induced intestinal permeability in healthy humans: What is the evidence?

An impaired intestinal barrier function can be detrimental to the host as it may allow the translocation of luminal antigens and toxins into the subepithelial tissue and bloodstream. In turn, this may cause local and systemic immune responses and lead to the development of pathologies. In vitro and animal studies strongly suggest that psychosocial stress is one of the factors that can increase intestinal permeability via mast-cell dependent mechanisms. Remarkably, studies have not been able to yield unequivocal evidence that such relation between stress and intestinal permeability also exists in (healthy) humans. In the current Review, we discuss the mechanisms that are involved in stress-induced intestinal permeability changes and postulate factors that influence these alterations and that may explain the translational difficulties from in vitro and animal to human studies. As human research differs highly from animal research in the extent to which stress can be applied and intestinal permeability can be measured, it remains difficult to draw conclusions about the presence of a relation between stress and intestinal permeability in (healthy) humans. Future studies should bear in mind these difficulties, and more research into in vivo methods to assess intestinal permeability are warranted.

Possible relationship between the gut leaky syndrome and musculoskeletal injuries: the important role of gut microbiota as indirect modulator

This article aims to examine the evidence on the relationship between gut microbiota (GM), leaky gut syndrome and musculoskeletal injuries. Musculoskeletal injuries can significantly impair athletic performance, overall health, and quality of life. Emerging evidence suggests that the state of the gut microbiota and the functional intestinal permeability may contribute to injury recovery. Since 2007, a growing field of research has supported the idea that GM exerts an essential role maintaining intestinal homeostasis and organic and systemic health. Leaky gut syndrome is an acquired condition where the intestinal permeability is impaired, and different bacteria and/or toxins enter in the bloodstream, thereby promoting systemic endotoxemia and chronic low-grade inflammation. This systemic condition could indirectly contribute to increased local musculoskeletal inflammation and chronificate injuries and pain, thereby reducing recovery-time and limiting sport performance. Different strategies, including a healthy diet and the intake of pre/probiotics, may contribute to improving and/or restoring gut health, thereby modulating both systemically as local inflammation and pain. Here, we sought to identify critical factors and potential strategies that could positively improve gut microbiota and intestinal health, and reduce the risk of musculoskeletal injuries and its recovery-time and pain. In conclusion, recent evidences indicate that improving gut health has indirect consequences on the musculoskeletal tissue homeostasis and recovery through the direct modulation of systemic inflammation, the immune response and the nociceptive pain.

Reduced running performance and greater perceived exertion, but similar post-exercise neuromuscular fatigue in tropical natives subjected to a 10 km self-paced run in a hot compared to a temperate environment

Environmental heat stress impairs endurance performance by enhancing exercise-induced physiological and perceptual responses. However, the time course of these responses during self-paced running, particularly when comparing hot and temperate conditions, still needs further clarification. Moreover, monitoring fatigue induced by exercise is paramount to prescribing training and recovery adequately, but investigations on the effects of a hot environment on post-exercise neuromuscular fatigue are scarce. This study compared the time course of physiological and perceptual responses during a 10 km self-paced treadmill run (as fast as possible) between temperate (25°C) and hot (35°C) conditions. We also investigated the changes in countermovement jump (CMJ) performance following exercise in these two ambient temperatures. Thirteen recreational long-distance runners (11 men and 2 women), inhabitants of a tropical region, completed the two experimental trials in a randomized order. Compared to 25°C, participants had transiently higher body core temperature (TCORE) and consistently greater perceived exertion while running at 35°C (p < 0.05). These changes were associated with a slower pace, evidenced by an additional 14 ± 5 min (mean ± SD) to complete the 10 km at 35°C than at 25°C (p < 0.05). Before, immediately after, and 1 h after the self-paced run, the participants performed CMJs to evaluate lower limb neuromuscular fatigue. CMJ height was reduced by 7.0% (2.3 ± 2.4 cm) at 1 h after the race (p < 0.05) compared to pre-exercise values; environmental conditions did not influence this reduction. In conclusion, despite the reduced endurance performance, higher perceived exertion, and transiently augmented TCORE caused by environmental heat stress, post-exercise neuromuscular fatigue is similar between temperate and hot conditions. This finding suggests that the higher external load (faster speed) at 25°C compensates for the effects of more significant perceptual responses at 35°C in inducing neuromuscular fatigue.

The effects of betaine supplementation on fluid balance and heat tolerance during passive heat stress in men

INTRODUCTION

Consuming intracellular osmolytes, like betaine (BET), may attenuate symptoms of heat stress. The purpose of this study was to examine the effects of BET supplementation on fluid balance and heat tolerance after a 7-day loading period and during passive heat exposure.

METHODS

A double-blind, placebo controlled, crossover study compared BET or placebo consumption (50 mg·kg-1 , twice daily) for 7 days in young, recreationally active men (N = 11).

RESULTS

During the loading period, no significant interactions were found for any marker of fluid balance between or within conditions. During heat exposure, significant time effects but no condition x time interactions, were found for plasma characteristics (i.e., volume, osmolality, sodium, albumin, and total protein). Plasma volume was significantly increased by min 30 in both conditions (PLA: +6.9. ± 5.0%, BET: +10.2 ± 7.4%) and remained elevated for the remainder of the experimental trial, but was not significantly different between conditions. After 60 min of passive heat exposure, both conditions experienced a similar increase in core temperature (PLA: +0.32 ± 0.22°C, BET: +0.31 ± 0.21°C; p = 0.912).

CONCLUSIONS

Supplemental BET did not improve markers of fluid balance or heat tolerance during 7 days of loading or during passive heat exposure.

The increase in core body temperature in response to exertional-heat stress can predict exercise-induced gastrointestinal syndrome

Utilizing metadata from existing exertional and exertional-heat stress studies, the study aimed to determine if the exercise-associated increase in core body temperature can predict the change in exercise-induced gastrointestinal syndrome (EIGS) biomarkers and exercise-associated gastrointestinal symptoms (Ex-GIS). Endurance-trained individuals completed 2 h of running exercise in temperate (21.2-30.0°C) to hot (35.0-37.2°C) ambient conditions (n = 132 trials). Blood samples were collected pre- and post-exercise to determine the change in gastrointestinal integrity biomarkers and systemic inflammatory cytokines. Physiological and thermoregulatory strain variables were assessed every 10-15 min during exercise. The strength of the linear relationship between maximal (M-Tre) and change (Δ Tre) in rectal temperature and EIGS variables was determined via Spearman's rank correlation coefficients. While the strength of prediction was determined via simple and multiple linear regression analyses dependent on screened EIGS and Ex-GIS confounding factors. Significant positive correlations between Tre maximum (M-Tre) and change (Δ Tre) with I-FABP (rs = 0.434, p < 0.001; and rs = 0.305, p < 0.001; respectively), sCD14 (rs = 0.358, p < 0.001; and rs = 0.362, p < 0.001), systemic inflammatory response profile (SIR-Profile) (p < 0.001), and total Ex-GIS (p < 0.05) were observed. M-Tre and Δ Tre significantly predicted (adjusted R2) magnitude of change in I-FABP (R2(2,123)=0.164, p < 0.001; and R2(2,119)=0.058, p = 0.011; respectively), sCD14 (R2(2,81)=0.249, p < 0.001; and R2(2,77)=0.214, p < 0.001), SIR-Profile (p < 0.001), and total Ex-GIS (p < 0.05). Strong to weak correlations were observed between M-Tre and Δ Tre with plasma concentrations of I-FABP, sCD14, SIR-Profile, and Ex-GIS in response to exercise. M-Tre and Δ Tre can predict the magnitude of these EIGS variables and Ex-GIS in response to exercise.

Ovariectomy aggravates the pathophysiological response to exertional heat stroke in mice

Female mice have a greater capacity for exercising in the heat than male mice, reaching greater power output and longer times of heat exposure before succumbing to exertional heat stroke (EHS). Differences in body mass, size, or testosterone do not explain these distinct sex responses. Whether the ovaries could account for the superior exercise capacity in the heat in females remains unknown. Here, we determined the influence of ovariectomy (OVX) on exercise capacity in the heat, thermoregulation, intestinal damage, and heat shock response in a mouse EHS model. We performed bilateral OVX (n = 10) or sham (n = 8) surgeries in young adult (4 mo) female C57/BL6J mice. Upon recovery from surgeries, mice exercised on a forced wheel placed inside an environmental chamber set at 37.5 °C and 40% relative humidity until experiencing loss of consciousness (LOC). Terminal experiments were performed 3 h after LOC. OVX increased body mass by the time of EHS (sham = 3.8 ± 1.1, OVX = 8.3 ± 3.2 g, P < 0.05), resulted in shorter running distance (sham = 753 ± 189, OVX = 490 ± 87 m, P < 0.05), and shorter time to LOC (sham = 126.3 ± 21, OVX = 99.1 ± 19.8 min, P < 0.05). Histopathological assessment of the intestines revealed damage in the jejunum (sham = 0.2 ± 0.7, OVX = 2.1 ± 1.7 AU, P < 0.05) and ileum (sham = 0.3 ± 0.5, OVX = 1.8 ± 1.4 AU, P < 0.05). OVX increased mesenteric microvascular density (sham = 101 ± 25, OVX = 156 ± 66 10-2 mm/mm2, P < 0.05) and decreased concentration of circulatory heat shock protein 72 (HSP72) (sham = 26.7 ± 15.8, OVX = 10.3 ± 4.6 ng/mL, P < 0.05). No differences were observed in cytokines or chemokines between groups. Our findings indicate that OVX aggravates the pathophysiological response to EHS in mice.NEW & NOTEWORTHY Females outperform males in a mouse model of exertional heat stroke (EHS). Here, we show for the first time the impact of ovariectomy (OVX) on EHS pathophysiology. OVX resulted in a shorter exercise capacity in the heat, greater intestinal damage, and lower heat shock response following EHS.

Amino acid solution mitigates hypothermia response and intestinal damage following exertional heat stroke in male mice

Increased gut permeability is implicated in the initiation and extent of the cytokine inflammatory response associated with exertional heat stroke (EHS). The primary objective of this study was to determine if a five amino acid oral rehydration solution (5AAS), specifically designed for the protection of the gastrointestinal lining, would prolong time to EHS, maintain gut function and dampen the systemic inflammatory response (SIR) measured during EHS recovery. Male C57/BL6J mice instrumented with radiotelemetry were gavaged with 150 μL of 5AAS or H2 O, and ≈12 h later were either exposed to an EHS protocol where mice exercised in a 37.5°C environmental chamber to a self-limiting maximum core temperature (Tc,max) or performed the exercise control (EXC) protocol (25°C). 5AAS pretreatment attenuated hypothermia depth and length (p < 0.005), which are indicators of EHS severity during recovery, without any effect on physical performance or thermoregulatory responses in the heat as determined by percent body weight lost (≈9%), max speed (≈6 m/min), distance (≈700 m), time to Tc,max (≈160 min), thermal area (≈550°C∙min), and Tc,max (42.2°C). EHS groups treated with 5AAS showed a significant decrease in gut transepithelial conductance, decreased paracellular permeability, increased villus height, increased electrolyte absorption and changes in tight junction protein expression pattern suggestive of improved barrier integrity (p < 0.05). No differences were witnessed between EHS groups in acute phase response markers of liver, circulating SIR markers, or indicators of organ damage during recovery. These results suggest that a 5AAS improves Tc regulation during EHS recovery through maintaining mucosal function and integrity.

The effects of low and normal dose ice slurry ingestion on endurance capacity and intestinal epithelial injury in the heat

OBJECTIVES

Compare the effects of ice slurry ingestion at low and normal doses on endurance capacity and exertional heat stress-induced gastrointestinal perturbations.

DESIGN

Randomised, cross-over design.

METHODS

Twelve physically active males completed four treadmill running trials, ingesting ice slurry (ICE) or ambient drink (AMB) at 2 g·kg^-1 (Normal; N) or 1 g·kg^-1 (Low; L) doses every 15-min during exercise and 8 g·kg^-1 (N) or 4 g·kg^-1 (L) pre- and post-exercise. Pre-, during and post-exercise serum intestinal fatty-acid binding protein ([I-FABP]) and lipopolysaccharide ([LPS]) concentrations were determined.

RESULTS

Pre-exercise gastrointestinal temperature (T_gi) was lower in L + ICE than L + AMB (p < 0.05), N + ICE than N + AMB (p < 0.001) and N + ICE than L + ICE (p < 0.001). Higher rate of T_gi rise (p < 0.05) and lower estimated sweat rate (p < 0.001) were observed in N + ICE than N + AMB. Rate of T_gi rise was similar at low dose (p = 0.113) despite a lower estimated sweat rate in L + ICE than L+AMB (p < 0.01). Time-to-exhaustion was longer in L + ICE than L + AMB (p < 0.05), but similar between N + ICE and N + AMB (p = 0.142) and L + ICE and N + ICE (p = 0.766). [I-FABP] and [LPS] were similar (p > 0.05).

CONCLUSIONS

L + ICE elicited a lower heat dissipation compensatory effect with similar endurance capacity as N + ICE. Ice slurry conferred no protection against exertional heat stress-induced gastrointestinal perturbations.

Resveratrol attenuated high intensity exercise training-induced inflammation and ferroptosis via Nrf2/FTH1/GPX4 pathway in intestine of mice

BACKGROUND

Moderate exercise has beneficial effects for human health and is helpful for the protection against several diseases. However, high intensity exercise training caused gastrointestinal syndrome. Resveratrol, a plant extract, plays a vital role in protecting various organs. However, whether resveratrol protected mice against high intensity exercise training-induced intestinal damage remains unclear. In this study, our objective was to investigate the protective effects and mechanism of resveratrol in high intensity exercise training-treated mice.

METHODS

Mice were treated with swimming exercise protocol and/or resveratrol (15 mg/kg/day) for 28 consecutive days. Then, the mice were sacrificed, and a series of evaluation indicators, including inflammatory factors and intestinal permeability of the gut, were measured based on this model. The expressions of inflammatory factors (tumor necrosis factor (TNF)-α; interferon (IFN)-γ, interleukin (IL)-6 and IL-10), oxidative stress (Nrf2, glutathione (GSH), hydrogen peroxide (H2 O2), catalase (CAT) and malondialdehyde(MDA)), intestinal barrier (gut permeability, ZO-1, Occludin and Claudin-1 as well as ferroptosis (Fe2+, Fe3+, SLC7A11, glutathioneperoxidase 4 (GPX4) and ferritin heavy chain 1 (FTH1)) were measured, respectively.

RESULTS

High intensity exercise training induced colon damage, manifested as inflammation (increased TNF-α, IFN-γ and IL-6 concentrations, and decreased IL-10 concentration), oxidative stress (the increase of H2O2 and MDA concentration, and the reduced CAT and GSH activities), intestinal barrier injury (increased gut permeability and intestinal fatty-acid binding protein concentration,and inhibited ZO-1, Occludin and Claudin-1 expressions) and ferroptosis (the increased of Fe2+ and Fe3+ concentrations, and suppressed phosphorylated Nrf2, SLC7A11, GPX4 and FTH1), which was relieved by resveratrol treatment in mice.

DISCUSSION

Resveratrol attenuated high intensity exercise training-induced inflammation and ferroptosis through activating Nrf2/ FTH1/GPX4 pathway in mouse colon, which providing new ideas for the prevention and treatment of occupational disease in athlete.

The Effect of Exercise Prescription on the Human Gut Microbiota and Comparison between Clinical and Apparently Healthy Populations: A Systematic Review

This study systematically reviewed all human longitudinal exercise interventions that reported changes in the gut microbiota; frequency, intensity, duration and type of exercise were assessed to determine the influence of these variables on changes to the gut microbiota in both healthy individuals and clinical populations (PROPERO registration: CRD42022309854). Using PRISMA guidelines, trials analysing gut microbiota change with exercise interventions were included independent of trial randomisation, population, trial duration or analysis technique. Studies were excluded when microbiota abundance was not reported or when exercise was combined with other interventions. Twenty-eight trials were included, of which twelve involved healthy populations only and sixteen involved mixed or clinical-only populations. The findings show that participation in exercise of moderate to high-intensity for 30-90 min ≥3 times per week (or between 150-270 min per week) for ≥8 weeks is likely to produce changes in the gut microbiota. Exercise appears to be effective in modifying the gut microbiota in both clinical and healthy populations. A more robust methodology is needed in future studies to improve the certainty of the evidence.

Differential Effects of Oligosaccharides, Antioxidants, Amino Acids and PUFAs on Heat/Hypoxia-Induced Epithelial Injury in a Caco-2/HT-29 Co-Culture Model

(1) Exposure of intestinal epithelial cells to heat and hypoxia causes a (heat) stress response, resulting in the breakdown of epithelial integrity. There are indications that several categories of nutritional components have beneficial effects on maintaining the intestinal epithelial integrity under stress conditions. This study evaluated the effect of nine nutritional components, including non-digestible oligosaccharides (galacto-oligosaccharides (GOS), fructo-oligosaccharides (FOS), chitosan oligosaccharides (COS)), antioxidants (α-lipoic acid (ALA), resveratrol (RES)), amino acids (l-glutamine (Glu), l-arginine (Arg)) and polyunsaturated fatty acids (PUFAs) (docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA)), on heat/hypoxia-induced epithelial injury. (2) Two human colonic cell lines, Caco-2 and HT-29, were co-cultured and pre-treated with the nutritional components for 48 h. After pre-treatment, the cells were exposed to heat/hypoxia (42 °C, 5% O₂) for 2 h. Epithelial integrity was evaluated by measuring trans-epithelial electrical resistance (TEER), paracellular Lucifer Yellow (LY) permeability, and tight junction (TJ) protein expression. Heat stress and oxidative stress levels were evaluated by determining heat-shock protein-70 (HSP-70) expression and the concentration of the lipid peroxidation product malondialdehyde (MDA). (3) GOS, FOS, COS, ALA, RES, Arg, and EPA presented protective effects on epithelial damage in heat/hypoxia-exposed Caco-2/HT-29 cells by preventing the decrease in TEER, the increase in LY permeability, and/or decrease in TJ proteins zonula occludens-1 (ZO-1) and claudin-3 expression. COS, RES, and EPA demonstrated anti-oxidative stress effects by suppressing the heat/hypoxia-induced MDA production, while Arg further elevated the heat/hypoxia-induced increase in HSP-70 expression. (4) This study indicates that various nutritional components have the potential to counteract heat/hypoxia-induced intestinal injury and might be interesting candidates for future in vivo studies and clinical trials in gastrointestinal disorders related to heat stress and hypoxia.

Exercise sustains the hallmarks of health

Exercise has long been known for its active role in improving physical fitness and sustaining health. Regular moderate-intensity exercise improves all aspects of human health and is widely accepted as a preventative and therapeutic strategy for various diseases. It is well-documented that exercise maintains and restores homeostasis at the organismal, tissue, cellular, and molecular levels to stimulate positive physiological adaptations that consequently protect against various pathological conditions. Here we mainly summarize how moderate-intensity exercise affects the major hallmarks of health, including the integrity of barriers, containment of local perturbations, recycling and turnover, integration of circuitries, rhythmic oscillations, homeostatic resilience, hormetic regulation, as well as repair and regeneration. Furthermore, we summarize the current understanding of the mechanisms responsible for beneficial adaptations in response to exercise. This review aimed at providing a comprehensive summary of the vital biological mechanisms through which moderate-intensity exercise maintains health and opens a window for its application in other health interventions. We hope that continuing investigation in this field will further increase our understanding of the processes involved in the positive role of moderate-intensity exercise and thus get us closer to the identification of new therapeutics that improve quality of life.

No effect of a dairy-based, high flavonoid pre-workout beverage on exercise-induced intestinal injury, permeability, and inflammation in recreational cyclists: A randomized controlled crossover trial

BACKGROUND

Submaximal endurance exercise has been shown to cause elevated gastrointestinal permeability, injury, and inflammation, which may negatively impact athletic performance and recovery. Preclinical and some clinical studies suggest that flavonoids, a class of plant secondary metabolites, may regulate intestinal permeability and reduce chronic low-grade inflammation. Consequently, the purpose of this study was to determine the effects of supplemental flavonoid intake on intestinal health and cycling performance.

MATERIALS AND METHODS

A randomized, double-blind, placebo-controlled crossover trial was conducted with 12 cyclists (8 males and 4 females). Subjects consumed a dairy milk-based, high or low flavonoid (490 or 5 mg) pre-workout beverage daily for 15 days. At the end of each intervention, a submaximal cycling trial (45 min, 70% VO2max) was conducted in a controlled laboratory setting (23°C), followed by a 15-minute maximal effort time trial during which total work and distance were determined. Plasma samples were collected pre- and post-exercise (0h, 1h, and 4h post-exercise). The primary outcome was intestinal injury, assessed by within-subject comparison of plasma intestinal fatty acid-binding protein. Prior to study start, this trial was registered at ClinicalTrials.gov (NCT03427879).

RESULTS

A significant time effect was observed for intestinal fatty acid binding protein and circulating cytokines (IL-6, IL-10, TNF-α). No differences were observed between the low and high flavonoid treatment for intestinal permeability or injury. The flavonoid treatment tended to increase cycling work output (p = 0.051), though no differences were observed for cadence or total distance.

DISCUSSION

Sub-chronic supplementation with blueberry, cocoa, and green tea in a dairy-based pre-workout beverage did not alleviate exercise-induced intestinal injury during submaximal cycling, as compared to the control beverage (dairy-milk based with low flavonoid content).

A narrative review of the moderating effects and repercussion of exercise intervention on osteoporosis: ingenious involvement of gut microbiota and its metabolites

Osteoporosis (OP) is a systemic bone disease characterized by the decreased bone mass and destruction of bone microstructure, which tends to result in the enhanced bone fragility and related fractures, as well as high disability rate and mortality. Exercise is one of the most common, reliable and cost-effective interventions for the prevention and treatment of OP currently, and numerous studies have revealed the close association between gut microbiota (GM) and bone metabolism recently. Moreover, exercise can alter the structure, composition and abundance of GM, and further influence the body health via GM and its metabolites, and the changes of GM also depend on the choice of exercise modes. Herein, combined with relevant studies and based on the inseparable relationship between exercise intervention-GM-OP, this review is aimed to discuss the moderating effects and potential mechanisms of exercise intervention on GM and bone metabolism, as well as the interaction between them.

β-Glucan "Trained Immunity" Immunomodulatory Properties Potentiate Tissue Wound Management and Accelerate Fitness Recover

Introduction

It is well established that moderate physical activity can improve the immune status, rather excess or high-intensity physical exercise can cause damage to the immune system. In addition, muscle injuries resulting from increased frequency and intensity of exercises compromise innate immune activity and may decrease tissue regeneration. Thus, β-glucans, a natural compound, may represent an important substance with strong immunomodulatory properties acting as an immunostimulant therapy known as "trained immunity". This immune stimulating therapeutic is an immunological memory phenomenon linked to the innate immune system, triggering cellular changes at epigenetic, transcriptional, and functional levels, to regulate the immune system and recover its homeostasis with clinical benefits.

Conclusion

This narrative review works with the current evidence regarding β-glucans as a possible alternative therapy for wound healing and its safety and efficacy in the treatment of muscle injuries and physical recovery including other chronic conditions and diseases.

Biomarkers of heatstroke-induced organ injury and repair

NEW FINDINGS

What is the topic of this review? The status and potential role of novel biological markers (biomarkers) that can help identify the patients at risk of organ injury or long-term complications following heatstroke. What advances does it highlight? Numerous biomarkers were identified related to many aspects of generalized heatstroke-induced cellular injury and tissue damage, and heatstroke-provoked cardiovascular, renal, cerebral, intestinal and skeletal muscle injury. No novel biomarkers were identified for liver or lung injury.

ABSTRACT

Classic and exertional heatstroke cause acute injury and damage across numerous organ systems. Moreover, heatstroke survivors may sustain long-term neurological, cardiovascular and renal complications with a persistent risk of death. In this context, biomarkers, defined as biological samples obtained from heatstroke patients, are needed to detect early organ injury, and predict outcomes to develop novel organ preservation therapeutic strategies. This narrative review provides preliminary insights that will guide the development and future utilization of these biomarkers. To this end, we have identified numerous biomarkers of widespread heatstroke-associated cellular injury, tissue damage and repair (extracellular heat shock proteins 72 and 60, high mobility group box protein 1, histone H3, and interleukin-1α), and other organ-specific biomarkers including those related to the cardiovascular system (cardiac troponin I, endothelium-derived factors, circulation endothelial cells, adhesion molecules, thrombomodulin and von Willebrand factor antigen), the kidneys (plasma and urinary neutrophil gelatinase-associated lipocalin), the intestines (intestinal fatty acid-binding protein 2), the brain (serum S100β and neuron-specific enolase) and skeletal muscle (creatine kinase, myoglobin). No specific biomarkers have been identified so far for liver or lung injury in heatstroke. Before translating the identified biomarkers into clinical practice, additional preclinical and clinical prospective studies are required to further understand their clinical utility, particularly for the biomarkers related to long-term post-heatstroke health outcomes.

Assessment of Exercise-Associated Gastrointestinal Perturbations in Research and Practical Settings: Methodological Concerns and Recommendations for Best Practice

Strenuous exercise is synonymous with disturbing gastrointestinal integrity and function, subsequently prompting systemic immune responses and exercise-associated gastrointestinal symptoms, a condition established as "exercise-induced gastrointestinal syndrome." When exercise stress and aligned exacerbation factors (i.e., extrinsic and intrinsic) are of substantial magnitude, these exercise-associated gastrointestinal perturbations can cause performance decrements and health implications of clinical significance. This potentially explains the exponential growth in exploratory, mechanistic, and interventional research in exercise gastroenterology to understand, accurately measure and interpret, and prevent or attenuate the performance debilitating and health consequences of exercise-induced gastrointestinal syndrome. Considering the recent advancement in exercise gastroenterology research, it has been highlighted that published literature in the area is consistently affected by substantial experimental limitations that may affect the accuracy of translating study outcomes into practical application/s and/or design of future research. This perspective methodological review attempts to highlight these concerns and provides guidance to improve the validity, reliability, and robustness of the next generation of exercise gastroenterology research. These methodological concerns include participant screening and description, exertional and exertional heat stress load, dietary control, hydration status, food and fluid provisions, circadian variation, biological sex differences, comprehensive assessment of established markers of exercise-induced gastrointestinal syndrome, validity of gastrointestinal symptoms assessment tool, and data reporting and presentation. Standardized experimental procedures are needed for the accurate interpretation of research findings, avoiding misinterpreted (e.g., pathological relevance of response magnitude) and overstated conclusions (e.g., clinical and practical relevance of intervention research outcomes), which will support more accurate translation into safe practice guidelines.

Core body temperatures of rats subjected to treadmill exercise to fatigue or exhaustion: The journal Temperature toolbox

This study systematically reviewed the literature reporting the changes in rats' core body temperature (TCORE) induced by either incremental- or constant-speed running to fatigue or exhaustion. In addition, multiple linear regression analyses were used to determine the factors contributing to the TCORE values attained when exercise was interrupted. Four databases (EMBASE, PubMed, SPORTDiscus, and Web of Science) were searched in October 2021, and this search was updated in August 2022. Seventy-two studies (n = 1,538 rats) were included in the systematic review. These studies described heterogeneous experimental conditions; for example, the ambient temperature ranged from 5 to 40°C. The rats quit exercising with TCORE values varying more than 8°C among studies, with the lowest and highest values corresponding to 34.9°C and 43.4°C, respectively. Multiple linear regression analyses indicated that the ambient temperature (p < 0.001), initial TCORE (p < 0.001), distance traveled (p < 0.001; only incremental exercises), and running speed and duration (p < 0.001; only constant exercises) contributed significantly to explaining the variance in the TCORE at the end of the exercise. In conclusion, rats subjected to treadmill running exhibit heterogeneous TCORE when fatigued or exhausted. Moreover, it is not possible to determine a narrow range of TCORE associated with exercise cessation in hyperthermic rats. Ambient temperature, initial TCORE, and physical performance-related variables are the best predictors of TCORE at fatigue or exhaustion. From a broader perspective, this systematic review provides relevant information for selecting appropriate methods in future studies designed to investigate exercise thermoregulation in rats.

Pterostilbene attenuates intestinal epithelial barrier loss induced by high loading intensity of exercise

Mounting evidence suggested that high loading intensity of exercise might be detrimental to human health, especially the gastrointestinal tract. Pterostilbene (PTE), derived from grapes and blueberries, might reach a high concentration of intestinal contents. Our study aimed to evaluate PTE’s ability to prevent the loss of intestinal epithelial barrier in high loading intensity of exercise. The exercise model was established by the forced running of mice. An effective HPLC-UV method was developed to quantify PTE concentration in intestinal content. The mRNA changes were detected by quantitative polymerase chain reaction (qPCR). The structure of intestinal flora was analyzed by 16S rRNA sequencing. The PTE (100 mg/kg/d) could significantly attenuate exercise-induced intestinal epithelial barrier loss. Moreover, the HPLC-UV assay showed that the PTE concentration of intestinal content could last 12 h. Furthermore, the exercise increased the abundance of Alistipes, which was related to lipopolysaccharide (LPS) production but could not be reversed by PTE intervention. Besides, cell experiments showed that PTE could promote the expression of intestinal epithelial tight junction (TJ) molecules in vitro. In conclusion, PTE has a significant interest in preventing exercise-induced intestinal damage.

Heat stroke in dogs: Literature review

Heat stroke is a clinical emergency secondary to a severe temperature increase due to the inefficiencies of heat dissipation mechanisms, causing central nervous system dysfunctions, which may lead to multiple organ dysfunction syndrome. Heat disturbances can be classified as "classic" or "exertional" ones and several predisposing factors are involved in their development. This review provides a broad approach to the pathophysiological mechanisms of this syndrome, and the diagnostic and treatment methods to facilitate their approach in clinical routine and increase the survival rate of patients.

A Cocoa Diet Can Partially Attenuate the Alterations in Microbiota and Mucosal Immunity Induced by a Single Session of Intensive Exercise in Rats

Background

Following intensive sports events, a higher rate of upper respiratory tract infections and the appearance of gastrointestinal symptomatology have been reported. We aimed to evaluate the effect of a cocoa-enriched diet on the cecal microbiota and mucosal immune system of rats submitted to high-intensity acute exercise, as well as to elucidate the involvement of cocoa fiber in such effects.

Methods

Wistar rats were fed either a standard diet, a diet containing 10% cocoa providing 5% fiber and a diet containing only 5% cocoa fiber. After 25 days, half of the rats of each diet performed an exhaustion running test. Sixteen hours later, samples were obtained to assess, among others, the cecal microbiota and short chain fatty acids (SCFAs) composition, mesenteric lymph nodes (MLNs) and Peyer’s patches (PPs) lymphocyte composition, and immunoglobulin (Ig) content in salivary glands.

Results

The intake of cocoa, partially due to its fiber content, improved the SCFA production, prevented some changes in PPs and in MLNs lymphocyte composition and also decreased the production of proinflammatory cytokines. Cocoa diet, contrary to cocoa fiber, did not prevent the lower salivary IgM induced by exercise.

Conclusion

A cocoa dietary intake can partially attenuate the alterations in microbiota and mucosal immunity induced by a single session of intensive exercise.

Anthocyanin-Rich Blackcurrant Extract Preserves Gastrointestinal Barrier Permeability and Reduces Enterocyte Damage but Has No Effect on Microbial Translocation and Inflammation After Exertional Heat Stress

This study investigated the effects of 7 days of 600 mg/day anthocyanin-rich blackcurrant extract intake on small intestinal permeability, enterocyte damage, microbial translocation, and inflammation following exertional heat stress. Twelve recreationally active men (maximal aerobic capacity = 55.6 ± 6.0 ml·kg-1·min-1) ran (70% VO2max) for 60 min in an environmental chamber (34 °C, 40% relative humidity) on two occasions (placebo/blackcurrant, randomized double-blind crossover). Permeability was assessed from a 4-hr urinary excretion of lactulose and rhamnose and expressed as a ratio of lactulose/rhamnose. Venous blood samples were taken at rest and 20, 60, and 240 min after exercise to measure enterocyte damage (intestinal fatty acid-binding protein); microbial translocation (soluble CD14, lipopolysaccharide-binding protein); and interleukins 6, interleukins 10, and interleukins 1 receptor antagonist. Exercise increased rectal temperature (by ∼2.8 °C) and heart rate (by ∼123 beats/min) in each condition. Blackcurrant supplementation led to a ∼12% reduction in lactulose/rhamnose ratio (p < .0034) and enterocyte damage (∼40% reduction in intestinal fatty acid-binding protein area under the curve; p < .0001) relative to placebo. No between-condition differences were observed immediately after exercise for lipopolysaccharide-binding protein (mean, 95% confidence interval [CI]; +80%, 95% CI [+61%, +99%]); soluble CD14 (+37%, 95% CI [+22%, +51%]); interleukins 6 (+494%, 95% CI [+394%, +690%]); interleukins 10 (+288%, 95% CI [+105%, +470%]); or interleukins 1 receptor antagonist (+47%, 95% CI [+13%, +80%]; all time main effects). No between-condition differences for these markers were observed after 60 or 240 min of recovery. Blackcurrant extract preserves the GI barrier; however, at subclinical levels, this had no effect on microbial translocation and downstream inflammatory processes.

Effects of oral cystine and glutamine on exercise-induced changes in gastrointestinal permeability and damage markers in young men

PURPOSE

Although acute prolonged strenuous exercise has been shown to increase markers of gastrointestinal permeability and damage, little is known regarding the efficacy of nutritional supplement interventions on the attenuation of exercise-induced gastrointestinal syndrome. This study addressed the effects of oral amino acid supplementation on markers of gastrointestinal permeability and damage in response to exercise.

METHODS

Sixteen active men aged 22.7 ± 2.6 years (mean ± standard deviation) completed placebo or cystine and glutamine supplementation trials in random order. Participants received either a placebo or cystine and glutamine supplements, three times a day for 5 days, separated by a 2-week washout period. On day 6, participants took their designated supplements 30 min before running at a speed corresponding to 75% of maximal oxygen uptake for 1 h, followed by a 4-h rest period. Blood samples were collected pre-exercise, immediately post-exercise, 30 min post-exercise, and 1, 2 and 4 h post-exercise on day 6. The plasma lactulose to mannitol ratio (L:M) and plasma intestinal fatty acid-binding protein (I-FABP) were used as markers of gastrointestinal permeability and damage, respectively.

RESULTS

Plasma L:M (linear mixed model, coefficient ± standard error: - 0.011 ± 0.004, P = 0.0090) and changes (i.e., from pre-exercise) in plasma I-FABP (linear mixed model, - 195.3 ± 65.7 coefficient ± standard error (pg/mL), P = 0.0035) were lower in the cystine and glutamine supplementation trial than in the placebo trial.

CONCLUSION

Oral cystine and glutamine supplementation attenuated the markers of gastrointestinal permeability and damage after 1 h of strenuous running in young men.

TRIAL REGISTRATION NUMBER

UMIN000026008.

DATE OF REGISTRATION

13 December 2018.

No protective benefits of low dose acute L-glutamine supplementation on small intestinal permeability, epithelial injury and bacterial translocation biomarkers in response to subclinical exertional-heat stress: A randomized cross-over trial

Exertional heat stress disrupts gastrointestinal permeability and, through subsequent bacterial translocation, can result in potentially fatal exertional heat stroke. Glutamine supplementation is a potential countermeasure although previously validated doses are not universally well tolerated. Ten males completed two 80-minute subclinical exertional heat stress tests (EHSTs) following either glutamine (0.3 g kg FFM^-1) or placebo supplementation. Small intestinal permeability was assessed using the lactulose/rhamnose dual sugar absorption test and small intestinal epithelial injury using Intestinal Fatty-Acid Binding Protein (I-FABP). Bacterial translocation was assessed using the total 16S bacterial DNA and Bacteroides/total 16S DNA ratio. The glutamine bolus was well tolerated, with no participants reporting symptoms of gastrointestinal intolerance. Small intestinal permeability was not influenced by glutamine supplementation (p = 0.06) although a medium effect size favoring the placebo trial was observed (d = 0.73). Both small intestinal epithelial injury (p < 0.01) and Bacteroides/total 16S DNA (p = 0.04) increased following exertional heat stress, but were uninfluenced by glutamine supplementation. Low-dose acute oral glutamine supplementation does not protect gastrointestinal injury, permeability, or bacterial translocation in response to subclinical exertional heat stress.

Exertional heat stroke: pathophysiology and risk factors

Exertional heat stroke, the third leading cause of mortality in athletes during physical activity, is the most severe manifestation of exertional heat illnesses. Exertional heat stroke is characterised by central nervous system dysfunction in people with hyperthermia during physical activity and can be influenced by environmental factors such as heatwaves, which extend the incidence of exertional heat stroke beyond athletics only. Epidemiological data indicate mortality rates of about 27%, and survivors display long term negative health consequences ranging from neurological to cardiovascular dysfunction. The pathophysiology of exertional heat stroke involves thermoregulatory and cardiovascular overload, resulting in severe hyperthermia and subsequent multiorgan injury due to a systemic inflammatory response syndrome and coagulopathy. Research about risk factors for exertional heat stroke remains limited, but dehydration, sex differences, ageing, body composition, and previous illness are thought to increase risk. Immediate cooling remains the most effective treatment strategy. In this review, we provide an overview of the current literature emphasising the pathophysiology and risk factors of exertional heat stroke, highlighting gaps in knowledge with the objective to stimulate future research.

Intestinal Barrier in Human Health and Disease

The intestinal mucosa provides a selective permeable barrier for nutrient absorption and protection from external factors. It consists of epithelial cells, immune cells and their secretions. The gut microbiota participates in regulating the integrity and function of the intestinal barrier in a homeostatic balance. Pathogens, xenobiotics and food can disrupt the intestinal barrier, promoting systemic inflammation and tissue damage. Genetic and immune factors predispose individuals to gut barrier dysfunction, and changes in the composition and function of the gut microbiota are central to this process. The progressive identification of these changes has led to the development of the concept of ‘leaky gut syndrome’ and ‘gut dysbiosis’, which underlie the relationship between intestinal barrier impairment, metabolic diseases and autoimmunity. Understanding the mechanisms underlying this process is an intriguing subject of research for the diagnosis and treatment of various intestinal and extraintestinal diseases.

Impact of successive exertional heat injuries on thermoregulatory and systemic inflammatory responses in mice

The purpose of the study was to determine if repeated exertional heat injuries (EHIs) worsen the inflammatory response. We assessed the impact of a single EHI bout (EHI0) or two separate EHI episodes separated by 1 (EHI1), 3 (EHI3), and 7 (EHI7) days in male C57BL/6J mice (n = 236). To induce EHI, mice underwent a forced running protocol until loss of consciousness or core temperature reached ≥ 42.7°C. Blood and tissue samples were obtained 30 min, 3 h, 1 day, or 7 days after the EHI. We observed that mice undergoing repeated EHI (EHI1, EHI3, and EHI7) had longer running distances before collapse (∼528 m), tolerated higher core temperatures (∼0.18°C higher) before collapse, and had higher minimum core temperature (indicative of injury severity) during recovery relative to EHI0 group (∼2.18°C higher; all P < 0.05). Heat resilience was most pronounced when latency was shortest between EHI episodes (i.e., thermal load and running duration highest in EHI1), suggesting the response diminishes with longer recoveries between EHI events. Furthermore, mice experiencing a second EHI exhibited increased serum and liver HSP70, and lower corticosterone, FABP2, MIP-1β, MIP-2, and IP-10 relative to mice experiencing a single EHI typically at 30 min to 3 h after EHI. Our findings indicate that an EHI event may initiate some adaptive processes that provide acute heat resilience to subsequent EHI conditions. NEW & NOTEWORTHY Mice undergoing repeated exertional heat injuries, within 1 wk of an initial heat injury, appear to have some protective adaptations. During the second exertional heat injury, mice were able to run longer and sustain higher body temperatures before collapse. Despite this, the mice undergoing a second exertional heat injury were more resilient to the heat as evidenced by attenuated minimum body temperature, higher HPS70 (serum and liver), lower corticosterone, and lower FABP2.

Development and Validation of Test for "Leaky Gut" Small Intestinal and Colonic Permeability Using Sugars in Healthy Adults

BACKGROUND

Oral monosaccharides and disaccharides are used to measure in vivo human gut permeability through urinary excretion.

AIMS

The aims were as follows: (1) to obtain normative data on small intestinal and colonic permeability; (2) to assess variance on standard 16 g fiber diet performed twice; (3) to determine whether dietary fiber influences gut permeability measurements; and (4) to present pilot data using 2 selected probes in patients with diarrhea-predominant irritable bowel syndrome (IBS-D).

METHODS

Sixty healthy female and male adults, age 18-70 years, participated in 3 randomized studies (2 studies on 16.25 g and 1 study on 32.5 g fiber) in otherwise standardized diets. At each test, the following sugars were ingested: 12C-mannitol, 13C-mannitol, rhamnose (monosaccharides), sucralose, and lactulose (disaccharides). Standardized meals were administered from 24 hours before and during 24 hours post-sugars with 3 urine collections: 0-2, 2-8, and 8-24 hours. Sugars were measured using high-performance liquid chromatography-tandem mass spectrometry. Eighteen patients with IBS-D underwent 24-hour excretion studies after oral 13C-mannitol and lactulose.

RESULTS

Baseline sugars (>3-fold above lower limits of quantitation) were identified in the 3 studies: 12C-mannitol in all participants; sucralose in 4-8, and rhamnose in 1-3. Median excretions/24 h (percentage of administered dose) for 13C-mannitol, rhamnose, lactulose, and sucralose were ∼30%, ∼15%, 0.32%, and 2.3%, respectively. 13C-mannitol and rhamnose reflected mainly small intestinal permeability. Intraindividual saccharide excretions were consistent, with minor differences with 16.25 g vs 32.5 g fiber diets. Median interindividual coefficient of variation was 76.5% (10-90 percentile: 34.6-111.0). There were no significant effects of sex, age, or body mass index on permeability measurements in health. 13C-mannitol measurements are feasible in IBS-D.

CONCLUSIONS

Baseline 12C-mannitol excretion precludes its use; 13C-mannitol is the preferred probe for small intestinal permeability.

Exercise hyperthermia induces greater changes in gastrointestinal permeability than equivalent passive hyperthermia

Hyperthermia and exertional heat illness increase gastrointestinal (GI) permeability, although whether the latter is only via hyperthermia is unclear. The aim of this pilot study was to determine whether different changes in GI permeability, characterized by an increased plasma lactulose:rhamnose concentration ratio ([L:R]), occurred in exercise hyperthermia in comparison to equivalent passive hyperthermia. Six healthy adult male participants (age 25 ± 5 years, mass 77.0 ± 6.7 kg, height 181 ± 6 cm, peak oxygen uptake [ V · O 2 peak ] 48 ± 8 ml.kg-1 .min-1 ) underwent exercise under hot conditions (Ex-Heat) and passive heating during hot water immersion (HWI). Heart rate (HR), rectal temperature (TCORE ), rating of perceived exertion (RPE), and whole-body sweat loss (WBSL) were recorded throughout the trials. The L:R ratio, peak HR, change in HR, and change in RPE were higher in Ex-Heat than HWI, despite no differences in trial duration, peak core temperature or WBSL. L:R was strongly correlated (p < 0.05) with HR peak (r = 0.626) and change in HR (r = 0.615) but no other variable. The greater L:R in Ex-Heat, despite equal TCORE responses to HWI, indicates that increased cardiovascular strain occurred during exercise, and exacerbates hyperthermia-induced GI permeability at the same absolute temperature.

Sauna dehydration as a new physiological challenge model for intestinal barrier function

The intestinal barrier plays a crucial role in maintaining gut health, and an increased permeability has been linked to several intestinal and extra-intestinal disorders. There is an increasing demand for interventions aimed at strengthening this barrier and for in vivo challenge models to assess their efficiency. This study investigated the effect of sauna-induced dehydration on intestinal barrier function (clinicaltrials.gov: NCT03620825). Twenty healthy subjects underwent three conditions in random order: (1) Sauna dehydration (loss of 3% body weight), (2) non-steroidal anti-inflammatory drug (NSAID) intake, (3) negative control. Intestinal permeability was assessed by a multi-sugar urinary recovery test, while intestinal damage, bacterial translocation and cytokines were assessed by plasma markers. The sauna dehydration protocol resulted in an increase in gastroduodenal and small intestinal permeability. Presumably, this increase occurred without substantial damage to the enterocytes as plasma intestinal fatty acid-binding protein (I-FABP) and liver fatty acid-binding protein (L-FABP) were not affected. In addition, we observed significant increases in levels of lipopolysaccharide-binding protein (LBP), IL-6 and IL-8, while sCD14, IL-10, IFN-ɣ and TNF-α were not affected. These results suggest that sauna dehydration increased intestinal permeability and could be applied as a new physiological in vivo challenge model for intestinal barrier function.

Sports Dietitians Australia Position Statement: Nutrition for Exercise in Hot Environments

It is the position of Sports Dietitians Australia (SDA) that exercise in hot and/or humid environments, or with significant clothing and/or equipment that prevents body heat loss (i.e., exertional heat stress), provides significant challenges to an athlete's nutritional status, health, and performance. Exertional heat stress, especially when prolonged, can perturb thermoregulatory, cardiovascular, and gastrointestinal systems. Heat acclimation or acclimatization provides beneficial adaptations and should be undertaken where possible. Athletes should aim to begin exercise euhydrated. Furthermore, preexercise hyperhydration may be desirable in some scenarios and can be achieved through acute sodium or glycerol loading protocols. The assessment of fluid balance during exercise, together with gastrointestinal tolerance to fluid intake, and the appropriateness of thirst responses provide valuable information to inform fluid replacement strategies that should be integrated with event fuel requirements. Such strategies should also consider fluid availability and opportunities to drink, to prevent significant under- or overconsumption during exercise. Postexercise beverage choices can be influenced by the required timeframe for return to euhydration and co-ingestion of meals and snacks. Ingested beverage temperature can influence core temperature, with cold/icy beverages of potential use before and during exertional heat stress, while use of menthol can alter thermal sensation. Practical challenges in supporting athletes in teams and traveling for competition require careful planning. Finally, specific athletic population groups have unique nutritional needs in the context of exertional heat stress (i.e., youth, endurance/ultra-endurance athletes, and para-sport athletes), and specific adjustments to nutrition strategies should be made for these population groups.

Enhanced intestinal permeability and intestinal co-morbidities in heat strain: A review and case for autodigestion

Enhanced intestinal permeability is a pervasive issue in modern medicine, with implications demonstrably associated with significant health consequences such as sepsis, multiorgan failure, and death. Key issues involve the trigger mechanisms that could compromise intestinal integrity and increase local permeability allowing the passage of larger, potentially dangerous molecules. Heat stress, whether exertional or environmental, may modulate intestinal permeability and begs interesting questions in the context of global climate change, increasing population vulnerabilities, and public health. Emerging evidence indicates that intestinal leakage of digestive enzymes and associated cell dysfunctions--a process referred to as autodigestion--may play a critical role in systemic physiological damage within the body. This increased permeability is exacerbated in the presence of elevated core temperatures. We employed Latent Dirichlet Allocation (LDA) topic modeling methods to analyze the relationship between heat stress and the nascent theory of autodigestion in a systematic, quantifiable, and unbiased manner. From a corpus of 11,233 scientific articles across four relevant scientific journals (Gut, Shock, Temperature, Gastroenterology), it was found that over 1,000 documents expressed a relationship between intestine, enhanced permeability, core temperature, and heat stress. The association has grown stronger in recent years, as heat stress and potential autodigestion are investigated in tandem, yet still by a limited number of specific research studies. Such findings justify the design of future studies to critically test novel interventions against digestive enzymes permeating the intestinal tract, especially the small intestine.

Military nutrition research: Contemporary issues, state of the science and future directions

The importance of diet and nutrition to military readiness and performance has been recognized for centuries as dietary nutrients sustain health, protect against illness, and promote resilience, performance and recovery. Contemporary military nutrition research is increasingly inter-disciplinary with emphasis often placed on the broad topics of (1) determining operational nutrition requirements in all environments, (2) characterizing nutritional practices of military personnel relative to the required (role/environment) standards, and (3) developing strategies for improving nutrient delivery and individual choices. This review discusses contemporary issues shared internationally by military nutrition research programmes, and highlights emerging topics likely to influence future military nutrition research and policy. Contemporary issues include improving the diet quality of military personnel, optimizing operational rations, and increasing understanding of biological factors influencing nutrient requirements. Emerging areas include the burgeoning field of precision nutrition and its technological enablers.

Nutritional considerations to counteract gastrointestinal permeability during exertional heat stress

Intestinal barrier integrity and function are compromised during exertional heat stress (EHS) potentially leading to consequences that range from minor gastrointestinal (GI) disturbances to fatal outcomes in exertional heat stroke or septic shock. This mini-review provides a concise discussion of nutritional interventions that may protect against intestinal permeability during EHS and suggests physiological mechanisms responsible for this protection. Although diverse nutritional interventions have been suggested to be protective against EHS-induced GI permeability, the ingestion of certain amino acids, carbohydrates, and fluid per se is potentially effective strategy, whereas evidence for various polyphenols and pre/probiotics is developing. Plausible physiological mechanisms of protection include increased blood flow, epithelial cell proliferation, upregulation of intracellular heat shock proteins, modulation of inflammatory signaling, alteration of the GI microbiota, and increased expression of tight junction (TJ) proteins. Further clinical research is needed to propose specific nutritional candidates and recommendations for their application to prevent intestinal barrier disruption and elucidate mechanisms during EHS.

Cooling During Endurance Cycling in the Heat: Blunted Core Temperature but Not Inflammatory Responses

PURPOSE

This study assessed the effects of cooling during endurance cycling (percooling) on changes in core body temperature (Tcore), inflammatory, and metabolic responses.

METHODS

A total of 12 male cyclists (peak oxygen uptake 60 [4] mL·kg-1·min-1) completed a 60-minute constant workload trial (55% of peak power output and ambient temperature 30.4°C [0.6°C]) in a randomized order both with (ICE) and without (CON) an ice vest. An ingestible capsule was used to measure Tcore. Blood samples were collected immediately before and after each trial to determine concentrations of blood lactate, serum cortisol, interleukin-6, and reactive oxygen and nitrogen species.

RESULTS

Tcore increased statistically (P < .001) both in CON (7.0% [1.4%], effect size [ES] = 6.3) and ICE (5.1% [1.1%], ES = 5.7). The increase in CON was statistically larger compared with ICE (P = .006, ES = 1.4). Concentrations of blood lactate (CON: 163% [63%], ES = 1.3; ICE: 149% [91%], ES = 1.3), cortisol (CON: 138% [123%], ES = 1.7; ICE: 81% [102%], ES = 1.0), and interleukin-6 (CON: 661% [324%], ES = 2.1; ICE: 624% [368%], ES = 1.2) statistically increased in both conditions (P < .01) to a similar extent. In addition, reactive oxygen and nitrogen species statistically decreased in both conditions (CON: -19.2% [14.9%], P = .002, ES = 0.9; ICE: -15.1% [16.5%], P = .02, ES = 0.9). No correlations were found between the changes of Tcore and blood parameters across the conditions.

CONCLUSIONS

Despite attenuated Tcore, similar inflammatory and metabolic responses were observed. Our findings suggest percooling to be a promising strategy to attenuate thermal strain without compromising physiological function.

Effects of Ibuprofen during Exertional Heat Stroke in Mice

Intestinal injury is one of the most prominent features of organ damage in exertional heat stroke (EHS). However, whether damage to the intestine in this setting is exacerbated by ibuprofen (IBU), the most commonly used nonsteroidal anti-inflammatory drug in exercising populations, is not well understood.

PURPOSE

We hypothesized that IBU would exacerbate intestinal injury, reduce exercise performance, and increase susceptibility to heat stroke.

METHODS

To test this hypothesis, we administered IBU via diet to male and female C57/BL6J mice, over 48 h before EHS. Susceptibility to EHS was determined by assessing exercise response using a forced running wheel, housed inside an environmental chamber at 37.5°C. Core temperature (Tc) was monitored by telemetry. Mice were allocated into four groups: exercise only (EXC); EHS + IBU; EXC + IBU; and EHS only. Exercise performance and Tc profiles were evaluated and stomachs, intestines and plasma were collected at 3 h post-EHS.

RESULTS

The EHS + IBU males ran approximately 87% longer when Tc was above 41°C (P < 0.03) and attained significantly higher peak Tc (P < 0.01) than EHS-only mice. Histological analyses showed decreased villi surface area throughout the small intestine for both sexes in the EXC + IBU group versus EXC only. Interestingly, though EHS in both sexes caused intestinal injury, in neither sex were there any additional effects of IBU.

CONCLUSIONS

Our results suggest that in a preclinical mouse model of EHS, oral IBU at pharmacologically effective doses does not pose additional risks of heat stroke, does not reduce exercise performance, and does not contribute further to intestinal injury, though this could have been masked by significant gut injury induced by EHS alone.

Changes in gastrointestinal cell integrity after marathon running and exercise-associated collapse

PURPOSE

Endurance exercise and hyperthermia are associated with compromised intestinal permeability and endotoxaemia. The presence of intestinal fatty acid-binding protein (I-FABP) in the systemic circulation suggests intestinal wall damage, but this marker has not previously been used to investigate intestinal integrity after marathon running.

METHODS

Twenty-four runners were recruited as controls prior to completing a standard marathon and had sequential I-FABP measurements before and on completion of the marathon, then at four and 24 h later. Eight runners incapacitated with exercise-associated collapse (EAC) with hyperthermia had I-FABP measured at the time of collapse and 1 hour later.

RESULTS

I-FABP was increased immediately on completing the marathon (T0; 2593 ± 1373 ng·l^-1) compared with baseline (1129 ± 493 ng·l^-1; p < 0.01) in the controls, but there was no significant difference between baseline and the levels at four hours (1419 ± 1124 ng·l^-1; p = 0.7), or at 24 h (1086 ± 302 ng·l^-1; p = 0.5). At T0, EAC cases had a significantly higher I-FABP concentration (15,389 ± 8547 ng.l^-1) compared with controls at T0 (p < 0.01), and remained higher at 1 hour after collapse (13,951 ± 10,476 ng.l^-1) than the pre-race control baseline (p < 0.05).

CONCLUSION

I-FABP is a recently described biomarker whose presence in the circulation is associated with intestinal wall damage. I-FABP levels increase after marathon running and increase further if the endurance exercise is associated with EAC and hyperthermia. After EAC, I-FABP remains high in the circulation for an extended period, suggesting ongoing intestinal wall stress.