Influence of aerobic fitness on gastrointestinal barrier integrity and microbial translocation following a fixed-intensity military exertional heat stress test

Heat stroke: Pathogenesis, diagnosis, and current treatment

Recently, the incidence of heat-related illnesses has exhibited a steadily upward trend, which is closely associated with several environmental factors such as climate change and air pollution. The progression of heat-related illnesses is a continuous process and can progress to the terminal period when it transforms into heat stroke, the most severe form. Heat stroke is markedly by a core body temperature above 40°C and central nervous system dysfunction. Current knowledge suggests that the pathogenesis of heat stroke is complex and varied, including inflammatory response, oxidative stress, cell death, and coagulation dysfunction. This review consolidated recent research progress on the pathophysiology and pathogenesis of heat stroke, with a focus on the related molecular mechanisms. In addition, we reviewed common strategies and sorted out the drugs in various preclinical stages for heat stroke, aiming to offer a comprehensive research roadmap for more in-depth researches into the mechanisms of heat stroke and the reduction in the mortality of heat stroke in the future.

Why are we doing this Staff? Justification and implications of aerobic fitness testing in the military

BACKGROUND

Physical fitness is a key tenet of military organisations worldwide. Specifically, many consider aerobic fitness (AF) an essential physical attribute for ensuring optimal military performance and readiness. However, the intricate relationship between AF and various facets of military job performance necessitates comprehensive review to ascertain the appropriateness and effectiveness of its assessment.

OBJECTIVE

This narrative review aims to describe the relationship between AF and factors influencing individual military performance and readiness, and explores the implications of the enforcement of in-service, generic AF test standards in military populations.

METHODS

Databases (PubMed and Google Scholar) were searched for all relevant published peer-reviewed literature as at August 2023.

RESULTS

Inconsistent associations were found between AF and outcomes influencing individual military performance (physical capabilities, cognitive capabilities, presenteeism and productivity, resilience, and technical/tactical capabilities) and readiness (mental health and wellbeing and physical health). Consequently, the level of AF needed for acceptable or optimal military performance remains uncertain.

CONCLUSIONS

AF is a cornerstone of health and performance, yet linking generic AF test standards to job performance is complex, with multiple factors interacting to influence outcomes. From existing literatures, there does not appear to be a specific level of AF at, and/or above, which acceptable military performance is achieved. As such, the enforcement of and emphasis on in-service, pass/fail, generic AF test standards in military populations is questionable and requires thoughtful re-evaluation. Role/task-specific AF should be assessed through evidence-based PES and the use of generic AF tests limited to the monitoring and benchmarking of health-related fitness.

Epithelial barrier theory in the context of nutrition and environmental exposure in athletes

Exposure to toxic substances, introduced into our daily lives during industrialization and modernization, can disrupt the epithelial barriers in the skin, respiratory, and gastrointestinal systems, leading to microbial dysbiosis and inflammation. Athletes and physically active individuals are at increased risk of exposure to agents that damage the epithelial barriers and microbiome, and their extreme physical exercise exerts stress on many organs, resulting in tissue damage and inflammation. Epithelial barrier-damaging substances include surfactants and enzymes in cleaning products, laundry and dishwasher detergents, chlorine in swimming pools, microplastics, air pollutants such as ozone, particulate matter, and diesel exhaust. Athletes' high-calorie diet often relies on processed foods that may contain food emulsifiers and other additives that may cause epithelial barrier dysfunction and microbial dysbiosis. The type of the material used in the sport equipment and clothing and their extensive exposure may increase the inflammatory effects. Excessive travel-related stress, sleep disturbances and different food and microbe exposure may represent additional factors. Here, we review the detrimental impact of toxic agents on epithelial barriers and microbiome; bring a new perspective on the factors affecting the health and performance of athletes and physically active individuals.

Exertional Stress-induced Pathogenic Luminal Content Translocation - Friend or Foe?

The incidence of perturbed gastrointestinal integrity, as well as resulting systemic immune responses and gastrointestinal symptoms, otherwise known as exercised-induced gastrointestinal syndrome (EIGS), is common among individuals who partake in prolonged exercise. EIGS may cause the translocation of pathogenic material, including whole bacteria and bacterial endotoxins, from the lumen into circulation, which may progress into clinical consequences such as sepsis, and potentially subsequent fatality. However, further investigation is warranted to assess the possibility of food allergen and/or digestive enzyme luminal to circulatory translocation in response to exercise, and the clinical consequences. Findings from this narrative literature review demonstrate evidence that whole bacteria and bacterial endotoxins translocation from the gastrointestinal lumen to systemic circulation occurs in response to exercise stress, with a greater propensity of translocation occurring with accompanying heat exposure. It has also been demonstrated that food allergens can translocate from the lumen to systemic circulation in response to exercise stress and initiate anaphylaxis. To date, no research investigating the effect of exercise on the translocation of digestive enzymes from the lumen into systemic circulation exists. It is evident that EIGS and consequential pathogenic translocation presents life-threatening clinical implications, warranting the development and implementation of effective management strategies in at-risk populations.

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.

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.

Reliability of pathophysiological markers reflective of exercise-induced gastrointestinal syndrome (EIGS) in response to 2-h high-intensity interval exercise: A comprehensive methodological efficacy exploration

The study aimed to determine the test-retest reliability of exercise-induced gastrointestinal syndrome (EIGS) biomarkers, and assess the association of pre-exercise short chain fatty acid (SCFA) concentration with these biomarkers in response to prolonged strenuous exercise. Thirty-four participants completed 2 h of high-intensity interval training (HIIT) on two separate occasions with at least 5-days washout. Blood samples were collected pre- and post-exercise, and analysed for biomarkers associated with EIGS [i.e., cortisol, intestinal fatty-acid binding protein (I-FABP), sCD14, lipopolysaccharide binding protein (LBP), leukocyte counts, in-vitro neutrophil function, and systemic inflammatory cytokine profile]. Fecal samples were collected pre-exercise on both occasions. In plasma and fecal samples, bacterial DNA concentration was determined by fluorometer quantification, microbial taxonomy by 16S rRNA amplicon sequencing, and SCFA concentration by gas-chromatography. In response to exercise, 2 h of HIIT modestly perturbed biomarkers indicative of EIGS, including inducing bacteremia (i.e., quantity and diversity). Reliability analysis using comparative tests, Cohen's d, two-tailed correlation, and intraclass correlation coefficient (ICC) of resting biomarkers presented good-to-excellent for IL-1ra (r = 0.710, ICC = 0.92), IL-10 (r = 0.665, ICC = 0.73), cortisol (r = 0.870, ICC = 0.87), and LBP (r = 0.813, ICC = 0.76); moderate for total (r = 0.839, ICC = 0.44) and per cell (r = 0.749, ICC = 0.54) bacterially-stimulated elastase release, IL-1β (r = 0.625, ICC = 0.64), TNF-α (r = 0.523, ICC = 0.56), I-FABP (r = 0.411, ICC = 0.21), and sCD14 (r = 0.409, ICC = 0.38), plus fecal bacterial α-diversity; and poor for leukocyte (r = 0.327, ICC = 0.33) and neutrophil (r = 0.352, ICC = 0.32) counts. In addition, a medium negative correlation was observed between plasma butyrate and I-FABP (r = -0.390). The current data suggest a suite of biomarkers should be used to determine the incidence and severity of EIGS. Moreover, determination of plasma and/or fecal SCFA may provide some insight into the mechanistic aspects of EIGS instigation and magnitude in response to exercise.

The Impact of Heat Acclimation on Gastrointestinal Function following Endurance Exercise in a Hot Environment

To determine the effects of heat acclimation on gastrointestinal (GI) damage and the gastric emptying (GE) rate following endurance exercise in a hot environment. Fifteen healthy men were divided into two groups: endurance training in hot (HOT, 35 °C, n = 8) or cool (COOL, 18 °C, n = 7) environment. All subjects completed 10 days of endurance training (eight sessions of 60 min continuous exercise at 50% of the maximal oxygen uptake (V·O2max). Subjects completed a heat stress exercise tests (HST, 60 min exercise at 60% V·O2max) to evaluate the plasma intestinal fatty acid-binding protein (I-FABP) level and the GE rate following endurance exercise in a hot environment (35 °C) before (pre-HST) and after (post-HST) the training period. We assessed the GE rate using the ¹³C-sodium acetate breath test. The core temperature during post-HST exercise decreased significantly in the HOT group compared to the pre-HST (p = 0.004) but not in the COOL group. Both the HOT and COOL groups showed exercise-induced plasma I-FABP elevations in the pre-HST (p = 0.002). Both groups had significantly attenuated exercise-induced I-FABP elevation in the post-HST. However, the reduction of exercise-induced I-FABP elevation was not different significantly between both groups. GE rate following HST did not change between pre- and post-HST in both groups, with no significant difference between two groups in the post-HST. Ten days of endurance training in a hot environment improved thermoregulation, whereas exercise-induced GI damage and delay of GE rate were not further attenuated compared with training in a cool environment.

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.

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.

Acute L-Glutamine Supplementation does not improve Gastrointestinal Permeability, Injury or Microbial Translocation in Response to Exhaustive High Intensity Exertional-Heat Stress

PurposeExertional-heat stress adversely distrupts (GI) barrier integrity and, through subsequent microbial translocation (MT), can result in potentially fatal exertional-heat stroke. Acute glutamine (GLN) supplementation is a potential nutritional countermeasure, although the practical value of current supplementation regimens is questionable. Method: Ten males completed two high-intensity exertional-heat stress tests (EHST) involving running in the heat (40°C and 40% relative humidity) at lactate threshold to volitional exhaustion. Participants ingested GLN (0.3 g·kg·FFM^-1) or a non-calorific placebo (PLA) one hour prior to the EHST. Venous blood was drawn pre-, post- and one-hour post-EHST. GI permeability was assessed using a serum dual-sugar absorption test (DSAT) and small intestinal epithelial injury using plasma Intestinal Fatty-Acid Binding Protein (I-FABP). MT was assessed using the Bacteroides/total 16S DNA ratio. Results: Volitional exhaustion occurred after 22:19 ± 2:22 (minutes: seconds) in both conditions, during which whole-body physiological responses and GI symptoms were not different (p ˃ 0.05). GI permeability (serum DSAT) was greater following GLN (0.043 ± 0.020) than PLA (0.034 ± 0.019) (p = 0.02; d = 0.47), but small intestine epithelial injury (I-FABP) increased comparably (p = 0.22; η²p = 0.16) following the EHST in both trials (GLN Δ = 1.25 ± 0.63 ng·ml^-1; PLA Δ= 0.92 ± 0.44 ng·ml^-1). GI MT (Bacteroides/total 16S DNA ratio) was unchanged in either condition following the EHST (p = 0.43). Conclusion: Acute low-dose (0.3 g·kg^-1 fat free mass) GLN supplementation ingested one hour before high-intesity exertional-heat stress worsened GI permeability, but did not influence either small intestinal epithilial injury or microbial translocation.Highlights: The pathophysiology of exertional-heat stroke is widely hypothesised to be at least in part attributable to a systemic inflammatory response caused by the leak of gastrointestinal microbes into the circulating blood.Acute high-dose (0.9 g·kg·FFM^-1) _L-glutamine supplementation is widely promoted as a practical strategy to protect gastrointestinal barrier integrity during exertional-heat stress. However, previously validated doses are often poorly tolerated and cannot be recommended for widespread implementation.This study examined the efficacy of low-dose (0.30 g·kg·FFM^-1; ∼20 grams) acute _L-glutamine supplementation on small intestinal injury, permeability, and microbial translocation in response a high-intensity exertional-heat stress test to exhaustion (20 - 30 minutes). This type of exercise accounts for the majority of exertional-heat stroke cases in the military.Despite being universally well-tolerated across all participants, acute low-dose _L-glutamine supplementation worsened gastrointestinal permeability, without influencing either small intestinal injury or microbial translocation. These findings do not support the application of low-dose _L-glutamine supplementation to help prevent exertional-heat stroke.