Abstract
Background
Firefighters are frequently exposed to high temperatures, environmental toxicants, and strenuous physical demands. The health impacts of these occupational exposures on processes including inflammation and kidney function as well as on the gut microbiota are poorly understood. A firefighter training course may provide a controlled environment to assess these health risks.
Methods
Basic health measures, stool, and blood samples were obtained from 24 firefighters participating in a one-week, heat-intensive training course. Indicators of inflammation, gut permeability, kidney health, and stool microbiota composition were measured before and after the training course in 18 participants. Urine specific gravity was measured before and after a heat-intensive training day to evaluate dehydration.
Results
The majority of firefighters in this cohort were categorized as hypertensive and experienced multiple heat-related illness symptoms during the training week and dehydration after the heat-intensive training day. While plasma IL-1β, CXCL8, and NGAL decreased over the training week, other indicators of inflammation and acute kidney injury increased, and estimated kidney function declined. Microbiota composition shifted over the course of the training week, with changes in Peptostreptococcus anaerobius and Streptococcus.
Conclusions
This pilot study conducted in a controlled field setting suggests that the occupational environment of firefighters may increase their risk for systemic inflammation and kidney disease.
Keywords: firefighter, heat-related illness, dehydration, inflammation, kidney injury, microbiome
Graphical Abstract
Introduction
Occupational heat stress represents one of the many hazards experienced by firefighters. Both environmental and physiological heat stress contribute to increased core body temperatures for firefighters (Games et al., 2020; McEntire et al., 2013) and can lead to dehydration and multiple heat-related-illness (HRI) symptoms (Nerbass et al., 2017). Firefighters are also exposed to numerous toxicants such as wood smoke (Fabian et al., 2011), polycyclic aromatic hydrocarbons (Stec et al., 2018), and heavy metals (Al-Malki, 2009). This unique combination of occupational heat and toxicant exposure has been associated with changes in the body’s inflammatory profile, such as increased interleukin (IL)-6, IL-1β, immunoglobulin G, and C-reactive protein (CRP) (Watkins et al., 2021).
There is also a growing body of literature that connects occupational heat exposure with kidney dysfunction, and this phenomenon has been documented among agricultural workers (Smith et al., 2021), kitchen workers (Singh et al., 2016), and also firefighters (Schlader et al., 2017). The mechanisms linking heat and kidney injury are not well understood, though dehydration is likely involved (Houser et al., 2021; Roncal-Jimenez et al., 2015), and inflammation-related mechanisms have been proposed (Sato and Yanagita, 2018). The role of the microbiome in inducing these inflammatory changes and their sequelae also warrants investigation. Heat stress has been associated with changes in the microbiome in animal models such as cattle (Chen et al., 2018), ducks (He et al., 2019), and pigs (Pearce et al., 2013). It has also been reported that heat stress decreases the integrity of intestinal tight junctions, allowing translocation of inflammatory bacterial products such as lipopolysaccharide to the blood stream (Dokladny et al., 2006).
Workplace hazards, including heat stress, can be challenging to examine in firefighter groups due to the unpredictable frequency of fire calls and the need for rapid response which often prevents the collection of biological samples in temporal proximity to an acute exposure. This pilot study utilized a repeated measures design to assess the effects of a physically demanding, heat-intensive training course designed to mimic real-life firefighting scenarios on heat illness, inflammation, kidney health, and the gut microbiome of firefighters. This design enabled us to detect changes in bacterial populations, immune mediators, and indicators of kidney damage following exposure to high heat and hazardous conditions.
Materials and Methods
Study Design and Population
Study participants were attendees or instructors of the Georgia Smoke Diver Association’s (GSD) week-long training program in January-February 2017. The class hones the skills of firefighters through drills designed to mimic real-life situations, with the goal of improving safety during critical incidents. Drills, called evolutions, are taught incrementally, beginning with the basics early in the week and becoming more complex as the week progresses until they are completed in a simulated fireground environment with smoke, heat, fire, zero visibility, running water, and loud noises. Because evolutions are often designed based on past incidents where there may have been a line-of-duty death, there is also an emotional component in the drill which contributes to the stress level during evolutions, yielding a high-fidelity training environment. Each day of class begins with high intensity physical training (PT) in full gear. PT is followed by a strenuous strength-based obstacle course and a three-mile run. The purpose of this regimen is to tire the student in an effort to compromise decision-making ability, adding another level of stress (Glick-Smith, 2016). All participants were provided the same meals for lunch and dinner during the training days; however, breakfast and evening snacks may have differed among participants.
Forty students and 6 instructors were approached during in-processing for the training course, and 18 students and 6 instructors were enrolled. Participants provided informed consent at the on-site study office and were compensated with $100 gift cards if completing baseline and follow-up data collection. All procedures were carried out in accordance with the Declaration of Helsinki and approved by the Emory University Institutional Review Board (IRB00093822).
Data Collection
Immediately before the training week began, during orientation, medical screenings, and in-processing, participants completed a questionnaire covering demographics and health-related practices, and waist circumference, body mass index (BMI), and blood pressure (bp) were obtained, along with blood glucose, triglycerides, and cholesterol through a fingerstick blood sample. A combination of these measures was used to represent pseudo-metabolic syndrome (pseudo- due to the non-fasting status of participants). We considered participants with at least three of the following to meet criteria for pseudo-metabolic syndrome: central abdominal obesity (waist circumference >= 102cm), stage 2 hypertension (systolic bp >=140 OR diastolic bp >=90), elevated blood glucose (>= 140), elevated triglycerides (>=200), or low high-density lipoprotein (HDL) (<40).
Additional baseline measures included cytokines, chemokines, and other indicators of inflammation, gut permeability, and kidney health which were measured in plasma and in stool collected via Catch-All rectal swabs (Epicentre). These inflammation measures were repeated immediately after the conclusion of the training week (day 5), and participants provided information about HRI symptoms experienced during the week. Analytes were measured by the Emory Multiplexed Immunoassay Core on the QuickPlex instrument (Meso Scale Diagnostics) using V-PLEX Proinflammatory Panel 1 (human), Kidney Injury Panel 5 (human), Human LBP, and V-PLEX Human CRP kits (Meso Scale Discovery). Glomerular filtration rate was estimated (eGFR) from plasma cystatin C levels using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) cystatin C equation (Inker et al., 2012) and evaluated using Kidney Disease Improving Global Outcomes threshold for GFR indicative of kidney disease (KDIGO, 2013).
During the training, a single day was denoted as especially heat-intensive, in which firefighters were exposed to increased environmental heat compared to the other days (burn building temperature 150°F-500°F). On this day, urine specific gravity (USG) was measured to assess dehydration before and after the training. For USG readings which indicated dehydration (USG >=1.020) (Sawka et al., 2007), we informed the participant and instructed them to follow up with the onsite medical team who advised them on rehydration. For other out-of-range study-related lab values or anthropometric readings, participants were notified of the result and given a copy to share with their healthcare provider at their next routine visit.
16S sequencing was performed by the Emory Integrated Genomics Core. DNA was isolated from stool samples collected before and after the training week and from clean swabs as negative controls using the MoBio PowerSoil DNA Isolation Kit per the manufacturer’s protocol. The 16S rRNA gene V3 and V4 regions were amplified and tailed with Illumina sequencing adapters and barcodes per the Illumina 16S Metagenomic Sequencing Library Preparation guide (version 15044223-b). Quantified libraries were pooled and sequenced at 10pM loading density on the Illumina MiSeq using the 600cycle MiSeq Reagent Kit v3 and 20% Phix Control Kit v3 spike-in.
Data Analysis
Analyses were performed in R (R Core Team, 2021) and RStudio (RStudio Team, 2020). Data were formatted and summarized using packages “tidyverse” (v1.3.0) (Wickham et al., 2019), “arsenal” (v3.6.2) (Heinzen et al., 2021), and “ggpubr” (v0.4.0) (Kassambara, 2020). Data were summarized as mean and standard deviation (SD) and percentages of total. Wilcoxon signed-rank tests were used to compare measures pre- and post-training week and pre- and post-heat-intensive day for each participant.
For microbiome analysis, raw 16S sequences were trimmed, de-replicated, and processed including chimera removal, paired reads merging, and taxonomic assignment using the “dada2” package (v1.16.0) (Callahan et al., 2016). Taxonomy was assigned by alignment with Silva database (v132) sequences (Quast et al., 2013). The “decontam” package (v1.6.0) (Davis et al., 2018) was used to identify and remove sequences associated with contaminants (50) based on both presence in negative control samples and frequency in different sizes of sequence libraries. One pre-training sample had relatively few reads that met quality thresholds, so it and its post-training paired sample were excluded from analysis. Alpha diversity (Shannon and inverse Simpson indices) and beta diversity (non-metric multidimensional scaling [NMDS] method, Bray-Curtis dissimilarity) were evaluated using the “phyloseq” package (v1.30.0) (McMurdie and Holmes, 2013). Differences in bacterial composition pre- and post-training week were assessed by PERMANOVA (adonis2, 999 permutations). After removing taxa with counts of three or fewer in at least 20% of the samples, taxa which differed significantly in abundance before and after the training week were identified using packages “DESeq2” (v1.26.0) (Love et al., 2014) and “apeglm” (v1.8.0) (Zhu et al., 2019).
For all tests, p<0.05 was considered statistically significant. Analysis code and output are provided in Supplementary File 1.
Results
Eighteen participants completed the study protocol with attrition (n=6) due only to training course failure, meaning participants did not complete the week-long program and thus did not complete the study. Participants were males aged 23-46 with 1-19 years in fire service, and the majority were non-smokers (Table 1).
Table 1.
Participant demographics and health-related practices
| Instructor (N=6) |
Student (N=18) |
Total (N=24) |
|
|---|---|---|---|
| Age (years) | |||
| - Mean (SD) | 32.7 (5.4) | 30.1 (6.3) | 30.8(6.0) |
| - Min − Max | 24.0 - 38.0 | 23.0 - 46.0 | 23.0 - 46.0 |
| Years in fire service | |||
| - Mean (SD) | 9.5 (4.9) | 7.6 (5.2) | 8.0 (5.1) |
| - Min − Max | 3.0 - 15.0 | 1.0 - 19.0 | 1.0 - 19.0 |
| Smoked more than 100 cigarettes in lifetime | |||
| - No | 4 (80.0%) | 13 (72.2%) | 17 (73.9%) |
| - Yes | 1 (20.0%) | 5 (27.8%) | 6 (26.1%) |
| - Missing | 1 | 0 | 1 |
| Current smoking frequency | |||
| - Not at all | 5 (83.3%) | 17 (94.4%) | 22 (91.7%) |
| - Some days | 1 (16.7%) | 1 (5.6%) | 2 (8.3%) |
| - Every day | 0 (0.0%) | 0 (0.0%) | 0 (0.0%) |
| Current use of chewing tobacco | |||
| - Not at all | 4 (80.0%) | 11 (64.7%) | 15 (68.2%) |
| - Some days | 1 (20.0%) | 2 (11.8%) | 3 (13.6%) |
| - Every day | 0 (0.0%) | 4 (23.5%) | 4 (18.2%) |
| - Missing | 1 | 1 | 2 |
Pseudo-metabolic syndrome rare among participants
The majority of participants were categorized as either overweight or obese by BMI, though only one student and two instructors met the criterion for central abdominal obesity by waist circumference. The majority of students had HDL in the acceptable or desirable range and low-density lipoprotein (LDL) and total cholesterol in the desirable range, but half of instructors were classified as having high total cholesterol. Most participants had normal triglyceride and blood glucose levels, but none had normal blood pressure according to the 2017 American College of Cardiology/American Heart Association Task Force guidelines (Whelton et al., 2018); in fact, over half of participants were classified as having stage II hypertension. Nonetheless, only one student and one instructor met criteria for pseudo-metabolic syndrome based on the combination of these variables (Supplementary Table 1). GSD is an elite firefighter training program requiring applicants to complete physical fitness tests and receive recommendations from the leadership of their home stations in order to qualify for the course. It is not unexpected, therefore, that these participants are, on average, younger and have higher cardiometabolic fitness relative to the broader career firefighter population in the U.S. (Moffatt et al., 2021).
Dehydration prevalent after heat-intensive training
Before the heat-intensive training day, half of instructors and two-thirds of students were euhydrated; at the end of the day, only a third of instructors and students were adequately hydrated, and 25% of students were severely dehydrated (Table 2). The difference in USG bordered on statistical significance (p=0.052) (Supplementary File 1).
Table 2.
Participant Dehydration Status Pre- and Post-Heat-Intensive Day
| Instructor (N=6) | Student (N=12*) | Total (N=18) | |
|---|---|---|---|
| Urine specific gravity pre-heat day | |||
| - Mean (SD) | 1.018 (0.010) | 1.018 (0.009) | 1.018 (0.009) |
| - Min − Max | 1.005 - 1.030 | 1.007 - 1.033 | 1.005 - 1.033 |
| Pre-heat day dehydration status | |||
| - Euhydrated (< 1.020) | 3 (50.0%) | 8 (66.7%) | 11 (61.1%) |
| - Dehydrated (1.020-1.030) | 1 (16.7%) | 2 (16.7%) | 3 (16.7%) |
| - Severely dehydrated (> 1.030) | 2 (33.3%) | 2 (16.7%) | 4 (22.2%) |
| Urine specific gravity post-heat day | |||
| - Mean (SD) | 1.023 (0.010) | 1.022 (0.009) | 1.023 (0.009) |
| - Min − Max | 1.008 - 1.036 | 1.004 - 1.034 | 1.004 - 1.036 |
| Post-heat day dehydration status | |||
| - Euhydrated (< 1.020) | 2 (33.3%) | 4 (33.3%) | 6 (33.3%) |
| - Dehydrated (1.020-1.030) | 3 (50.0%) | 5 (41.7%) | 8 (44.4%) |
| - Severely dehydrated (> 1.030) | 1 (16.7%) | 3 (25.0%) | 4 (22.2%) |
| Percent change in urine specific gravity pre- to post-heat day | |||
| - Mean (SD) | 0.496 (0.998) | 0.404 (0.724) | 0.434 (0.797) |
| - Min − Max | −0.485 - 2.069 | −0.883 - 1.390 | −0.883 - 2.069 |
Data are missing from 6 students who provided baseline data
HRI symptoms common during firefighter training
Two-thirds of participants reported experiencing at least two symptoms consistent with HRI during the training week (Table 3). The most common were heavy sweating (reported by all participants), cramps, and headache. One student and one instructor reported experiencing dizziness, and one student reported experiencing both confusion and gastrointestinal symptoms (nausea and vomiting).
Table 3.
Heat-Related Illness (HRI) Symptoms Experienced by Participants During Training Week
| Instructor (N=6) | Student (N=12*) | Total (N=18) | |
|---|---|---|---|
| Cramps | |||
| - No | 5 (83.3%) | 6 (50.0%) | 11 (61.1%) |
| - Yes | 1 (16.7%) | 6 (50.0%) | 7 (38.9%) |
| Nausea and vomiting | |||
| - No | 6 (100.0%) | 11 (91.7%) | 17 (94.4%) |
| - Yes | 0 (0.0%) | 1 (8.3%) | 1 (5.6%) |
| Heavy sweating | |||
| - No | 0 (0.0%) | 0 (0.0%) | 0 (0.0%) |
| - Yes | 6 (100.0%) | 12 (100.0%) | 18 (100.0%) |
| Confusion | |||
| - No | 6 (100.0%) | 11 (91.7%) | 17 (94.4%) |
| - Yes | 0 (0.0%) | 1 (8.3%) | 1 (5.6%) |
| Dizziness | |||
| - No | 5 (83.3%) | 11 (91.7%) | 16 (88.9%) |
| - Yes | 1 (16.7%) | 1 (8.3%) | 2 (11.1%) |
| Fainting | |||
| - No | 6 (100.0%) | 12 (100.0%) | 18 (100.0%) |
| - Yes | 0 (0.0%) | 0 (0.0%) | 0 (0.0%) |
| Headache | |||
| - No | 4 (66.7%) | 9 (75.0%) | 13 (72.2%) |
| - Yes | 2 (33.3%) | 3 (25.0%) | 5 (27.8%) |
| Number of HRI symptoms | |||
| - Mean (SD) | 1.7 (0.5) | 2.0 (1.0) | 1.9 (0.8) |
| - Min − Max | 1.0 - 2.0 | 1.0 - 4.0 | 1.0 - 4.0 |
| Two or more HRI symptoms | |||
| - No | 2 (33.3%) | 4 (33.3%) | 6 (33.3%) |
| - Yes | 4 (66.7%) | 8 (66.7%) | 12 (66.7%) |
| HRI with CNS involvement | |||
| - No | 3 (50.0%) | 8 (66.7%) | 11 (61.1%) |
| - Yes | 3 (50.0%) | 4 (33.3%) | 7 (38.9%) |
Data are missing from 6 students who provided baseline data
Changes in indicators of inflammation and kidney injury after firefighter training
After the training week, plasma levels of IL-1β, CXCL8, and neutrophil gelatinase-associate lipocalin (NGAL) had decreased (Figure 1A). Levels of plasma lipopolysaccharide-binding protein (LBP), an indicator of the levels of bacterial lipopolysaccharide in circulation, rose, as did levels of CRP in plasma and stool (Figure 1B). Kidney injury markers cystatin C, epidermal growth factor (EGF), and osteopontin (OPN) in plasma had increased (Figure 1C). Cystatin C levels in particular can serve as an indicator of kidney function and can be used to estimate glomerular filtration rate (Inker et al., 2012) (Supplementary Table 2). Immediately prior to the training week, one third of participants (3 of 6 instructors and 3 of 12 students) had cystatin C levels that exceeded normal reference ranges (Erlandsen and Randers, 2018). After the training week, 89% of participants – all but one instructor and one student – had plasma cystatin C levels that exceeded the reference range.
Figure 1:
Levels of Inflammatory Factors and Kidney Function Indicators Change During Training Week Measurements obtained before and after the training week of A) plasma cytokines, B) lipopolysaccharide-binding protein (LBP) in plasma and C-reactive protein (CRP) in plasma and stool, and C) kidney injury markers in plasma and eGFR calculated from cystatin C. Wilcoxon signed-rank test, n=18.
The average eGFR of the study cohort prior to training was 90.29 mL/min/1.73 m2 of body surface area, in the range considered normal according to KDIGO criteria (KDIGO, 2013) (Supplementary Table 2). None of the students had eGFR less than 60 mL/min/1.73 m2, the commonly utilized threshold for impaired kidney function, before the training week, although two instructors did. Participants’ eGFR declined significantly from the beginning to the end of the training week (Figure 1C), with the average dropping to 66.86 mL/min/1.73 m2 and 50% of both students and instructors having eGFR less than 60 mL/min/1.73 m2.
Changes in stool microbiota composition after firefighter training
While we did not observe statistically significant differences in alpha diversity from the beginning of the training week to the end, there was a trend for a reduction in the inverse Simpson index (p=0.057) (Figure 2A). Microbial composition did shift over the course of the week (Figure 2B), and PERMANOVA indicated a significant effect of timepoint (p=0.012). Specifically, the abundance of the bacterium Peptostreptococcus anaerobius increased over the training week while the abundance of Streptococcus decreased to undetectable levels in most individuals but increased for two participants (Figure 2C).
Figure 2:
Subtle Microbiota Composition Changes Detected Following Training Week Sequencing data from before and after the training week were used to calculate A) Shannon and inverse Simpson indices representing alpha diversity of bacteria in stool (Wilcoxon signed-rank test, n=17), B) non-metric multidimensional scaling (NMDS) using Bray-Curtis dissimilarity representing beta diversity of stool bacteria (n=17), and C) abundance of bacteria belonging to two taxa, Peptostreptococcus anaerobius and Streptococcus, which differed significantly between the two timepoints. Each subdivision of a bar represents relative abundance in a different participant’s stool (n=17).
Discussion
Firefighters’ occupation entails regular physical activity and exertion as well as frequent exposure to extreme heat and environmental toxicants. The week-long GSD training course presented a unique opportunity to study the potential impact of these circumstances on firefighters’ health in a controlled environment in which participants’ activities, exposures, and even diet were consistent. This pilot study also highlighted the value of field training programs for the reliable collection of biospecimens and assessment of firefighters’ physiological state in close temporal proximity to acute exposure to occupational hazards. Along with other recent studies examining physiologic effects during multiple bouts of firefighting activity (Smith et al., 2022; Watkins et al., 2021), this pilot demonstrates the importance of monitoring the potential impacts of their occupational environment on firefighters’ health. While the small sample size of this pilot study necessitates the validation of its findings in larger cohorts, we anticipate that the results will guide the design and implementation of future studies. As the number of participants in a firefighter training course is usually limited, efforts should be made to increase study enrollment by enhancing the appeal of research participation to training course students, and data could be collected from multiple cohorts of training course participants to increase the sample size for future studies and ensure the consistency of research findings.
Despite the diverse occupational hazards that firefighters face, cardiovascular disease remains a leading cause of on-duty fatalities (Kahn et al., 2019; Kales et al., 2007). The prevalence of hypertensive blood pressure readings, overweight and obesity, and other cardiometabolic risk factors in this study cohort highlight the importance of continued efforts to promote cardiovascular health among firefighters. This is especially true since our study participants represent a select group that is generally younger and in better health than the average career firefighter (Moffatt et al., 2021), and as such, adverse health outcomes observed in this study likely represent an underestimate of those in the full firefighter population. Moreover, our findings regarding gastrointestinal and renal impacts add to the growing evidence of pathophysiologic changes to multiple body systems [e.g. cardiovascular (Kales and Smith, 2017)] and hematologic (Smith et al., 2022; Watkins et al., 2021) that can occur in fireground environments.
Over the course of the training week, plasma LBP rose significantly, in keeping with what is known regarding increases in gut permeability following heat exposure and strenuous exercise (Armstrong et al., 2018). CRP is commonly used as an indicator of inflammation, and higher levels of CRP were found in plasma and stool after the training week. This suggests that the training activities may have induced gut inflammation and gut leakiness which likely contributed to systemic inflammation. However, levels of certain inflammation-related molecules – IL-1β, CXCL8, and NGAL – were significantly reduced in plasma after the training week. This may reflect the anti-inflammatory impact of regular physical activity. While plasma NGAL levels typically rise immediately after exercise, there is some evidence suggesting that this may not be true following repetitive bouts of exercise, such as the daily runs and strength challenges during the training course, or in highly athletic individuals (Andreazzoli et al., 2017; Moghadasi and Mohammadi Domieh, 2014). IL-1β is often produced in the early stages of inflammatory immune responses, but its production is reportedly suppressed in response to lipopolysaccharide in the context of exercise (Nielsen et al., 2016).
Despite these adaptive anti-inflammatory changes, levels of kidney injury markers cystatin C, EGF, and OPN in the plasma had increased after the training week, and eGFR calculated using cystatin C values had decreased, with half of participants having eGFR values indicative of impaired kidney function at the end of the week. While eGFR derived from cystatin C is an imperfect measure of kidney function, the clear change in it and other renal markers raises concern about acute kidney injury and long-term kidney health among firefighters. This concern is amplified because the majority of firefighters in this cohort met the criteria for hypertension and overweight or obesity, both factors that are known to increase the risk for chronic kidney disease (Herrington et al., 2017). This study also demonstrated that after a day involving intense heat exposure, the majority of participants were dehydrated. Dehydration also increases the risk for kidney injury (Chapman et al., 2020; Houser et al., 2021; Roncal-Jimenez et al., 2015), and it impairs adaptive heat stress responses (Hillman et al., 2011), increasing the likelihood of HRI. Two-thirds of participants in this pilot study experienced multiple symptoms consistent with HRI during the training week, and more than a third had symptoms indicating central nervous system involvement. While it is expected that heat exposure contributed markedly to these symptoms and to dehydration, and while the training environment did seek to limit exposure to some hazards which would be encountered in a real fireground (e.g. using only untreated wood for burning), this study could not distinguish between the effects of heat and of other exposures. Further studies will be needed to delineate the contributions of exertion and hazards such as noise, products of combustion in smoke particles, residual chemical compounds on hoods and turnout gear, and fire itself to the physiological responses observed.
A novel component of this research was the evaluation of the gut microbiome in participants before and after the training course. Even in this small cohort, we observed a trend for a reduction in the inverse Simpson index, which reflects the richness and evenness of bacterial taxa in an environment, and changes in the composition of the gut microbiota over the week. We observed an increase in the relative abundance of Peptostreptococcus anaerobius and changes in Streptococcus levels. Though Peptostreptococcus anaerobius is a common component of the human gut microbiota, it is also considered an opportunistic pathogen, and it has been implicated in development of colorectal cancer (Long et al., 2019). The genus Streptococcus includes species with diverse functions. They can be associated with opportunistic infections and are also over-represented in colorectal cancer patients (Wang et al., 2012), but some species are used as probiotics and may have particular benefits in instances of physical exertion (Pane et al., 2018) and in individuals with chronic kidney disease (Vitetta et al., 2019). Interestingly, bacteria of the family Streptococcaceae reportedly increased in abundance after heat stress in chickens (Suzuki et al., 1983). It will be important to confirm and expand upon these findings in a larger cohort and to examine the relationships between gut microbiota composition and HRI and kidney function in firefighters.
Conclusions
This pilot study demonstrates the importance of examining associations between exposure to occupational hazards and firefighters’ inflammatory profiles, kidney health, and gut microbiota composition, and it highlights factors that may contribute to long-term health concerns such as chronic kidney disease. Our findings emphasize the need to ensure adequate hydration during trainings, which will take place repeatedly during a firefighter’s career, as well as during any exposure to high heat conditions on the job. They also highlight the need for monitoring and further studies of systemic inflammation and metabolic health among firefighters to determine their potential contributions to chronic kidney damage in this population.
Supplementary Material
Highlights.
After a week-long firefighter training course:
Dehydration was common after heat-intensive training
Plasma inflammatory markers IL-1β, CXCL8, and NGAL decreased
Stool and plasma CRP and plasma LBP increased
Plasma kidney injury markers cystatin C, EGF, and OPN increased and eGFR decreased
Significant changes in gut microbiota composition
Acknowledgements
The authors wish to extend our thanks to Jacqueline Mix, Nathan Mutic, Chris Hullender, Chris Cook, Steve Tatum, Randy Shephard, and the Georgia Smoke Diver Association for their involvement in this research.
Funding
This study was supported in part by the Emory Integrated Genomics Core (EIGC) shared resource of Winship Cancer Institute of Emory University and NIH/NCI under award number P30CA138292 and by the Emory Multiplexed Immunoassay Core (EMIC), which is subsidized by the Emory University School of Medicine and is one of the Emory Integrated Core Facilities. Additional support was provided by the National Center for Georgia Clinical & Translational Science Alliance of the National Institutes of Health under award number UL1TR002378 and the National Institute of Nursing Research under award number T32NR012715. The content is solely the responsibility of the authors and does not necessarily reflect the official views of the National Institutes of Health.
Footnotes
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Conflict of Interest
Declarations of interest: none
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