Association between metabolic syndrome and physical fitness in firefighters: a protocol for a systematic review and meta-analysis

Abstract

Introduction

Among the emergency services, firefighters have the highest percentage of mortality (45%) due to sudden cardiac death, with the majority related to underlying cardiovascular disease. This necessitates that firefighters stay in good physical condition and maintain adequate cardiovascular fitness to cope with these stressors and perform their duties with minimal health risks. Therefore, this study aims to determine the association between metabolic syndrome and physical fitness in firefighters.

Methods

The authors will search the following electronic databases: PubMed/Medline, SCOPUS and Web of Science, with no limitations to publication year. For data extraction, the two principal reviewers will use a general data extraction form to retrieve the key characteristics of each study. The Rayyan intelligent systematic review tool will be used to screen and select studies for inclusion. Thereafter, information from the included studies will be captured on the researcher-generated data extraction form. The Joanna Briggs Institute (JBI) Critical Appraisal Tools for quantitative studies will be used to conduct the methodological assessment of each study included. Data will be analysed using Review Manager 5.3 to determine the exposure effects and MedCalc statistical software Ltd and will be used to determine the pooled correlation effects. The results will be presented using figures, graphs and tables.

Ethics and dissemination

Details for this systematic review protocol can be accessed on PROSPERO (CRD42024535088). The authors will disseminate this protocol and the findings of the systematic review and meta-analysis in peer-reviewed journals and in national and international conferences. In addition, this review will add significantly to the body of knowledge in the scientific community worldwide and assist academics in exploring research gaps on this topic.

PROSPERO registration number

CRD42024535088.

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• A strength of this review is that it will be the first systematic review to use a meta-analysis to estimate the association between metabolic syndrome and physical fitness in firefighters.

• A strength of this review is that it will use a rigorous approach to the methodology, adhering to the guidelines for Meta-analysis of Observational Studies in Epidemiology studies (MOOSE) and Quality of Reporting of Meta-analysis (QUOROM).

• A limitation is that the age and gender distribution of the firefighters, along with the inconsistent methodology, may introduce a significant amount of heterogeneity, which may affect the meta-analysis.

• A limitation of this study is that only English articles will be included in this review, which may exclude other relevant studies.

Introduction

Approximately 43–45% of on-duty firefighter-related deaths are due to poor cardiovascular health and increased cardiovascular risk.^{1 2} Firefighting is a physically demanding occupation requiring firefighters to be physically fit to fulfil on-duty emergencies, particularly fire suppression.³,⁵ If firefighters are not in optimal cardiometabolic and physical condition, this predisposes them to significant morbidity and mortality while on duty.³

Metabolic syndrome is a condition described by the clustering of cardiovascular disease (CVD) risk factors, requiring at least three of the five components.⁶,⁸ These conditions include hyperglycaemia, hypertension, hypertriglyceridaemia, dyslipidaemia and abdominal obesity.^{6 9} A systematic review by Beckett et al⁶ reported that 22.3% of firefighters were reported to have metabolic syndrome. In addition, the study found that 39.1% were hypertensive, 37.9% were abdominally obese, 30.2% had hypertriglyceridaemia, 30.1% were dyslipidaemic and 21.1% had hyperglycaemia.⁶ Globally, the prevalence of metabolic syndrome has been rising in firefighters, which has been attributed to negative attitudes toward their occupation, lack of physical activity and poor eating habits.^{7 8 10} This is problematic, given the physical nature of their occupations. Studies have indicated that metabolic syndrome has been associated with poor physical performance.^{7 9 11} This accentuates the physical stress of an already high-risk occupation group, especially in older firefighters. Studies have indicated that CVD risk factors have a negative impact on the occupational performance of firefighters.¹²,¹⁴

This study originated from the challenges firefighters face globally, particularly in South Africa.^{15 16} A concerning number of firefighters are at increased cardiometabolic risk and have poor physical fitness levels, which negatively impacts their overall health, well-being and occupational performance.¹⁷,²¹ To the authors’ knowledge, no systematic review has investigated the association between metabolic syndrome and physical fitness in firefighters, which motivated the current systematic review. Therefore, the authors aim to determine the association between metabolic syndrome and physical fitness in firefighters, which will inform policymakers in South Africa of the need for corrective action and develop strategies to improve and maintain cardiometabolic health and physical fitness levels of firefighters.

Protocol

This protocol was first designed based on the ‘priori’ approach and then registered in PROSPERO.

Primary objective

Investigating the association between metabolic syndrome and physical fitness in firefighters.

Methods and analysis

The guidelines for Meta-analysis of Observational Studies in Epidemiology studies (MOOSE) and Quality of Reporting of Meta-analysis (QUOROM) will guide the methods when conducting the review.^{22 23} When considering studies for this review, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for systematic reviews will be followed, and the outcomes for each step will be described in a flow-diagram.²⁴

Eligibility criteria of primary studies

The authors have chosen to address firefighters’ metabolic syndrome and its association with physical fitness. The study design of choice is a quantitative systematic review assessing the relationship between metabolic syndrome and physical fitness in adult, full-time firefighters. All study types and designs will be included and appraised accordingly.

Types of participants

Full-time, part-time and volunteer firefighters 18 years and older.

Types of exposures

Metabolic syndrome risk factors which include: (1) a non-fasting blood glucose level of 7.7 mmol/L or higher; (2) elevated triglyceride levels of 1.7 mmol/L or higher; (3) reduced high-density lipoprotein (HDL) cholesterol levels below 1.0 mmol/L; (4) blood pressure readings with values for systolic pressure of 130 mm Hg or higher, or a diastolic pressure of 85 mm Hg or higher; (5) central or abdominal obesity defined by a waist circumference greater than 94 cm in males and 80 cm in females.^{6 8 25}

Types of outcomes

Physical fitness measures as continuous variables for cardiorespiratory fitness (VO_2max or Metabolic Equivalent (METs)), leg strength (kgs), upper body strength (kg), upper body endurance (repetitions), abdominal endurance (repetitions) and flexibility (cm) in firefighters.

Inclusion criteria

• Studies that recruit full-time and voluntary adult firefighters.

• All study designs including observational and experimental (intervention) will be included.

• Studies investigating the association between metabolic syndrome and their risk factors and physical fitness in firefighters.

• Studies available in full text.

Exclusion criteria

• Studies focusing on other outcome measures rather than the main exposures or outcomes, such as CVD, pulmonary disease and other associated conditions.

• Systematic reviews or other types of reviews.

• Non-English articles.

Search strategy for identification of studies

A detailed literature search will be conducted to identify studies investigating the association between metabolic syndrome and metabolic syndrome risk factors on the cardiorespiratory fitness of firefighters. Relevant studies, irrespective of publication date, will be searched, with guidance from a specialist librarian. The team will be made up of two main contributors. The first reviewer (JR) will be the primary investigator, who will take responsibility for all aspects of the review and independently extract the data, verify the data collected, analyse the results, grade the data quality and write up the first review draft. Reviewer II (JG) will be responsible for independently extracting the data, verifying the data collected, analysing the results and grading the data quality. Reviewers will use Rayyan Tools²⁶ to compare inclusion and exclusion criteria and discuss the final data-extraction sheets when selecting studies. If the instance that a dispute arises, an arbitrator (LL) will be recruited to make the final decision.

Electronic literature search

A comprehensive literature search will be conducted for this systematic review to enable the capturing of as many relevant articles as possible but limited to English papers only. The following journal databases will be searched: PubMed/Medline, SCOPUS, Web of Science and CINAHL. Keywords and medical subject heading (MeSH) terms will be used in various arrangements depending on the specific database. This review will use a combination of the appropriate terms (search string) to ensure the inclusion of the relevant components of the participants, exposure, comparison and outcome (PECO) will be obtained. The details of the search strategy are given below under the subheading ‘search terms’.

Search terms in PubMed

#1 “firefighter” OR “fire and rescue personnel” OR “fire fighters” OR “fire fighter” OR “firefight” OR “firemen”

#2 “cardiovascular system”[MeSH] OR (“cardiovascular” [All Fields] AND “system” [All Fields]) OR “cardiovascular system” [All Fields] OR “cardiovascular*” [All Fields] OR “cardiovascular abnormalities” [MeSH] OR “CVD” [All Fields] “metabolic syndrome”[All Fields] OR “syndrome X” [All Fields] OR “insulin resistance syndrome” [All Fields] OR “cardiometabolic syndrome” [All Fields] OR “MetSyn” [All Fields] OR “lipid profile” [All Fields] OR “cholesterol” [MeSH] OR “dyslipidaemia” OR “hypercholesteremia” OR “triglycerides” [All Fields] OR “LDL” [All Fields] OR “HDL” [All Fields] OR “diabetes” AND “mellitus” OR “blood glucose” OR “insulin” OR “obesity” OR “central obesity” OR “hypertension” OR “blood pressure” OR “hyperglycaemia” OR “cardiometabolic”

#3 “fitness” [MeSH] OR “physical fitness”[MeSH] OR “exercise” [All Fields] OR “physical exertion” [All Fields] OR “muscular strength” OR “muscular endurance” OR “aerobic fitness” OR “cardiorespiratory fitness” OR “cardiorespiratory capacity” OR “V̇O₂max” OR “aerobic fitness” OR “power” OR “anaerobic power”

#4 (#1 AND #2) OR (#1 AND #3) OR (#1 AND #2 AND #3)

The search strategies for the other databases are presented in online supplemental appendix 1.

Additional searches for grey literature

The following databases will be searched for grey literature to complete the search strategy: Google, Google Scholar and Networked Digital Library of Theses and Dissertations. JR and JG will look through the recognised articles’ reference lists to find probable titles of papers that might fit the inclusion requirements.

Selection of studies

All studies that fit the inclusion criteria will be chosen for full-text screening. For full-text articles or missing data, every effort will be taken to get in touch with the authors. The two lead reviewers will then use the Rayyan intelligent systematic review ²⁶ tool to independently evaluate the full-text articles. Three categories will be used when screening and classifying the studies: included, excluded and unsure. The two reviewers will debate any doubts about the inclusion of a study; if they cannot agree, they will confer with the third reviewer, who will settle the disagreement.

Steps involved in the selection and screening

The selection and screening process will be carried out using the subsequent steps: (1) finding and screening potential studies’ titles and abstracts for eligibility through a search of all preselected databases; (2) gathering search results into Mendeley Desktop V. 1.19.8 for reference; (3) eliminating duplicates; (4) comparing full-text articles to the inclusion criteria and selecting the final studies for inclusion in the review; (5) extracting data from the included studies using a pre-designed data extraction form; (6) Review Manager 5.3²⁷ and MedCalc statistical software Ltd will be used for the meta-analysis in order to analyse and publish the review’s findings.

Data extraction and data management

The primary reviewers will extract data using an initial form created by the researcher (online supplemental appendix 2) to obtain the relevant details of each study. The comprehensive researcher-generated data extraction form (online supplemental appendix 3) will then be used to record details about the included studies. The initial data to be retrieved will consist of the study’s overall details, including the authors, study title, study design, study country, assessed exposure and outcome measures. Second, data about the study’s characteristics, such as sample size and sampling technique, as well as participant details, such as age, height, weight, body mass index, gender, years of experience and core job description, if relevant. The final specifics regarding exposure and the study must report on at least one exposure variable (MetS risk factors) connected to physical fitness to extract the outcome variables.

Critical appraisal of included studies (risk of bias assessment)

The Joanna Briggs Institute (JBI) Critical Appraisal Tools for quantitative studies, including observational (cross-sectional and cohort) and experimental study designs, will be used to conduct the methodological quality assessment of each study included. The JBI Critical Appraisal Tools have been shown to be valid and reliable in assessing the quality of quantitative studies.²⁸ A score will be given for each major question of the appraisal tool. All criteria for grading the studies will be given a score which will be added to provide an accumulated score, presented in a table indicating each major section score and the tallied up score. This will be used to assess the overall quality of each article included in the systematic review. The overall score will be calculated, and the following criteria will be used to assess the overall quality of the articles: poor, if the appraisal checklist has fewer than 50% of the specified criteria; good: if the appraisal checklist contains between 50% and 65% of the specified criteria; and excellent: if the appraisal checklist has more than 65% of the specified criteria.

Data synthesis and analysis

The data will be analysed and synthesised after completing the systematic literature search and identifying all pertinent documents. A methodical synthesis of the findings from the literature will be used in this review. Using a systematic review synthesis, the researcher can uncover, assess and compile comparable study findings from all pertinent individual studies.²⁹,³¹ We will perform a meta-analysis if there is sufficient data available to perform a meta-analysis. The association between the metabolic syndrome risk factors and firefighters’ physical fitness will be estimated using the risk ratio and OR for dichotomous data and the mean difference and standardised mean difference of estimation for continuous data.³² Review Manager 5.3 will be used to import and analyse the data.²⁷³¹,³³ The inverse meta-analysis strategy will be favoured for data analysis.³⁴ The association between metabolic syndrome and physical fitness variables will be ascertained using a meta-analysis of correlations. It will be preferred to use MedCalc statistics software Ltd (V. 20.104).³⁴ The meta-analysis will produce the pooled r values between each cardiovascular risk factor and cardiorespiratory fitness using the original r values and sample sizes.³⁴ The Fisher’s r to z transformation will be applied to convert the original r values to a standard test metric³⁴:

$${\displaystyle {\displaystyle {\mathrm{Z}}_{\mathrm{r}\mathrm{i}}=\frac{1}{2}\ln \left(\frac{1+\mathrm{r}\mathrm{i}}{1-\mathrm{r}\mathrm{i}}\right)\mathrm{s}\frac{2}{\mathrm{z}}=\frac{1}{\mathrm{n}-3}}}$$

The following will be used to indicate the strength of correlation: 0.90 to 1.00 (−0.90 to −1.00) will indicate a very high correlation; 0.70 to 0.90 (−0.70 to −0.90) will indicate a high correlation; 0.50 to 0.70 (−0.50 to −0.70) will indicate a moderate correlation; 0.30 to 0.50 (−0.30 to −0.50) will indicate a low correlation; and 0.00 to 0.30 (−0.00 to −0.30) will indicate a negligible correlation.³⁴

Assessment of heterogeneity

The I² and χ² tests will be used to assess heterogeneity.³³ Furthermore, heterogeneity will be detected by visually examining the forest plots to determine how much the individual research CI overlap. The I² statistics will be explained using the following: a range of 0–30% might not be significant; a range of 31–60% might suggest moderate heterogeneity; a range of 61–80% might suggest substantial heterogeneity; and a range of 81–100% might suggest significant heterogeneity. Individual studies and subgroup characteristics will be evaluated if significant heterogeneity exists to identify potential causes. A low level of heterogeneity will favour a meta-analysis, where the results will be more reliable.³³ A fixed-effects model will be employed if homogeneity is discovered in the studies, and a random-effects model will be used if heterogeneity is evident.³³

Subgroup analysis and investigation of heterogeneity

According to the authors, the following factors could introduce clinical heterogeneity: age, gender, marital status, experience, core job description, publication year, study design, geography (country, regions or continent), procedures used to measure metabolic syndrome characteristics and type of testing procedure used to measure cardiorespiratory fitness in firefighters. If possible, the authors plan to conduct subgroup analysis on these variables. Although all exposures and results will be measured using standardised tools and procedures, each study may have distinct protocols that need to be compared and converted in order to yield comparable results. If sufficient numbers of included studies exist, where two is the minimum amount required,³⁵ authors will use MedCalc statistical software Ltd³⁴ and Review Manager³¹,³³ for the subgroup analysis.

Presenting and reporting of results

Generated results will be presented using a combination of figures, graphs and tables. This will include the methods and steps of how studies were sourced and selected using the PRISMA guidelines.²⁴ Excluded studies and the reasons for exclusion will be tabulated and further explained in the methodology section of the systematic review. In addition, summary tables will be created along with the use of forest plots.

Discussion and conclusion

To the best of the authors’ knowledge, no conclusive evidence exists on the association between metabolic syndrome and the physical fitness of firefighters. In addition to helping policymakers create intervention plans to support the health, well-being and career longevity of firefighters in South Africa and around the world, this review is anticipated to significantly add to the body of knowledge in the scientific community worldwide. The planned review will also help academics who want to create original primary or secondary research on this topic and might even help discover research gaps for more investigations.

Ethics and dissemination

This study has been registered onto PROSPERO (CRD42024535088). Accessible and published data will be used in the study; thus, no confidentiality or ethical procedures need to be considered for this review.³⁶ The information gathered will be presented at conferences, webinars and to local firefighting organisations.

Study status

This systematic review and meta-analysis is currently in the search phase for information sources.

Risk and benefits

Due to the lack of participant involvement, this study is low risk. Consequently, nobody will suffer any injury. The study will contribute to understanding the relationship between metabolic syndrome and physical fitness in firefighters, which may lead to additional research in this understudied field. This systematic review is anticipated to shed light on how CVD risk factors impact firefighters’ cardiorespiratory fitness, hence demonstrating the necessity of intervention measures to promote the longevity and productivity of firefighting careers. Overall, the benefits of this study outweigh the risks or potential harms.

Data availability

All data obtained will be made publicly available and included in the published systematic review and meta-analysis.

Underlying data

The systematic review and meta-analysis will include and describe all underlying data. In the instances where the underlying data cannot be included, the data will be attached as appendices.

Timeline for completion of systematic review

Formal searching/study identification stage will be commencing from July to September 2025. The second stage of screening search results against inclusion criteria will commence from September to October 2025. The third stage, which is the data extraction, will take place from October to December 2025. The risk of bias/quality assessment will commence from December 2025 to February 2026. Data synthesis will take place from February 2026 to March 2026. The first rough draft will be completed by April 2026. Submission of the final paper will be completed by May 2026.