Association of Endogenous Testosterone with Physical Fitness Measures during Firefighter Occupational Health Evaluations
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1. Introduction
Firefighters (FFs) perform various duties, many of which involve strenuous physical work and emotional stress [1,2]. Although they vary worldwide, most often, FFs perform fire suppression, technical rescues, and emergency medical services. FFs’ routine tasks include stair climbing, forceable entry, crawling/searching, advancing hoselines and suppressing fire, and rescuing animals/persons. Importantly, these activities are done while wearing heavy protective equipment (~25 kg) and working in extreme environmental conditions with time urgency [1]. To perform these highly demanding activities, FFs need adequate physical fitness (PF), including both muscular and cardiorespiratory fitness (CRF) and proper body composition [1]. The US National Fire Protection Association (NFPA) recommends a minimum CRF of 12.0 metabolic equivalents (MET) for FFs to safely and efficiently perform their job-related tasks [3]. Also, this cut-off point has been widely used in firefighter fitness-related research [4,5,6]. Due to the hazards they face and the physiological strain of their work, firefighting is recognized as a hazardous profession with high on-duty mortality rates [7,8,9].
The US Fire Administration (USFA) reports that sudden cardiac events were responsible for almost 50% of on-duty deaths in the last 20 years among US FFs [9]. The odds of a cardiac event are highest following fire suppression activities [7,8]. While firefighting does not cause cardiac events, firefighting may trigger a cardiac event in individuals with underlying diseases [7,8]. A review of medical examiner records for duty-related cardiac fatalities of male FFs showed that almost 80% had evidence of coronary heart disease and increased heart size (cardiomegaly and/or left ventricular hypertrophy) at autopsy [8].
Furthermore, overweight/obese FFs and those with low or inadequate physical fitness (PF) have been shown to have worse job performance and undesirable cardiometabolic (CM) risk profiles [10,11]. Therefore, it is necessary to investigate further the relationship between cardiorespiratory and muscular fitness levels and the appropriate CM risk profiles required to safely perform the strenuous work of firefighting and decrease the risk of duty-related cardiac events [12,13,14,15].
In the context of complex CM risk assessments, endogenous serum testosterone (TT) levels have recently appeared as an emerging cardiovascular risk factor. In the general population, low TT has been associated with increased mortality [16,17]. In the first study to report on TT levels among US FFs, we found a proportion of 10.6% of low TT. FFs with lower testosterone were older and showed worse CM profiles than FFs with higher TT values [18]. Furthermore, we observed an association between borderline-low TT (264–319 ng/dL) and lower left ventricular wall thickness. In a more recent study among young/middle-aged career FFs, we observed a high proportion of fatty liver (FL; ≈40%), especially in those with low TT (78.1%). Having low TT also increased the odds of having a fatty liver and an unfavorable CM risk profile, even after adjusting for covariates [19]. However, despite the known physiological effects of TT on the cardiovascular and muscular systems [20], whether low TT levels influence PF remains unclear, even in the general population. We then hypothesized that low TT levels are associated with worse physical fitness measured in the context of occupational health assessment in firefighters. Therefore, the purpose of this study is to explore the relationship between TT with PF parameters among US career FFs.
4. Discussion
The main findings of this study are that the low and borderline TT groups had lower PF (push-ups and Bruce Treadmill test duration) compared to the TT reference range group. However, after adjusting for age and BF%, the differences in PF did not achieve statistical significance. Similarly, all significant odds of less-fit FFs having low TT lost significance after adjustment for the same confounders. Therefore, our data suggest that age and body fat may mediate the association between PF and TT.
TT deficiency and CM diseases share common risk factors such as obesity, age, and metabolic syndrome components [16,21,29,30,31]. These risk factors are also related to PF, although the directionality of the relationship is often unclear [32,33]. Although a direct or reverse causality is yet to be clarified, our data support the hypothesis of a complex and negative vicious cycle among these factors, similar to the one proposed by Genchi et al. According to this model, the relationship between obesity and hypogonadism potentiates a feedback loop so that each condition further exacerbates the other [34]. Specifically, excess body fat is associated with impaired TT levels while hypogonadism, in turn, is associated with fat accumulation, thus leading to a vicious cycle of obesity and hypogonadism [34]. Furthermore, our data support that low PF is associated with obesity and concurrent low TT, thus adding a novel reinforcing factor to the driving of the negative vicious cycle of obesity, low TT, and low PF, with age interacting with all these three components.
Despite the high physical demand of FFs’ job-related activities and formal recommendations for fitness assessment and minimum standards (NFPA 1582 and 1583) [3,35], the prevalence of obese and unfit FFs is surprisingly high [4,12]. Obesity prevalence in the fire service has been reported from ≈15–37% in Brazil and South Africa and up to 50% in the US [26,36,37,38]. Among US FFs, Storer observed that only 33% achieved the recommended CRF for FFs (CRF ≥ 12 MET), while this proportion was higher but still low (≈50–70%) among Brazilian, Spanish, and Korean FFs [5,6,38]. Also, studies have shown a strong association between PF and body composition (BC) in the general population and FFs [13,38,39].
The CM health of FFs has been investigated primarily by assessing traditional cardiovascular (CV) risk factors. Moffatt et al. found that the CV risk profile of a large cohort of US career FFs (4279 males) was similar to that of the general population, which is highly concerning in this workforce [40]. A study aiming to evaluate blood pressure (BP) by decade of life in US career FFs, in comparison with the general population, found a much higher prevalence of hypertension among male FFs compared to the general population in all ages [41]. When analyzing the effect of both BC and age on the overall CM risk factors assessment among US FFs, a study found that a higher BMI, independent of age, was associated with a higher prevalence of CM risk factors and metabolic syndrome [42]. Notably, a recent study among US FFs showed that the group of FFs who lost weight after 5 years of observation (>3% body weight) significantly improved their CM risk profile. At the same time, those who gained weight (>3% body weight) showed increases in total cholesterol, LDL cholesterol, blood glucose, and BMI [43], reinforcing the hypothesis that weight control could be a key CM risk profile mediator.
Obesity is an inflammatory condition that contributes to atherosclerosis, which is a disease that is responsible for millions of CV deaths per year [44,45]. Besides its direct effects on increasing CM risk, obesity-induced systemic inflammation is also associated with reduced TT levels [34,46]. In a previous study, we observed that the proportion of FFs with low TT that had a normal BMI was only 3.1%, while 59.4% of those with low TT were obese (BMI ≥ 30 km/m2) [19]. It is then plausible to consider that, although the link between TT levels and PF might be indirect, the potential of PF mitigating the negative effect of high BF% on testosterone is consistent and represents a positive health parameter. Importantly, despite the great progress of pharmacological alternatives developed in the last decades to treat obesity [47], preventing weight gain must be undoubtedly prioritized.
Because FFs perform strenuous work that requires muscular and cardiorespiratory fitness, low TT is of great concern in this workforce since testosterone exerts physiological effects on many sites [20], including the CV system, skeletal muscle, and adipose tissue. Symptoms associated with sustained low TT include fatigue, increased body fat, and lowered fat muscle mass [21,22]. The physiological effects of TT, in addition to the fact that PF may improve in men with testosterone deficiency after TRT [48,49], support the general idea that higher TT levels are associated with better PF or vice-versa. In fact, several studies have observed a positive relationship between TT and PF variables, although they often lacked adjustments for age or adiposity [50,51,52]. Others also supported the hypothesis that the relationship between TT levels and key PF variables is mediated by age and body composition. In a study that investigated the relationship between adiposity, CRF, and serum TT levels, bivariate correlations were moderately significant when CRF was controlled for BF%, or vice-versa, but remained weaker for testosterone itself [53]. Similar to our results, this finding also suggests that body fat mediates the association between CRF with serum TT. In a study performed by Koch (2011) with 624 men aged 25 to 85 years old, serum TT was not associated with aerobic performance, which was measured through a progressive incremental exercise protocol on a cycle ergometer [54]. In addition, a study comparing health-related physical fitness components in young and middle-aged men clinically diagnosed with congenital hypogonadotropic hypogonadism (CHH) and clinically healthy controls showed that acute fluctuations in TT levels did not correlate with muscle strength and endurance, neither in men with CHH nor in healthy individuals [55]. Furthermore, an acute intramuscular injection of testosterone esters in healthy eugonadal men did not acutely enhance strength and power [56].
In agreement with other studies, our data refuse the hypothesis of a primary association between a low TT measurement with poor PF, but also support the concurrent negative effects of age and body fat on TT levels and PF. These findings challenge the commonsense idea that higher blood concentrations of TT are associated with higher global strength and must be interpreted with caution. Our adjusted data support the previously proposed model of a pathological vicious cycle of obesity, low TT, and low PF with age interacting with all these components. However, in the face of the current research evidence, it is important to note that a consensus regarding the association of testosterone levels and PF is still lacking. A study using the Cooper Center data of 2994 healthy men aged 50–79 years found that men with high CRF (maximal treadmill test) had significantly lower odds (OR: 0.75, 95% CI: 0.71, 0.79) of having low TT (57]. The higher age of the Cooper Center study (50 to 79 years) and the difference in the CRF assessment may explain the different findings.
The decline of TT with aging is well accepted in the literature [16,22,58]. Although some uncertainties continue regarding the effects of age on total testosterone concentration [59], low TT in aging men has been shown to be a marker of CV risk [16]. Similarly, BC and PF also tend to worsen with aging [42,60,61]. Although age is a non-modifiable CM risk factor, it is a factor that needs more attention from the Fire Service. A recent study pointed out that fire departments will shortly have more FFs working beyond 60 years due to changes in retirement rules in different countries [33]. This study showed a significant decrease of CRF (−4.42 mL·(min·kg)−1; ≈−1.3 MET) and a simultaneous increase in BMI (+1.25 kg/m2) after 8 years of follow-up of a relatively young cohort of Brazilian military male FFs (36.2 years) [33].
Limitations
Some limitations in our study must be considered. Firstly, our analyses are from a cross-sectional study design, which precludes causal relationships and the exclusion of reverse causality. Secondly, inherent to the retrospective study design, we used a single testosterone measurement that precludes evidence of an unequivocal state of testosterone deficiency. However, our aim was not to establish a medical diagnosis of low TT but to perform an exploratory analysis of data that were collected with standard procedures in an occupational health setting that is expanding their routine health assessment on FFs. Also, the definition of TT cut-off points is still controversial and might influence the results. To address this inherent limitation, we conducted additional analyses with different TT cut-off points (analyses not shown for simplicity) to determine TT groups, and the results were essentially the same. That is, some PF variables showed poor values in the lowest TT groups in the crude analysis, but no statistical difference remained after adjustment for age and BF%. The assessment of CRF was based on a common practice of only having FF exercise to 85% of the predicted maximum, which may affect the accuracy of the data and affect our understanding of the relationship between CRF and TT. Our comprehensive analyses (recommended and alternative TT cut-off points, subgroups by age and body composition categories) mitigate the effects of these inherent limitations. Although our results are based on data from a convenience sample, which imposes an external validity limitation, our data came from the records of regular medical screening examinations in an entire department that was not subject to any selection bias. Finally, it is important to consider that although the PF tests are validated and widely used among fire service personnel, several of them are affected by BC, making it challenging to evaluate exclusively CRF or muscular endurance. We attempted to overcome such limitations by adjusting analyses for age and BF% and analyzing by body composition and age categories.
5. Conclusions
In this cross-sectional study among male career US FFs, we found significant associations between low TT and decreased muscular endurance and cardiorespiratory fitness; i.e., in crude analyses, the low and borderline TT groups had lower PF compared to the TT reference range group. However, after adjusting for age and BF%, no difference remained significant. Therefore, in the final adjusted model, there was no significant association between TT and PF. The adjusted data suggest that body fat and age interact to modulate the significance of testosterone association with a poor PF. In this context, it is important to highlight that excess body fat is responsive to lifestyle modification, which includes a healthy diet and PF improvement [62,63] and represents a priority action to prevent weight gain. Importantly, and considering how difficult it is to treat obesity, even in the context of the progress of pharmacological alternatives developed in the last decades [47,52], preventing weight gain must be prioritized.
Our exploratory and novel findings are relevant to the Fire Service, occupational clinics, and all health professionals who work with firefighters and also from a public health perspective. Our findings support the hypothesis that the association of low TT and impaired PF is mediated largely by age and body fat. Medical and allied health professionals (e.g., medical personnel, exercise physiologists, and dietitians) should be aware of the association between increased adiposity, age, and low testosterone and the potential positive role of reducing body fat and increasing PF as strategies to mitigate the effects of the negative vicious cycle of obesity, low TT, and low PF. Since age is a non-modifiable risk factor, these professionals should target weight loss to combat systemic inflammation, improve endocrine function, and increase physical fitness parameters.
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