Abstract
Background Obesity is a known risk factor for hypertension, but the extent to which physical activity mediates this relationship remains unclear.
Methods Cross-sectional data from medical exams of Swiss Armed Forces conscripts between 2007 and 2022 (N=382 583). Physical fitness was assessed via the Conscription Physical Test (CPT), which included five components, one of which was an endurance test (ET). Both CPT and ET results were categorised as ‘fit’ or ‘unfit’. Weight status was classified based on body mass index (BMI) into normal weight, overweight (BMI 25–29.99) and obesity (BMI≥30 kg/m²). Hypertension was defined as ≥140/90 mm Hg. The study explored CPT and ET as mediators between BMI and hypertension.
Results 20.6% of the conscripts had hypertension, 20.4% overweight and 4.5% obese. Conscripts with overweight or obesity had a higher risk of hypertension compared with normal weight (OR and (bias-corrected 95% CI) of natural direct effect: 1.803 (1.766 to 1.845) and 2.727 (2.570 to 2.865)), with a protective effect of being fit (natural indirect effect for CPT: 0.976 (0.971 to 0.982) and 0.917 (0.881 to 0.953)). When ET was assessed, similar findings were obtained: 1.765 (1.731 to 1.804) and 2.680 (2.482 to 2.887) for overweight and obesity, with a protective effect of being fit (0.991 (0.983 to 0.999) and 0.925 (0.861 to 0.991)).
Conclusions Male Swiss conscripts with overweight and obesity face an increasing risk of hypertension, with the protective benefit of physical fitness showing an increasing trend as BMI increases.
Introduction
Physical inactivity and obesity are well-established risk factors for cardiovascular diseases (CVD), which are the leading causes of death worldwide.1 Globally, one in three adults does not meet recommended physical activity (PA) guidelines,2 and 43% of the population is either overweight or obese.3 These modifiable factors are linked to higher risks of coronary heart disease, stroke and heart failure, contributing to an estimated 17.9 million deaths annually.1
Switzerland has faced a concerning trend in recent decades, with obesity rates doubling from 1992 to 2017 and currently staying at elevated levels,4 despite a more encouraging recent increase in PA levels.5 Considering the significant public health impact of these conditions, it is crucial to investigate the role of obesity regarding CVD risk factors, and to further understand the protective effect of PA in this relationship.
Hypertension is a major risk factor for heart disease and stroke and remains the leading preventable risk factor for CVD mortality in the world. Hypertension is closely tied to both physical inactivity and obesity.6 A large longitudinal study showed that young adults with low physical fitness had a sixfold increased risk of developing hypertension.7 Obesity increases blood pressure through various complex mechanisms, such as increased sympathetic activity and inflammation,8 and low physical fitness can worsen these effects. In fact, physically unfit obese individuals have poorer CVD outcomes compared with fit counterparts.9 Nevertheless, the exact interplay between these factors is not yet fully understood.
Recently, Crump et al10 examined the relationship between body mass index (BMI), physical fitness and hypertension among over 1.5 million Swedish military conscripts. The study revealed that both high BMI and low aerobic capacity were independently and synergistically associated with an increased risk of developing hypertension in adulthood, the highest risk being found in individuals who exhibited both risk factors. However, the authors did not investigate the mediating role of physical fitness in this relationship, particularly considering that people with obesity typically engage in less PA due to their health condition.11
Therefore, this study aims to address this gap by examining the mediation effect of physical fitness on the association between obesity and hypertension in a population-based sample of Swiss young men aged 18–23 years. Understanding these interactions is important for developing targeted strategies to prevent hypertension and CVD in the population.
Discussion
This study showed that Swiss male conscripts who present with overweight or obesity have an elevated risk of hypertension, with the protective effects of physical fitness (both musculoskeletal and cardiorespiratory) increasing as BMI increases.
Obesity and hypertension
Obesity is a well-established risk factor for hypertension, contributing to elevated blood pressure through complex endothelial, inflammatory and hormonal alterations.8 Our findings revealed a clear association between increased BMI and the presence of hypertension. Individuals classified as obese had a significantly higher odds of hypertension compared with those with normal weight, while individuals with overweight also showed an increased but less pronounced risk. This association remained significant after adjusting for confounding variables and controlling for the effect mediated through physical fitness. Our findings are consistent with two prior longitudinal studies conducted in similar military populations in Sweden that showed an independent association of obesity with hypertension10 and CVD.19
Mediation effect of physical fitness
Physical fitness is known to mitigate some of the adverse health effects of obesity, including its impact on blood pressure. We investigated how musculoskeletal and cardiorespiratory physical fitness mediates the association between obesity and hypertension, considering that obesity may contribute to reduced PA rather than reduced PA being the main cause of obesity.11 Our findings showed a protective effect of physical fitness, lowering the likelihood of hypertension in both overweight and obese individuals. Interestingly, this benefit increased as BMI increased, as the OR for ET was lower in participants with obesity than in participants with overweight. This suggests that physical fitness may be more effective in counteracting the harmful impact of obesity, compared with overweight, on blood pressure.
These findings align with previous studies showing that both BMI and cardiorespiratory fitness influence hypertension10 and CVD9 20, the highest risk being associated with having both a high BMI and a low fitness. Indeed, the meta-analysis by Barry et al 9suggested that high physical fitness could almost eliminate CVD risk in individuals with obesity, a finding in agreement with our results. It is important to note that not all previous studies are fully comparable to ours, as they assessed obesity and physical fitness as independent variables, while our mediation study examined both the direct effect of obesity and its effect mediated through physical fitness.
Strengths and limitations
As far as we know, this is the first study investigating the mediation of physical fitness in the association between weight status and hypertension. Importantly, our study included a large sample size of over 380 000 Swiss young men over 15 years and the use of standardised CPTs that assess various aspects of physical fitness.
However, our study also has limitations that should be considered. First, its cross-sectional design precludes the assessment of any causality between obesity, physical fitness and hypertension. Second, we cannot fully exclude the possibility that despite efforts to standardise data collection procedures, inherent differences may persist across different conscription centres, leading to variations in the measurement of weight, physical fitness or blood pressure. Third, as BMI was the sole indicator of weight status, we could not rule out that some athletes, who may have a high body weight due to higher muscle mass, have been mistakenly classified as overweight. This could have underestimated the magnitude of the association of obesity with hypertension; nevertheless, in the absence of other complementary markers, BMI and related WHO categorisation remain the most adequate measurement due to its well-established association with health outcomes. Fourth, a portion of conscripts did not undergo physical fitness assessments. This can be attributed, in part, to the presence of acute or chronic health conditions when they presented at the conscription centre. However, the absence of physical fitness tests only accounted for a relatively small percentage of observations, as illustrated in figure 1. Fifth, we observed a decrease in hypertension prevalence in our dataset, from 29.7% in 2007 to 6.6% in 2022, which may be explained by the reassessment of elevated readings in a quieter setting—a practice gradually introduced over time. Sixth, the available data did not allow us to conduct a comprehensive assessment of the socioeconomic level of the conscripts, therefore preventing the adjustment for this parameter. Sixth, by excluding conscripts with a BMI below 18.5 (underweight), we may have also excluded some athletic individuals; however, as underweight conscripts represented less than 0.4% of the study population, their exclusion is unlikely to have introduced bias. Seventh, the participants included were younger and presented with less obesity than the excluded ones, likely due to less missing and erroneous data in the database. This could have underestimated the mediation effect of physical fitness on the association between obesity and hypertension. Finally, the study population consists of young men, and the findings may not be generalisable to other populations without Swiss citizenship, other age groups and women.
Conclusions
Our study provides important evidence on the relationship between obesity, physical fitness and hypertension among Swiss male conscripts. Our findings indicate that overweight and obesity markedly increase hypertension risk, and that this association is partially mediated by a protective effect of physical fitness. Interestingly, as BMI increases, this protective mediation by physical fitness also increases, revealing a complex interaction between weight status and hypertension. Therefore, these findings suggest that addressing obesity remains a key public health priority for preventing hypertension, but promoting PA is also essential and may help counteract the adverse effects of obesity.
