Despite continuously improving diagnostic and therapeutic strategies, cardiovascular disease (CVD) still causes over 18.5 million deaths per year worldwide (1). The aging cardiovascular system is characterized by vascular changes, such as endothelial dysfunction, intimal thickening, and increased arterial stiffness. Ultimately, these age-related changes increase arterial pressure and impose mechanical stress on the heart. In response, the myocardium develops hypertrophy, leading to left ventricular (LV) diastolic, and eventually systolic, dysfunction (2–4). While cardiovascular aging is inevitable, lifestyle factors, such as diet and physical activity, have a major impact on this complex process. Although there is substantial evidence demonstrating adverse effects of unhealthy eating habits, obesity, and physical inactivity on the cardiovascular system, the underlying mechanisms are still insufficiently understood. Especially the interaction between dietary habits and physical activity is poorly studied. In Western societies, many people try to compensate unhealthy eating habits with exercise, but evidence for the efficacy of this approach is lacking.

A hypercaloric nutrition with high intake of saturated fat in combination with a sedentary lifestyle leads to obesity and metabolic syndrome (5). Both conditions are associated with increased mortality due to premature CVD (6), LV hypertrophy (7), and diastolic dysfunction, that, coupled with left atrial (LA) enlargement, is a risk factor for atrial fibrillation (8). In contrast, aerobic training has favorable effects on vascular and myocardial structure and function (9), which reduces CVD risk and cardiovascular mortality (10, 11). Large studies have consistently shown that 1 h per day of walking or 3 h per week of moderate-intensity jogging reduces mortality risk by up to 40% (10, 11). Regular exercise favors physiologic over pathologic hypertrophy (12), promotes myocardial autophagy (13), and ensures a healthy vascularization of the heart with adequate vascular density (14).

Recently, senescent cells have been identified as important promotors of cardiac aging and CVD (15). Cellular senescence refers to the reversible or irreversible withdrawal of cells from the cell cycle in response to a variety of stressors, such as telomere attrition, oncogene activation, genotoxic agents, mitochondrial dysfunction, inflammatory cytokines, and oxidative stress (16). In vivo experiments have shown that the removal of senescent cells prolongs the median life span of mice (17). These observations have sparked speculations that an unhealthy nutrition and a sedentary lifestyle promote cardiac aging and CVD through accelerated telomere shortening. Critically short telomeres are known to induce senescence and apoptosis (18). Physical activity is believed to preserve telomere length by increasing telomerase activity (19–22), an enzyme that counteracts telomere shortening through the addition of new nucleotides to telomeric ends. An upregulation of Tert expression in pre-senescent cells has been shown also to slow down senescence and tumorigenesis (23). Furthermore, exercise seems to modulate the expression of shelterin proteins, such as TRF2, which are essential for the protection of telomeres and their function (24).

Although the concept that lifestyle interventions may modulate telomere shortening and cardiac senescence by regulating telomerase activity and shelterin expression has attracted much attention, existing evidence is limited. Human studies that show differences in telomere biology between athletes and sedentary controls are mostly cross-sectional and focused on blood leucocytes rather than cells of the cardiovascular system (21, 25). So far, only one intervention study has shown a preservation of leucocyte telomere length and an induction of telomerase after 6 months of aerobic endurance exercise or high-intensity interval training, but not resistance training (21). Furthermore, results from the very few existing animal studies are also inconsistent. While voluntary wheel-running prevented cellular senescence in the thoracic aorta of C57/BL6 mice (25), it had no impact on the cardiac telomere length (26). In contrast, 1 year of voluntary wheel running attenuated the age-related shortening of telomeres in cardiomyocytes of CAST/EiJ mice (27).

Considering the lack of long-term studies that addressed the interaction between western-type diet and regular physical activity on telomere biology and cardiac aging, here we explored the impact of regular moderate exercise and a hypercaloric high-fat diet on cardiovascular remodeling, myocardial telomeres, senescence, and apoptosis.

The long-term consumption of HFD induced obesity without affecting cardiac cells TL. In addition, there were only small alterations of the myocardium including moderate hypertrophy, a reduction of capillary density and a trend toward more apoptotic cardiomyocytes as well as interstitials cells. Moderate endurance exercise improved the HFD-induced reduction of myocardial vascularization but did not reverse hypertrophy or apoptosis. Furthermore, exercising obese animals had shorter telomeres than exercising lean animals. In lean animals, the exercise protocol had neither significant effect on myocardial RTL nor on various histological indices of aging.

Long-term HFD feeding did not affect cardiac telomeres and induced only minor structural changes in the myocardium. Such small effects may appear paradoxical when considering the broad spectrum of obesity-induced metabolic alterations in humans, such as dyslipidemia, hypertension, type 2 diabetes, and insulin resistance (8, 29). Obesity is further characterized by a hyperdynamic circulatory status with higher blood volume, increased heart rate, and an activated angiotensin-renin axis. The resulting hemodynamic stress triggers myocardial remodeling with myocardial hypertrophy and fibrosis. Ultimately, the progression of these metabolic and hemodynamic effects leads to cardiovascular disease, myocardial dysfunction, and heart failure (HF) (8, 29, 30). Some observational studies also suggest accelerated telomere attrition in the context of obesity and metabolic syndrome (31–33), whereas weight loss seems to preserve telomere length (34). However, existing studies in humans and rodents are inconsistent, and controversies remain (35–38). The modest cardiovascular alterations observed in the HFD animals of our study are most likely explained by the beef-tallow-based HFD that has a high content of saturated and polyunsaturated fat, but only moderate amounts of carbohydrates. In a previous study, this diet has been shown to provoke obesity with only small metabolic and inflammatory changes (39). This rather mild metabolic phenotype is probably due to species-specific differences in lipid trafficking and adipocyte lipid-storage capacity (40). The modest cardiovascular alterations despite marked obesity, support the concept that dyslipidemia and inflammation are crucial for the development and progression of coronary atherosclerosis, cardiac remodeling, and HF. In the absence of additional metabolic and inflammatory factors, an excess of dietary lipids may be of limited harm as they represent the predominant substrate for oxidative phosphorylation in cardiac mitochondria (41, 42). This concept is supported by human studies where isolated obesity was associated with a lower risk for CVD and cardiac dysfunction (43, 44). In the present study, HFD induced a significant increase in body weight (Figure 1A), but only mild LV hypertrophy without evidence of fibrosis (Figure 2). Moreover, myocardial hypertrophy is characterized by enlarged cardiomyocytes that push interstitial capillaries apart resulting in capillary rarefaction (45), which has also been observed in our HFD animals (Figure 5).

In contrast to the experimental model of our study, human obesity is usually the result of a diet that is high in saturated fat (SFA) and refined carbohydrates. Although a high dietary uptake of lipids with only a small fraction of polyunsaturated fatty acid (PUFA) species promotes inflammation and lipotoxicity (46, 47), excessive carbohydrate intake seems to dominate the detrimental cardiac effects in obese individuals (48–50). For example, hypertensive rats fed for 8 weeks a diet rich in carbohydrates showed an elevated end-systolic volume with a lower left ventricular ejection fraction, a higher number of apoptotic cardiomyocytes, and increased mortality when compared to rats on HFD (48). Also, nurturing men for several weeks with a high-sugar diet resulted in unfavorable changes in lipid metabolism and signs of hepatic lipid accumulation (51). In addition to metabolic, hemodynamic, and lipotoxic effects, also increased leptin concentrations promote myocardial hypertrophy and incident HF in obese individuals (52). For example, leptin treatment of cardiomyocytes from humans and neonatal rats has been found to induce TNF-α-/IL-6-mediated redox stress and cardiac hypertrophy (53–55). Despite high serum leptin, the HFD animals in the present study showed only a minor increase in oxidative-nitrosative stress (39), which may explain the lack of telomeric and apoptotic effects.

Regular exercise is believed to improve obesity, metabolic dysfunction, and chronic inflammation and slow down cellular aging, which ultimately reduces mortality. In a previous study we have shown that exercise ameliorated the pro-inflammatory profile of HFD-fed rats (39). While the present results indicate that exeHFD animals had also improved body weight and myocardial vascularization (Figures 1, 6), LV hypertrophy, interstitial fibrosis, and RTL were not altered (Figures 2, 7). Although there was no change in capillary density, the number of small arteries increased in response to exercise suggesting improved myocardial vascularization (Figure 6C). This concept is substantiated by an increase in RTL, Terf-1, and Terf-2 expression in the aortic tissue of exercising animals (56). While moderate exercise has only mild cardiac effects, intensive exercise seems to induce cardiac hypertrophy, fibrosis, apoptosis, and inflammation in SD rats (57). Exercise-induced myocardial hypertrophy is a well-known physiologic reaction to high volume endurance exercise with a significant proportion of high-intensity training that is controlled by C/EBPbeta (12, 58). For example, 6-weeks of interval exercise with increasing intensity induced eccentric cardiac hypertrophy without fibrosis in 3-month-old male SD rats (59). In contrast, the metabolic stimulus of mild exercise, such as in the present study, is apparently insufficient to induce hypertrophy, but still has multiple beneficial anti-inflammatory, metabolic and vascular effects (58). However, a direct comparison of studies is often hampered by differences in age, sex, exercise protocol and nutrition.

Previous studies by Werner et al. demonstrated significant effects of voluntary wheel running on telomerase and shelterin expression as well as on survival pathways in blood leucocytes, aortic tissue, and the myocardium of 6-month-old C57/BI6 mice. Similar to the present study, these effects were not accompanied by alterations in myocardial RTL (25, 26). Others have shown an exercise-induced attenuation of myocardial telomere erosion in 1-year old CAST/EiJ mice (27). In this study, natural telomere shortening was accompanied by a decreased expression of the shelterins Terf-1 and Terf-2. Therefore, contrasting results in existing studies may be due to differences in species, exercise protocols, study duration, and group size. While some studies worked with voluntary wheel running (25–27), others used motorized treadmills, where continuous running is imposed by mild electric stimuli (57). Although exercise volume and intensity are well standardized with this approach, the perceived psychological stress may be higher, which could explain some of the controversial effects. The fact that telomere shortening in cardiomyocytes is only detectable at older age may also be due to the post-mitotic nature of this cell type, where the end-replication problem does not exist. In such cells, telomere shortening is probably the result of other mechanisms that involve typical age-related effects, including metabolic dysfunction, chronic inflammation, and oxidative stress. Therefore, also the beneficial effects of lifestyle factors, such as exercise, may become more evident at older age. Notably, there is some evidence suggesting that the association between age and RTL is stronger in males than in females (60). Therefore, it cannot be excluded that age and lifestyle factors have more pronounced effects on the cardiovascular system in males than in females. However, this hypothesis is purely speculative, and the use of both sexes is warranted in future aging studies.

While all studies discussed before investigated the effects of exercise in lean animals, here we explored the effects of regular moderate running exercise in the context of obesity. Our exercise protocol reversed the reduction of myocardial vascularization in obese HFD animals (Figure 6) but did not affect the size of cardiomyocytes or myocardial apoptosis (Figures 2–4). Despite the improvement of myocardial vascularization and the lack of fibrotic tissue, exercise was accompanied by evidence of arterial remodeling (Figure 6E) which is one of the earliest signs of target organ damage in hypertension (61). Together with shorter telomeres (Figure 7A) and higher numbers of p16^INK4A positive cardiomyocytes (Figure 4B), the higher wall-to-lumen ratio in HFD animals questions the beneficial nature of forced endurance exercise in obese individuals. In analogy to observations by Agrimi et al., the psychological stress induced by frequent forced running might have caused an adaptive stress response with increased myocardial apoptosis, senescence, and oxidative-nitrosative stress (39, 62). Increased oxidative-nitrosative stress might be a potential mechanism that mediates adaptive stress responses in our animals including telomeric dysfunction. This hypothesis is supported by a positive correlation between the serum NOx concentration and cardiac Terf-1 expression in our animals. A relationship between eNOS and telomere function in exercising mice has also been reported by Werner et al. (25). However, the exeHFD animals also showed a moderate upregulation of Tert expression (Figure 7B), which might reflect an altered DNA-damage-repair and checkpoint response to short-telomere-induced DNA damage (23). Thus, the upregulation of Tert expression in these animals could reflect a protective mechanism aiming to prevent myocardial degeneration due to premature telomere attrition and chronic cellular senescence.

In conclusion, isolated obesity induced by a hypercaloric HFD with low carbohydrate content has no telomeric effects in the myocardium and induces only mild morphologic alterations suggesting that this treatment does not significantly accelerate cardiac aging. Also, long-term moderate endurance exercise was not associated with clear cardioprotective effects. Therefore, obesity without additional risk factors or comorbidities may be less harmful than expected. Furthermore, moderate exercise appears to be of limited relevance for myocardial telomere biology and cardiovascular remodeling. Of note, the results from this study cannot easily be translated to humans where cardiovascular aging is usually a multifactorial process. In conjunction with the existing literature, our findings emphasize once more the importance of the experimental setting, which may profoundly impact the results.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The animal study was reviewed and approved by the responsible National Ethics Committee (GZ: 66.010/0070-V/3b/2018) and conducted in accordance with the guidelines of the Animal Care and Use Committee of the Ministry of Science and Research, Vienna, Austria.

Conceptualization: MH, H-JG, and GA. Methodology and validation: AB, WR, and H-JG. Software, writing—original draft preparation, and visualization: MS. Formal analysis: H-JG, AB, and MS. Investigation: MS, AB, H-JG, GA, MH, and FK. Resources: FC and MH. Data curation: MS and AB. Writing—review and editing: AB, GA, SS, WR, H-JG, FC, and MH. Supervision: MH and AB. Project administration and funding acquisition: MH. All authors have read and agreed to the published version of the manuscript.