Highlights
- High cardiorespiratory fitness in adolescence is associated with a small excess risk of atrial fibrillation (AF) during early adulthood.
- This AF risk is outweighed by a substantially larger reduction in non-AF cardiovascular disease (CVD) risk from middle age onward.
- Sibling-controlled analyses reveal that shared familial factors partly explain the observed associations, eliminating any early life cardiovascular disadvantage.
- Findings reinforce public health initiatives promoting improved adolescent fitness for long-term cardiovascular benefits.
Background
Cardiovascular diseases (CVD) remain the leading cause of morbidity and mortality globally. Cardiorespiratory fitness (CRF), a key marker of physical health, inversely correlates with CVD risk. However, paradoxically, emerging evidence suggests that high levels of physical fitness or endurance exercise might increase atrial fibrillation (AF) risk, notably in young athletes. AF is the most common cardiac arrhythmia linked to stroke and heart failure, posing significant clinical burdens.
Given that adolescence is a formative period for establishing health trajectories, clarifying the long-term cardiovascular impact of high CRF during youth, including AF risk, is crucial. Observational studies often face residual confounding, including shared genetic and environmental factors. Sibling-controlled cohort designs provide robust opportunities to control for familial confounders, enhancing causal inference.
Key Content
Large-Scale Sibling-Controlled Cohort Evidence
The pivotal study by Ballin et al. (2026) leveraged a nationwide Swedish cohort of 1,124,049 men conscripted between 1972 and 1995, with measured adolescent CRF at a mean age of 18.3 years. Outcomes included incident AF and major non-AF CVD events (stroke and ischemic heart disease), tracked through national health registers up to 2023. Flexible parametric survival models estimated age-specific standardized risk differences (RDs) between CRF deciles.
Population-wide analyses showed that men in the highest CRF decile experienced a modest excess AF risk in early adulthood compared with the lowest decile, exceeding the contemporaneous reductions in non-AF CVD risk. However, from age 45 onward, reductions in non-AF CVD risk became more pronounced, eclipsing AF risk. Median ages at first AF and non-AF CVD events were 54.8 and 54.4 years, respectively.
Crucially, sibling-controlled comparisons, accounting for shared genetics and familial environment, eliminated this early adulthood trade-off. By age 35, the benefit in preventing non-AF CVD (RD -0.11%, 95% CI -0.21 to -0.01) already surpassed the small AF excess (RD 0.06%, 95% CI -0.01 to 0.12). By age 65, the cardiovascular benefit further widened. These results suggest familial confounding inflates the observed early AF risk in population analyses.
Complementary Findings on Physical Fitness, Musculoskeletal and Respiratory Diseases
Additional epidemiological data (Öberg et al., 2025) collected on ~40,000 Swedish males conscripted in 1969–1970 add context. This study linked higher CRF and muscle strength in youth with increased musculoskeletal disease risk (notably osteoarthritis and back/shoulder conditions) by middle age, likely reflecting repetitive mechanical overload or trauma. Conversely, high CRF was protective against cardiovascular and respiratory diseases.
These complementary findings underscore that while high CRF confers cardiovascular protection, elevated musculoskeletal morbidity represents a non-cardiovascular trade-off warranting awareness in physical activity counseling.
Mechanistic Considerations
High CRF reflects adaptations in cardiac structure and autonomic tone, which may predispose to AF by promoting atrial remodeling and vagal dominance. Nonetheless, the protective vascular and metabolic effects of fitness reduce atherosclerosis and ischemic risk, translating into net cardiovascular benefits over time. Familial factors, including genetics influencing fitness and cardiac phenotype, can confound epidemiological associations but are mitigated in sibling analyses.
Expert Commentary
This large nationwide sibling-controlled study robustly addresses the longstanding clinical dilemma regarding high adolescent fitness, AF risk, and net cardiovascular health. The findings clarify that the modest increased AF risk observed in earlier population analyses likely reflects hereditary and environmental biases.
From a clinical and public health perspective, promoting adolescent CRF remains prudent, given the clear lifelong cardiovascular benefits observed. Clinicians should remain vigilant for arrhythmia symptoms in high-intensity exercisers but also reassured by evidence indicating no net long-term cardiovascular harm.
Remaining challenges include elucidating sex differences (given study population of men only), defining safe upper limits of exercise intensity, and addressing musculoskeletal considerations linked to high fitness.
Conclusion
High cardiorespiratory fitness in adolescence confers significant protective effects against non-AF cardiovascular disease in later life, despite a small relative increase in AF risk during early adulthood. This risk is attenuated after controlling for familial factors, leaving no age period with net cardiovascular harm. These findings endorse population-level strategies to enhance youth fitness and support clinical reassurance regarding the safety and cardiovascular advantages of high adolescent CRF.
References
- Ballin M, Carlsson AC, Wändell P, et al. Adolescent Cardiorespiratory Fitness and the Trade-Off Between Atrial Fibrillation Risk and Cardiovascular Benefits: A Nationwide Sibling-Controlled Cohort Study. Circulation. 2026;154(1):7-15. doi:10.1161/CIRCULATIONAHA.125.078250. PMID: 42165142.
- Öberg B, Willner S, Sundquist J, et al. Physical health in young males and risk of chronic musculoskeletal, cardiovascular, and respiratory diseases by middle age: A population-based cohort study. PLoS Med. 2025;22(1):e1004517. doi:10.1371/journal.pmed.1004517. PMID: 39836663.