Exercise is important for your heart and cardiovascular health. But for years, research has suggested that very fit people – particularly young men – have a higher risk of developing atrial fibrillation (an irregular heartbeat) later in life than less athletic people do.
But our new large-scale study of more than one million young Swedish men challenges this long-held concern about fitness and heart health. The study shows that atrial fibrillation risk in very fit people is smaller than previously thought. And, importantly, the benefits of being fit – such as having lower rates of other cardiovascular diseases, such as stroke and heart attack – clearly outweigh the potential downsides.
To conduct our study, we analysed data from over one million Swedish men. Each of these men had completed a military conscription test between 1972 and 1995 when they were around 18 years old. During conscription, fitness was estimated using a maximal ergometer bicycle test.
We followed the participants using national health registries until 2023. Information was collected on diagnoses in specialised outpatient and inpatient care, as well as information on cause of death where relevant.
Nearly half a million of these men were siblings, allowing the team to compare brothers with different fitness levels. This method also helped control for shared genetics and upbringing to better understand risk.
In the population-wide analysis, where all men were compared based on their fitness levels, the fittest men did show a higher risk of atrial fibrillation until age 40. After 45, however, the benefits – fewer other heart problems – started to outweigh the risk.
But the sibling analysis told a different story. When comparing brothers, the link between high fitness and atrial fibrillation weakened significantly. The net positive effect of fitness on overall heart and cardiovascular health, although also weakening in the sibling analysis, was still evident from the start.
These benefits grew larger as the men aged. Across five decades of follow-up – from the age of 18 until the age of 65 – the benefits of being highly fit outweighed the overall risk of developing atrial fibrillation at all ages.
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While the higher risk of atrial fibrillation in the fittest men weakened in the sibling analysis, an elevated risk did remain. However, this risk was small overall. The potential risk of developing atrial fibrillation was also far outweighed by the positive benefits that being fit had on reducing risk of other cardiovascular diseases.
There was also some evidence that the increased risk of atrial fibrillation seemed to be driven by typically milder forms of atrial fibrillation.
Cardiovascular benefits of fitness
These findings suggest that earlier studies may have overestimated the risk of developing atrial fibrillation if you’re very fit because they couldn’t fully account for shared familial factors – such as genes and shared environmental factors – that may influence both fitness and heart health.
Because our study was able to account for these shared factors, we were able to show that we should potentially tone down the message that being very fit or engaging in endurance exercise poses a big risk to heart health. The risk of atrial fibrillation isn’t zero, but the benefits to heart health more generally are far greater.
Our results have also been confirmed in previous studies. However, our research builds upon this prior evidence by incorporating the sibling comparisons in the analyses, showing that the atrial fibrillation risk becomes weaker.
Our study also performed a more detailed examination of risk over time by following up with participants over a longer period of time, while also considering the benefits of high fitness. As such, our research has performed a more direct comparison of benefits and harms throughout life.
Overall, our research illustrates just how beneficial physical fitness is for overall cardiovascular health. Although very fit people do have a slightly higher risk of developing atrial fibrillation compared to people who are less fit, this risk of far lower than previously thought.
Plus, we show that the overall net benefits of fitness on cardiovascular health more broadly far outweigh any potential risk.
