A large Swedish study is helping answer a question that has worried some athletes and doctors for years: can very high fitness increase the risk of an irregular heartbeat later in life?
The answer appears to be yes, but only partly. The same study also suggests that the overall benefits of high fitness are much greater than the possible risk.
The research was published in the journal Circulation. It examined the long-term health records of more than one million Swedish men who completed military conscription testing between 1972 and 1995.
The men were around 18 years old when their fitness was measured. Researchers then followed their health outcomes until 2023.
The main focus was atrial fibrillation, often called AF. This condition affects the rhythm of the heart. Instead of beating in a steady pattern, the heart can beat too fast, too slowly, or irregularly. Atrial fibrillation becomes more common with age and can increase the risk of stroke if it is not treated.
Previous studies have suggested that young endurance athletes and men with very high fitness levels may have a higher risk of atrial fibrillation later in life. This has led to a strange public health message. On one hand, exercise is clearly good for the heart. On the other hand, extreme fitness has sometimes been discussed as a possible risk factor for a heart rhythm disorder.
Researchers wanted to understand this relationship more clearly. They also wanted to compare the possible atrial fibrillation risk with the broader benefits of fitness, including protection against other serious cardiovascular diseases such as stroke and heart attack.
The study was led by Marcel Ballin. The researchers used national Swedish registers to track diagnoses over time. Because the study included such a large number of people, it gave researchers a rare opportunity to look at heart risks across many decades of life.
In the first part of the analysis, the findings seemed to support earlier concerns. Men with the highest levels of fitness at age 18 had a higher risk of atrial fibrillation than men with the lowest levels of fitness. Before age 40, this extra risk appeared to be larger than the benefit from lower rates of other cardiovascular diseases.
After age 40, however, the balance shifted. The protection from other heart and blood vessel diseases became stronger than the increased atrial fibrillation risk.
But the researchers did not stop there. They carried out a second analysis using full siblings. Almost half a million of the men in the study had a brother included in the same data. This allowed the researchers to compare brothers who grew up in similar family conditions but had different fitness levels.
This sibling comparison was important because families share many background factors. Brothers may share genes, childhood environment, food habits, family income, early life stress, and other influences. These factors can affect both fitness and future disease risk.
When the researchers compared brothers, the link between high fitness and atrial fibrillation became weaker. The overall benefits of high fitness became clear from the beginning, not only after age 40. The benefits also grew stronger with age.
This suggests that some of the increased atrial fibrillation risk seen in earlier research may not be caused by fitness alone. It may partly reflect other factors shared within families.
The finding is important because it helps reduce unnecessary fear around exercise. It does not mean that atrial fibrillation is unimportant. It also does not mean that athletes should ignore heart symptoms. Anyone who feels heart fluttering, sudden racing heartbeat, faintness, chest pain, or unexplained shortness of breath should seek medical advice.
However, the study suggests that the message should be more balanced. High fitness may be linked with a small increase in atrial fibrillation risk, but it is also linked with important protection against other cardiovascular problems. When researchers used a stronger study design by comparing siblings, the overall result favored fitness.
This matters because exercise has many health benefits beyond the heart. It supports mental health, sleep, blood sugar control, muscle strength, bone health, and healthy aging. For most people, avoiding exercise because of fear of atrial fibrillation would likely do more harm than good.
The study also shows why research design matters. A simple comparison between very fit and less fit people may not tell the whole story. People with different fitness levels may also differ in many other ways. By comparing siblings, researchers can get closer to understanding whether fitness itself is responsible for the result.
The researchers conclude that the risk of atrial fibrillation is not zero, but the wider benefits of high fitness are much greater. This gives a more reassuring message for athletes, active young people, and anyone who values exercise.
In the end, the study does not suggest that people should stop training or fear endurance exercise. Instead, it supports a sensible view: staying fit is still strongly good for long-term health, but people should listen to their bodies and speak with a healthcare professional if they notice unusual heart symptoms.
The findings are useful because they calm an exaggerated worry while still taking the possible risk seriously. High fitness may not be completely risk-free, but from a lifelong heart health perspective, it appears to be far more helpful than harmful.
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