It has previously been shown that young, very fit people have an increased risk of atrial fibrillation. A new study of over one million young Swedish men shows that this risk is less than previous calculations showed, and that the benefits in terms of reduced risk for all other cardiovascular diseases outweigh the risk.
A number of previous studies have shown that young male endurance athletes and young men in general with high fitness levels appear to have an increased risk of developing atrial fibrillation later in life compared to non-athletes and those with low fitness levels. But to what extent is this true?
“Previous studies have not provided a clear answer as to how much of the increased risk is due to fitness per se or to other factors. It has also remained unclear whether the risk varies across the life course, and while weighting in the benefits that a high level of fitness can also confer. When assessing the potential risks, they must be weighed against the potential benefits,” says Marcel Ballin, lead author of the study.
Based on data from over 1 million men
In the new study, the researchers conducted a more extensive analysis. In parallel with studying the risk for atrial fibrillation, they also looked at the risk for other cardiovascular diseases. The aim was to find out at what age the benefit begins to outweigh the risk.
The study was based on national service conscription data from 1972 to 1995, and covers more than one million young Swedish men. The men were on average 18 years old, and had just completed a test that measured their physical capacity. Using national registers, the researchers followed this cohort’s disease data up to 2023, while collecting data on diagnoses of atrial fibrillation and all other cardiovascular diseases, such as stroke and myocardial infarction.
Almost half a million of the conscripts were full siblings, and among these, the researchers conducted a separate analysis to try to get a more accurate picture of the effects. The sibling analyses compared full siblings with different fitness levels, making it possible to control for shared familial factors that might affect the results, such as genetic and environmental factors.
In the traditional analysis, the men with the highest levels of fitness had a higher risk of atrial fibrillation than those with the lowest fitness. Up to the age of 40, this increased risk of atrial fibrillation was greater than the positive effects in terms of a reduced risk for other cardiovascular disease. After that age, the benefits began to outweigh the risks.
Siblings study gave new findings
However, when the researchers compared full siblings, the link between a high level of fitness and the risk of atrial fibrillation turned out to be weaker. As a result, the balance between risk and benefit shifted in a positive direction.
“We were then able to see that the age-dependent effect disappeared. Instead, we saw that the net effect was positive from the start, and that the benefits were greater as age increased. From a life-long perspective, the findings show that the benefits of high levels of fitness outweigh the risks,” says Marcel Ballin
The results of the sibling analysis suggest that the relationships seen in previous studies may be partly explained by factors that siblings share, rather than by fitness per se.
“Siblings share about half of their genes, but also to some extent socioeconomic factors, environmental factors, early childhood environment, and behavioural factors. These factors could affect both fitness levels and the risk of various diseases, including cardiovascular diseases. Therefore, one can assume that the findings become be a bit more reliable when we take these factors into account, compared to a traditional analysis,” says Marcel Ballin.
“Our study shows that there are good reasons to nuance and tone down the message, which has been widespread at times, that high levels of fitness or participating in races would pose a big risk to a person’s cardiovascular health. The risk of atrial fibrillation is certainly not zero, but that said, the benefits are significantly greater,” he concludes.
Circulation
10.1161/CIRCULATIONAHA.125.078250
Observational study
People
Adolescent Cardiorespiratory Fitness and the Trade-Off between Atrial Fibrillation Risk and Cardiovascular Benefits: A Nationwide Sibling-Controlled Cohort Study.
21-May-2026