• Posted May 27, 2026

High Fitness Doesn’t Raise A-fib Risk In Young Men, Study Finds

Being incredibly fit shouldn’t increase a young adult’s risk of dangerous irregular heart rhythm, a new study says.

Young male athletes and fitness buffs aren’t more likely to develop atrial fibrillation, despite earlier studies that showed an apparent link, researchers reported May 21 in the journal Circulation.

“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,” said lead investigator Marcel Ballin, an associated researcher at Uppsala University in Sweden.

“The risk of atrial fibrillation is certainly not zero, but that said, the benefits are significantly greater,” he said in a news release.

For the new study, researchers analyzed data for more than 1 million young Swedish men collected between 1972 and 1995 from national service data.

Overall, initial results showed that the men with the highest levels of fitness did have a higher risk of A-fib compared to those with the least fitness, researchers found.

Up to age 40, this higher A-fib risk outweighed the positive heart benefits from fitness, researchers said. From age 45, the benefits began to outweigh the risks.

But researchers then took matters a step further, by looking at about a half-million men drafted for military service who were full siblings. 

“Siblings share about half of their genes, but also to some extent socioeconomic factors, environmental factors, early childhood environment and behavioral factors,” Ballin said. “These factors could affect both fitness levels and the risk of various diseases, including cardiovascular diseases.

“Therefore, one can assume that the findings become … a bit more reliable when we take these factors into account, compared to a traditional analysis,” he added.

In the sibling comparison, the link between fitness and A-fib grew noticeably weaker. As a result, the risk/benefit balance shifted in a positive direction, even for younger people.

“We were then able to see that the age-dependent effect disappeared,” Ballin said. “Instead, we saw that the net effect was positive from the start, and that the benefits were greater as age increased.”

The links between fitness and A-fib found in previous studies likely are at least partly explained by factors that siblings share, researchers concluded.

“From a life-long perspective, the findings show that the benefits of high levels of fitness outweigh the risks,” he said.

More information

The American Heart Association has more on atrial fibrillation.

SOURCE: Uppsala University, news release, May 21, 2026