For years, some studies have suggested that very fit young men, especially endurance athletes, may have a higher risk of developing atrial fibrillation later in life.
Atrial fibrillation is a common heart rhythm problem. It happens when the upper chambers of the heart beat in a fast or irregular way. Some people feel a racing heartbeat, tiredness, chest discomfort, or shortness of breath.
Others may not notice symptoms at all. The condition matters because it can raise the risk of stroke and other heart problems if it is not managed properly.
This possible link between high fitness and atrial fibrillation has sometimes created confusion. Exercise is usually seen as one of the best things people can do for their heart.
It helps lower blood pressure, improves cholesterol levels, supports a healthy weight, improves blood sugar control, and lowers the risk of many long-term diseases. So if very high fitness is linked to one heart rhythm problem, does that mean being extremely fit could be dangerous?
A new study published in the journal Circulation gives a more balanced answer. The research looked at more than one million Swedish men and found that high fitness may be linked to a slightly higher risk of atrial fibrillation, but the overall heart health benefits appear to be much larger than the risk, especially when family background is taken into account.
The study was led by Marcel Ballin and used Swedish national military conscription records from 1972 to 1995. At that time, many young Swedish men had to complete military service testing.
As part of this process, their physical capacity was measured when they were about 18 years old. Researchers then followed these men through national health records until 2023 to see who later developed atrial fibrillation or other cardiovascular diseases, such as stroke and heart attack.
This made the study unusually large and powerful. More than one million men were included, and almost half a million of them were full siblings. This sibling information was especially important because it allowed the researchers to ask a deeper question. Were fitter young men developing atrial fibrillation because of fitness itself, or because of other family-related factors that also affect health?
In the first analysis, the researchers compared men with the highest fitness levels to those with the lowest fitness levels. They found that the fittest men had a higher risk of atrial fibrillation. Before age 40, this increased risk seemed to outweigh the protection they gained against other heart and blood vessel diseases. After age 40, the benefits started to become stronger than the risk.
But the story changed when the researchers compared full siblings with different fitness levels. Siblings share many things, including part of their genes, early home environment, family habits, and some social and economic conditions. By comparing brothers within the same families, the researchers could reduce the effect of these shared background factors.
In this sibling analysis, the link between high fitness and atrial fibrillation became weaker. More importantly, the overall balance became positive from the beginning. In other words, the benefits of high fitness were greater than the risks across the life course, and the benefits became even stronger with age.
This is an important finding because it suggests earlier studies may have made the risk look larger than it really is. Some of the apparent danger linked to high fitness may actually come from other factors that run in families or are connected to lifestyle, environment, or genetics.
The study does not say that atrial fibrillation risk is zero. Very fit people, including endurance athletes, can still develop the condition. People who notice an irregular heartbeat, dizziness, chest pain, unexplained tiredness, or breathlessness should speak with a doctor. However, the study suggests that people should not be overly frightened by the idea that high fitness is harmful to the heart.
The research also reminds us that health risks and benefits need to be judged together. A small increase in one type of heart rhythm problem may not outweigh major reductions in stroke, heart attack, and other cardiovascular diseases. From a lifelong point of view, being fit still appears to be strongly beneficial.
The main message is reassuring. High fitness in young adulthood may be linked to a higher chance of atrial fibrillation, but when researchers looked more carefully at brothers from the same families, the overall picture became much more positive. The findings support the idea that physical fitness is still one of the best long-term investments people can make for heart health.
This study is valuable because it does not give a simple “exercise is good” or “exercise is risky” message. Instead, it shows that the truth is more nuanced.
High fitness may carry a small risk for some people, but the wider benefits for cardiovascular health are likely much greater. For most people, the answer is not to avoid exercise, but to stay active, train wisely, and pay attention to any unusual heart symptoms.
If you care about heart health, please read studies about how eating eggs can help reduce heart disease risk, and herbal supplements could harm your heart rhythm.
For more health information, please see recent studies about how drinking milk affects risks of heart disease and cancer, and results showing strawberries could help prevent Alzheimer’s disease.
Source: Uppsala University.
