1 Million Men and 50 Years of Data: How Fitness Affects the Heart

A major new study published in the journal Circulation provides new insight into the relationship between high physical fitness and the risk of developing atrial fibrillation, one of the most common heart rhythm disorders.

Researchers concluded that well-trained young men may indeed face a slightly higher risk of atrial fibrillation compared with less active individuals. However, the overall long-term cardiovascular benefits of high fitness levels substantially outweigh this risk.

The study analyzed data from more than 1 million Swedish men who underwent mandatory military service between 1972 and 1995. The average participant age was around 18 years, and their physical fitness levels were assessed at the time of enrollment. Researchers then tracked their medical records through 2023, including cases of atrial fibrillation, stroke, and heart attack.

The initial analysis showed that men with the highest levels of physical fitness experienced atrial fibrillation more often than the least fit participants. Up to around age 40, the potential risk of arrhythmia among highly trained individuals appeared to outweigh the protective effect from reduced rates of other cardiovascular diseases. However, after that age, the balance shifted: the long-term cardiovascular benefits of high fitness levels became greater than the potential risks.

The researchers also conducted an additional analysis involving nearly half a million brothers and siblings. This approach helped account for the influence of genetics and shared family environment. In this model, the association between high physical fitness and atrial fibrillation risk became weaker, while the balance between risks and benefits shifted toward a positive effect even from a younger age.

Scientists note that siblings share roughly half of their genes and often grow up under similar environmental and socioeconomic conditions, which may influence both physical fitness and heart health. As a result, family-based analysis is considered a more accurate way to estimate the true impact of physical fitness on cardiovascular outcomes.