High cardiorespiratory fitness in adolescents and young athletes has previously been associated with increased atrial fibrillation (AF) risk. A large cohort study with sibling control analysis published in Circulation evaluated the long-term relationship between adolescent fitness, AF, and broader cardiovascular outcomes.

The analysis included 1,124,049 Swedish men who underwent mandatory military conscription examinations and completed cardiorespiratory fitness testing. Flexible parametric regression models were used to estimate standardized cumulative risk differences across fitness deciles. Sibling comparisons were additionally performed to account for shared genetic and environmental factors.

Findings

• During follow-up, 45,179 participants (4.0%) experienced AF events and 96,404 (8.6%) developed non-AF cardiovascular disease.
• In population-wide analyses adjusted for measured confounders, the highest fitness decile demonstrated a small excess AF risk compared with the lowest fitness decile.
• During early adulthood, the increase in AF risk exceeded reductions in non-AF cardiovascular disease, but this pattern reversed after approximately 45 years of age.
• In sibling-controlled analyses accounting for familial confounding, the age-related cardiovascular trade-off disappeared.
• By 35 years of age, reductions in non-AF cardiovascular disease exceeded the increase in AF risk (risk difference [RD], −0.11%; 95% CI, −0.21% to −0.01% vs RD, 0.06%; 95% CI, −0.01% to 0.12%).
• By 65 years of age, reductions in non-AF cardiovascular disease were substantially greater than excess AF risk (RD, −3.91%; 95% CI, −5.40% to −2.42% vs RD, 2.30%; 95% CI, 1.15%–3.45%).

The findings suggest that although high adolescent cardiorespiratory fitness was associated with a modest increase in AF risk, this was outweighed by larger long-term reductions in non-AF cardiovascular disease after accounting for shared familial factors.