Association between body mass index and the risk of atrial fibrillation and ischemic stroke according to age: a nationwide population-based cohort study

Abstract Background Atrial fibrillation (AF) and ischemic stroke (IS) are two significant cardiovascular diseases that confer an enormous healthcare burden. A limited study comprehensively evaluated the association between full ranges of body mass index (BMI), including underweight, and AF or IS risks, especially in the different age subgroups. Purpose We investigated the association between BMI and AF and IS incidence according to the Korean population's age groups. Methods This was a nationwide population-based cohort study using data from the Korea National Health Insurance Service, including 9 194 477 healthy adults who underwent a medical examination in 2009. We stratified the study population into three age subgroups: age 20–39 (young, 33.1%), age 40–64 (middle-aged, 56.3%), and age over 65 years (elderly, 10.6%). In each age group, the individuals were categorized based on BMI (kg/m2) into underweight (<18.5), normal (18.5 to <23), overweight (23 to <25), obese I (25 to <30), and obese II (≥30). The first occurrences of AF and IS were followed up until December 31, 2018. According to BMI in each age group, the risks of AF and IS were analyzed by Cox proportional hazards regression with 95% confidence intervals (CI) by adjusting age, sex, lifestyle behaviors, and comorbidities. Results Overall, both underweight and higher BMI were associated with an increased risk of AF and stroke across all age groups. The increased risk of AF for patients with obese II was slightly accentuated compared to patients with normal BMI in the young population than elderly population (hazard ratio [HR] 1.78, 95% CI 1.63–1.94 for age 20–39 years; HR 1.55, 95% CI 1.48–1.61 for age ≥65 years, respectively). For underweight individuals, however, the increased risk of AF became more prominent in the elderly: HR and 95% CI was 1.12 (1.07–1.17) in the age over 65 years old, and 1.05 (0.94–1.16) in the age 20–39. Regarding IS, the young group presented a considerable increment in the magnitude of HRs in both underweight and higher BMI groups. However, the association between the BMI and stroke risk became attenuated in the elderly: HRs and 95% CI in underweight and obese II individuals were 1.10 (0.93–1.30) and 2.223 (1.99–2.49) in the age 20–39 group, whereas 0.97 (0.93–1.01) and 1.03 (0.98–1.08) in the age over 65 years old. Conclusions Underweight as well as obesity was associated with increased risks of AF and IS in the general population. In both AF and IS, the gradient of risks according to BMI was apparent at young ages; thus, maintaining normal body weight should be warranted in early life. An interplay of several factors other than BMI may contribute to ischemic stroke in the old ages, requiring integrated risk management in older patients. Funding Acknowledgement Type of funding sources: None.