AIMS While obesity is a well-known cardiovascular risk factor, little is known whether age has a modifying effect. The aim of this study is to determine the age-dependent associations of body mass index (BMI) with cardiovascular outcomes.

METHODS AND RESULTS A population-based cohort of 9 278 433 Koreans without prior cardiovascular disease were followed up for the incidence of myocardial infarction (MI), heart failure (HF), and all-cause death. The effect of BMI with optimal normal weight (18.5-22.9 kg/m2) as reference was analysed according to age groups [young (20-39 years), middle-aged (40-64 years), and elderly (≥65 years)] and age decades. During 8.2 years, MI, HF, and all-cause death occurred in 65 607 (0.71%), 131 903 (1.42%), and 306 065 (3.30%), respectively. Associations between BMI and all outcomes were significantly modified by age (P-for-interaction < 0.001). There was a proportional increase in incident MI according to BMI in young subjects; this relationship became U-shaped in middle-aged subjects and inversely proportional/plateauing in elderly subjects. A U-shaped relationship between BMI and incident HF was observed, but the impact of obesity was stronger in young subjects while the impact of underweight was stronger in middle-aged and elderly subjects. Meanwhile, lower BMI was associated with higher all-cause mortality in all ages, although this association was attenuated at the young age, and pre-obesity was associated with the greatest survival benefit. These associations were independent of sex, smoking, physical activity, and comorbidities.

CONCLUSION The impact of BMI on cardiovascular risk differs according to age. Weight loss may be recommended for younger overweight subjects, while being mildly overweight may be beneficial at old age.