Aims Elevated body mass index (BMI) is a known risk factor for heart failure (HF), however, the underlying mechanisms are incompletely understood. The aim of this study was to investigate the role of common HF risk factors as potential mediators. Methods and Results Electronic health record data from primary care, hospital admissions and death registrations in England were used to perform an observational analysis. Data for 1.5 million individuals aged 18 years or older, with BMI measurements and free from heart failure at baseline, were included between 1998 and 2016. Cox models were used to estimate the association between BMI and HF with and without adjustment for atrial fibrillation (AF), diabetes mellitus (DM), coronary heart disease (CHD), and hypertension (HTN). Univariable and multivariable two-sample Mendelian randomisation was performed to estimate causal effects. Among non-underweight individuals, BMI was positively associated with HF with a 1-SD (~ 4.8kg/m2) higher BMI associated with a hazard ratio (HR) of 1.31 (95% confidence interval [CI] 1.30, 1.32). Genetically predicted BMI yielded a causal odds ratio (OR) of 1.64 per 4.8 kg/m2 BMI (95% CI 1.58, 1.70) which attenuated by 41% (to OR of 1.38 (95% CI 1.31 - 1.45), when simultaneously accounting for AF, DM, CHD and SBP. Conclusion About 40% of the excess risk of HF due to adiposity is driven by SBP, AF, DM and CHD. These findings highlight the importance of the prevention and treatment of excess adiposity and downstream HF risk factors to prevent HF, even in people in whom the above risk factors are well managed.

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