Reply to the letter to the editor: Body fat composition and risk of rheumatoid arthritis.

Zhao et al. conducted a two-sample Mendelian randomization (MR) to investigate a causal relationship between body fat composition and rheumatoid arthritis (RA). Five anthropometric traits, including body mass index (BMI), body fat percentage, whole-body fat mass, whole-body fat-free mass and appendicular lean mass, were included as exposures. The study found evidence to support a putative causal role of fat mass and body fat percentage, rather than fat-free mass and appendicular lean mass, in the development of RA, which replicates and extends our previous findings on a causal association between higher BMI and an increased risk of RA (1).