Previous epidemiological evidence suggests rheumatoid arthritis is associated with sarcopenia-related features. However, most of the current evidence is from cross-sectional studies, and the causal link of this association is still to be determined. Therefore, this study was committed to a two-sample Mendelian randomization analysis to assess the causal effect of rheumatoid arthritis on sarcopenia-related features. In this two-sample Mendelian randomization study, instrumental variables for rheumatoid arthritis were obtained from the Non-Cancer Disease Study, and data for the five relevant characteristics of sarcopenia were pooled from UKBiobank. Inverse variance weighting is the primary analysis method for assessing causal effects. MR-Egger regression and weighted median are complementary analysis methods for causal effects. Leave-one-out analysis, horizontal pleiotropy test, and Heterogeneity test are applied as a sensitivity analysis to assess the robustness of causal effect estimates. The inverse variance weighted results for the five characteristics associated with sarcopenia and rheumatoid arthritis were: hand grip strength (right) (beta = - 2.309, se = 0.206, p = 3.340E-29), hand grip strength (left) (beta = - 2.046, se = 0.205, p = 2.166E-23), whole body lean mass (beta = - 0.843, se = 0.135, p = 4.67E-10), appendicular lean mass (beta = - 2.444, se = 0.208, p = 6.069E-32), Usual walking pace (OR 0.340, 95% CI (0.238, 0.484), p = 2.471E-09). The sensitivity analyses did not support that horizontal pleiotropy distorted causal effect estimates. The beta coefficient quantifies the number of standard deviations of the continuous outcome variables (hand grip strength, whole body lean mass, and appendicular lean mass) that change on average with each increase in the standard deviation of the binary exposure variable (rheumatoid arthritis). The odds ratios indicate the increased risk of the binary outcome variable (usual walking pace) per rheumatoid arthritis standard deviation increase. This study has demonstrated a negative causal effect of rheumatoid arthritis with five major sarcopenia-related features in a European population.
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