MSTN and rheumatoid arthritis: However, the association between increased lean mass and decreased muscle IL-6 association was stronger in prediabetes compared with RA (Fisher r-to-z P = 0.0004); in prediabetes but not RA, lean mass increases occurred in conjunction with reductions in muscle myostatin (r = −0.92; P <0.05; Fisher r-to-z P = 0.026).