FGF21 and COVID-19: In this comprehensive mediated MR study, we identified seven gut microbiota (class Actinobacteria, class Negativicutes, order Bifidobacteriales, order Selenomonadales, family Bifidobacteriaceae, genus Dorea, genus RikenellaceaeRC9gutgroup) and five inflammatory proteins (4EBP1, CCL2, TSLP, FGF-21, FGF-23) in a causal relationship to COVID-19.