NRG4 and diabetes mellitus: There were seven studies [20–26] regarding the association between circulating Nrg4 levels and DM, the P value of heterogeneity between these studies was significant, so we used the random-effect model indicating that there was no significant statistical difference between diabetes patients and their normal controls (SMD = 0.18, 95% CI = -0.06 to 0.42, P = 0.143) (Fig 2).