Probiotic supplementation in T2DM patients led to a significant reduction in insulin level (SMD = − 0.185, 95% CI  − 0.313 to − 0.056, Peffect = 0.004) without heterogeneity (Fig. 2b, I2 = 0%, Pheterogeneity = 0.500). Here, INS is linked to type 2 diabetes mellitus.