INS and type 2 diabetes mellitus: The study population was divided into three categories: insulin-treated type 1 diabetes mellitus (T1DM) included two studies (16.7%), insulin-naïve type 2 diabetes mellitus (insulin-naïve T2DM) included six studies (50%), and previously insulin-treated T2DM (non-insulin-naïve T2DM) included four studies (33.3%).