INS and type 1 diabetes mellitus: A few reports have indicated that insulin treatment prevents or delays DR in type 1 diabetes (T1D).10, 11, 12, 13, 14 A systematic review revealed that insulin treatment can also reduce DR in T2D, although the reduction was not as marked as that in T1D.15 The findings of an experimental animal study also suggested that insulin treatment benefited DR.16 However, some studies have suggested that insulin therapy could be harmful with regard to DR.2, 3, 4, 5, 6 These inconsistent results imply that the relationship between insulin treatment and DR is complicated.