APLN and proliferative diabetic retinopathy: Our results show that the serum apelin in diabetic patients was significantly lower than that in non-diabetic controls, which is different from the results of most other studies, which showed increased serum apelin levels in type 2 diabetes mellitus [13], and in Type 2 diabetes with diabetic peripheral neuropathy [26], proliferative diabetic retinopathy [27] and coronary artery stenosis [28].