APLN and type 2 diabetes mellitus: In T2DM patients, serum apelin levels were negatively correlated with diabetes duration (years) (r = -0.130, P = 0.047), TG (r = -0.177, p = 0.007), TC (r = -0.144, P = 0.027), LDL-C (r = -0.186, P = 0.004), Creatinine (r = -0.136, p < 0.001), hs-CRP (r = -0.159, P = 0.015), whereas positively correlated with HDL-C (r = -0.147, P = 0.024) and serum BDNF level (r = 0.190, P—0.004) (Table 2).