By obesity status in controls, the reduction in circulating apelin concentration was comparable between studies with normal weight controls (BMI < 25 kg/m2) (WMD: -1.336 ng/mL, P < 0.001) and obese controls (BMI ≥ 25 kg/m2) (WMD: -1.226 ng/mL, P = 0.007). This evidence concerns the gene APLN and obesity due to melanocortin 4 receptor deficiency.