DKK1 and obesity due to melanocortin 4 receptor deficiency: Obesity may affect BMD through the following mechanisms: increasing the mechanical load on bones; producing more estrogen to regulate the activity of osteoblasts and osteoclasts, but enriching vitamin D and testosterone to reduce the serum concentration [30]; and regulating bone strength through some biochemical factors, such as adiponectin and leptin [31, 32], increasing the expression of SOST, DKK1, and FGF23 and inhibiting the expression of Wnt pathway and the function of 1α-hydroxylase [30], resulting in the loss of bone mass.