Soy isoflavones and vitamin D (VD) can synergistically stimulate bone formation through inducing 1,25-dihydroxyvitamin D synthesis and vitamin D receptor (VDR) expression. 1,25(OH)2D bound to VDR can induce osteoblast activity and bone formation. Also, bone resorption may be reduced through inducing estrogen receptor (ER) expression, which is affected by VD and soy isoflavones. An increase in ER can lead to an increase in OPG formation and thus prevents osteoclast (OC) maturation, resulting in less bone resorption.