Leptin: brains and bones.

Leptin, is a 16 kDa protein derived mainly, though not entirely, from white fat cells. It regulates the size of the body's fat load (i.e., energy reserves) and has a growing list of other functions. The latest addition is bone growth. The evidence so far indicates that leptin controls bone growth in two ways. It stimulates the release of an undefined hypothalamic osteoblast-inhibiting factor(s), which limits the amount of bone matrix that osteoblasts can make. Interestingly, it is itself a bone anabolic factor that directly stimulates bone growth by inducing osteoblasts to make IGF-I (insulin-like growth factor-I) and inhibiting osteoclast generation.