According to current Geriatric theory, Osteoporosis is part of the "normal" aging process. According to the TOR hyperfunction theory of aging, Osteoporosis is caused by over-expression of TOR.
52 million individuals in U.S have low bone mass (9 million osteoporosis). 50% of women and 20% of men will have a fragility fracture (hips, vertebral bodies, wrists).
Bone has two types of cells, the Osteocyte and the Osteoclast. Osteocytes are long lived cells and comprise 90% of all bone cells. They keep bone healthy and make new bone. Osteoclasts are a kind of macrophage, they engulf stuff and they dissolve and break down bone. Osteoporosis is due to excessive bone reabsorption by osteoclasts. These cells operate under control of mTOR within the cell.
Autophagy and Apoptosis are the two key terms. Apoptosis is self-destruction with loss of cell; autophagy is repair with preservation of cell.
One study in rats showed rapamycin reduced senile osteoporosis by activating osteocyte autophagy and preserving Osteocytes (mTOR reduces autophagy) and by decrease in apoptosis of Osteocytes and decrease in number of Osteoclasts.
Another study with Everolimus (equivalent to rapamycin) in rats showed a decrease of 60% in cancellous bone loss. This was by inhibition of osteoclasts.
Both studies showed that rapamycin or a rapalog decrease osteoporosis by blocking mTOR and decreasing activity of osteoclasts and preservation of osteocytes by increasing autophagy.