Remodeling consists of tearing down small parts of the bones, and then re-forming them. The breaking down portion is termed "resorption" and is performed by large cells within the bones called "osteoclasts". Osteoclasts live in the central portion of the bone. They are continually removing small (microscopic) portions of bone at the edge of the bone surface. Nearby, bone forming cells called "osteoblasts" begin to fill in the holes left behind.
Here is the big problem: remodeling does not seem to be a perfect give and take of bone mass. The osteoblasts are less efficient at making bone than the osteoclasts are at removing it. Although the difference is slight, this is what accounts for the gradual loss of bone density (BMD) as a person ages. Any factor which causes a higher rate of bone remodeling will ultimately lead to a more rapid loss of bone mass and thus more fragile bones.
Amount of Strong Bones Formed During Youth
- subtract -
Amount of Bone Lost to Remodeling Over Many Years
- equals -
Level of Bone Mass as an Adult (or degree of osteoporosis)
The same factors which encouraged bone formation as a youth and teenager affect the maintenance of bone mass during the adult years. The most important influences are calcium intake, reproductive hormone status, normal parathyroid gland function, and physical activity.