Bone is a living, growing tissue that turns over at a rate of about 10% a year. It is made up largely of type-I collagen, a protein network that gives the bone its tensile strength and framework, and calcium phosphate, a mineralized complex that hardens the framework. This combination of collagen and calcium makes bone strong and yet flexible enough to bear weight and to withstand stress. More than 99% of the body's calcium is contained in the bones and teeth. The remaining 1% is found in the blood.

Throughout your lifetime, old bone is constantly being removed (resorption) and replaced by new bone (formation). During resorption, cells called osteoclasts dissolve small amounts of bone, and enzymes dissolve the protein network. Bone formation is then initiated by cells called osteoblasts. They secrete a variety of compounds that help form a new protein network, which is then mineralized to create new bone. This on-going process takes place on a microscopic scale throughout the body. During early childhood and in the teenage years, new bone is added faster than old bone is removed. As a result, bones become larger, heavier, and denser. Bone formation happens faster than bone resorption until you reach your peak bone mass (maximum bone density and strength), around age 30. After this peak, bone resorption slowly begins to happen faster than bone formation. Bone loss is most rapid in women in the first few years after menopause but continues into the postmenopausal years.