Bone is remodeled constantly, throughout life, by osteoblasts and osteoclasts. There is a positive balance, with bone building outweighing bone breakdown through somewhere between age 25 and 40 years. After this, bone breakdown outweighs bone building. The peak bone mass occurs at this age of transition and is one of the determinants of later osteoporosis.
Bones act as a calcium reservoir for the body and calcium is pulled out whenever there is excessive loss (hypercalciuria, acidosis) or decreased absorption (Crohn's disease, dietary insufficiency) of calcium. Calcium is added to bones in response to stress such as weight bearing exercise. Calcium can not be effectively absorbed or added to bone in the absence of adequate active vitamin D.
After the age of peak bone mass, bone is lost throughout life. The rate of loss becomes much greater at menopause, which is why osteoporosis is more common in women. Primary prevention includes encouraging the development of the maximum peak bone mass and avoiding conditions that would result in more rapid bone loss.
Primary prevention strategies include:
Adequate calcium intake in the form of foods rich in calcium or supplements (see section on calcium)
Taking in or making, through exposure to sunlight, adequate vitamin D (see section on vitamin D)
Regular weight bearing exercise (Exercise and Fall Prevention)
Avoiding tobacco use
Avoiding excessive alcohol use
Avoiding prolonged use of Depo-provera, corticosteroids and certain anti-convulsants
Avoiding prolonged periods of amenorrhea due to excessive exercise or weight loss
There is weaker evidence (from retrospective and epidemiologic studies) that high dietary protein, especially from animal sources, may increase risk of osteoporosis (1, 2)