The thinning of bone tissue and loss of bone density over time.  Osteoporosis is the most common type of bone disease.  There are currently an estimated 10 million Americans suffering from osteoporosis, as well as another 18 million who have low bone mass, or osteopenia.  Osteoporosis occurs when the body fails to form enough new bone, or when too much old bone is reabsorbed by the body, or both.  Calcium and phosphate are two minerals that are essential for normal bone formation.  Throughout youth, the body uses these minerals to produce bones.  If calcium intake is not sufficient, or if the body does not absorb enough calcium from the diet, bone production and bone tissues may suffer.  As people age, calcium and phospate may be reabsorbed back into the body from the bones, which makes the bone tissue weaker.  Both situations can result in brittle, fragile bones that are subject to fractures, even in the absence of trauma.  Researchers estimate that about 20% of American women over the age of 50 have osteoporosis.  In addition, another 30% of them have osteopenia, which is abnormally low bone density that may eventually deteriorate into osteoporosis, if not treated.  About half of all women over the age of 50 will suffer a fracture of the hip, wrist, or vertebra.