The most common type of bone disease, osteoporosis is characterized by fragile, thinning bones. When bones lose density, they also lose calcium and minerals, making them much more susceptible to fracture. Bone is a living tissue that must constantly replace itself. When the body is unable to form sufficient quantities of new bone tissue—or when the body reabsorbs too much existing tissue—osteoporosis results. Genetics appear to be the most important risk factor for developing osteoporosis. However, lack of calcium intake also is a substantial factor in many cases. Other factors include smoking, excessive alcohol consumption, low body weight, prolonged absence of menstrual periods (in women), and various diseases. Osteoporosis is particularly common in postmenopausal women as a decrease in estrogen can cause bone loss. In fact, approximately half of all women over the age of 50 will suffer a hip, wrist, or spine fracture at some point. Because there are no symptoms associated with the early stages of osteoporosis, many people are not aware that they have the disease until they suffer a fracture. Treatment of osteoporosis may involve making lifestyle changes, taking calcium and vitamin D supplements, taking certain medications, and participating in a physical therapy program aimed at preserving bone density through monitored weight-bearing exercises.