Under normal conditions, bone metabolism is accomplished through the formation of new bone by osteoblasts and the breakdown and resorption of bone by osteoclasts. Bone formation and resorption are in a dynamic balance throughout a person’s life. During childhood and adolescence, bone formation is greater than bone resorption; the peak of bone mass is reached around the age of 30. Thereafter, as age increases, bone resorption is greater than bone formation. This results in lower and lower bone density, and osteoporosis occurs when bone density is below a certain standard. Osteoporosis is more common in women than in men, and the age of onset is also earlier than in men, because women’s estrogen levels decline rapidly after menopause, resulting in a large loss of bone mass. In severe cases, it can lead to fracture, making the height shorter and hunchbacked. Sometimes a slight bending or coughing may cause a fracture in the back, resulting in severe pain.