Osteoporosis is a disease of the skeleton in which bones become brittle and prone to fracture.
In other words, the bone loses density . Osteoporosis is diagnosed when bone density has decreased to the point where fractures will happen with mild stress. Until a healthy person is around 40, the process of breaking down and building up bone by cells called osteoclasts and osteoblasts is a nearly perfectly coupled system, with one phase stimulating the other. As a person ages, however, or in the presence of certain conditions, this system breaks down and the two processes become out of sync. The reasons why this occurs during aging are not clear.