Osteoporosis occurs when the body fails to form enough new bone, or when too much old bone is reabsorbed by the body, or both. In normal healthy bone, remodeling occurs when osteoclasts eat away at the bone tissue and osteoblasts come in and refill the holes with new bone.
As part of this process, the body uses calcium and phosphate to produce strong bones during youth. 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. The bones become less dense and more susceptible to breakage.
In fact, in people with osteoporosis, bone fractures often occur with very little stress. Osteoporosis occurs gradually during aging and is more prevalent in women.