Osteoporosis is due to reduction in overall bone mass, resulting in weakened strength of bones. This leads to higher risk of fractures. Basically, the weaker the bone, the less able it is to hold up to high stresses. The reduction in bone mass can be caused due to many factors.
One cause that can occur even in young patients is failure to deposit enough normal bone mass during development--that is, prior to reaching skeletal maturity. This can occur with many disease states, typically endocrine (hormonal) disorders.
After skeletal maturity (usually about age 25), many factors can decrease bone mass, including diseases that cause excessive resorption of bone, impaired bone remodeling. In all patients, decreased bone mass may be worsened by smoking, drinking excessive soda or coffee, poor diet, lack of excerise and steroid use.