What are the Causes of Osteoporosis?

Osteoporosis occurs when bone remodelling - the balanced process by which bone is renewed and repaired - is disrupted by a variety of factors, some of which can be modified and some of which cannot at present. It is thought that the potential for any individual to develop osteoporosis is determined by both genetic and environmental factors. These in turn determine:

  • The peak bone mass achieved at maturity
  • The age at which bone loss starts
  • The rate of bone loss

A variety of risk factors increase the risk of fracture, and many of these do so by increasing the risk of osteoporosis. Factors influencing the development of osteoporosis are:

  • Age: Bone density begins to decline in both sexes around the age of 35. In women there is a phase of accelerated bone loss in the 10 years or so following the menopause, while in men there is a steady decline. Following this endocrine-mediated loss, there is a steady decline in bone density with advancing age.
  • Gender: Men have larger skeletons than women so their skeleton has a greater bone mass and hence greater mechanical strength. The pattern of bone loss differs between men and women, with women experiencing a phase of accelerated bone loss in the 10 years or so following the menopause.
  • Family history of osteoporosis: Genetic factors determine peak bone mass, and the timing and rate of bone loss in both sexes. It is thought that several susceptibility genes are involved. Maternal or paternal fragility fractures or kyphotic posture suggest the presence of a family history of osteoporosis.
  • Low body mass index (BMI): A BMI of ≤ 19 kg/m2 predisposes the individual to osteoporosis.
  • Hormones: Premature menopause (in women under the age of 45) - whether natural or induced by surgery, chemotherapy, radiotherapy, or endocrine therapy - increases risk. Risk is increased in men who have had orchidectomy or androgen deprivation therapy.
  • Medical conditions associated with bone loss: These include rheumatoid arthritis, inflammatory bowel disease (e.g. Crohn's disease, ulcerative colitis), malabsorption (e.g. coeliac disease, pancreatic insufficiency), cystic fibrosis, hyperthyroidism, hyperparathyroidism, vitamin D insufficiency, immobilisation (e.g. resulting from CVA or Parkinson's disease), chronic obstructive pulmonary disease, diabetes mellitus type 1 and chronic renal and hepatic disease.
  • Drug treatments associated with bone loss: These include oral corticosteroids, aromatase inhibitors, androgen deprivation therapy, some anti-epileptic medications and glitazones.
  • Lifestyle factors: Smoking and alcohol intake ≥ 3 units per day increase osteoporosis risk.