Bones are constantly changing. Old bone is removed by specialized bone cells, called osteoclasts, and new bone is laid down in its place by specialized cells, called osteoblasts. This process is known as bone remodeling.

A number of factors influence bone remodeling:

• Age
• Availability of calcium
• Exercise
• Hormones

During childhood and adolescence, the body is producing new bone faster than it is removing old bone. As a result, the skeletal system grows and becomes stronger. This usually reaches a peak between the age of 20 and 30. With aging, bone breakdown exceeds the rate of new bone formation. At this point the bones can become weaker and more susceptible to breaking. The spine, hips and wrists are most often affected.

Causes of osteoporosis include:

• Lack of calcium
• Lack of vitamin D
• Reduction in estrogen levels after menopause
• Any condition causing physical inactivity
• Hyperparathyroidism
• Hyperthyroidism
• Malnutrition
• Rheumatoid arthritis

Osteoporosis is uncommon in children and adolescents.

The most common causes of osteoporosis in children and adolescents include:

• Premature birth
• Cystic fibrosis
• Inflammatory bowel disease:
• Crohn's disease
• Ulcerative colitis
• Celiac sprue
• Hypogonadism
• Delayed puberty in females:
• Due to intense athletic activity