Bone and Calcium Metabolism: Prevention of Osteoporosis

Bone metabolism in the human body is a very dynamic process.  There is a constant lying down of bone by cells called osteoblasts and resorbtion of bone by cells called osteoclasts.  Initially the osteoblasts work much harder and faster that the osteoclasts and bone is laid down to facilitate growth.

Each person has a genetically determined peak bone mass which is attained in their twenties.  By age 17 ninety percent of the bone mass has been obtained.  The consequence of bone loss is the development of very weak and fragile bones, a medical condition known as osteoporosis.  Osteoporosis is associated with compression fracture of the bones of the spine called vertebra(ae) and fracture of the hip.  While these fractures from osteoporosis may be painful and cause shortening of stature, even difficulty breathing, the major consequence is complications and even death from surgical repair of the hip fracture.

Calcium and Vitamin D Play a Critical Role

Adequate calcium, exercise and vitamin D are required to attain and maintain bone mass. This is important in the prevention of osteoporosis.  Current data indicates that the majority of children do not receive adequate amounts of calcium or vitamin D.
Recommended amount of calcium vary for individuals.

Below is a table of adequate intakes as outlined by the National Academy of Science.

Recommended Calcium Intakes

Ages    Amount mg/day
Birth–6 months    210
6 months–1 year    270
1–3    500
4–8    800
9–13    1300
14–18    1300
19–30    1000
31–50    1000
51–70    1200
70 or older    1200
Pregnant & Lactating    1000
14–18    1300
19–50    1000

Role of Vitamin D

Vitamin D also plays an important role in healthy bone development. Vitamin D helps in the absorption of calcium (this is why milk is fortified with vitamin D).  In the past many people depended on the sun’s effect on the skin to make vitamin D.  Since more emphasis has been place on sunscreen, there is less sun effect and a greater need for vitamin D supplement.  The exact amount of vitamin D required is being debated, but the consensus is that Americans are generally deficient in vitamin D and require greater amounts than originally thought.  800 international units to 1000 international units is the current recommendation for adults.

Certain medical conditions may limit the amount of calcium that may be ingested.  As always, consulting your physician is suggested regarding your personal specific calcium needs.

Exercise is Necessary to Maintain Bone Health

Weight bearing exercise (walking, running, etc.) is an excellent stimulus for bone growth.  Current recommendations are:

Adults: Engage in at least 30 minutes of moderate physical activity [on] most, preferably all, days of the week

Children: Engage in at least 60 minutes of moderate physical activity [on] most, preferably all, days of the week

Bone Mineral Density

The evaluation of bone strength, the about of calcium in the bone, is measured by a dual density densitometer.  The purpose of this is to measure the exact bone mineral density of the spinal vertebrae, usually the four lumbar (lower back) vertebrae, and the femur, the large thigh bone that connects to the pelvis in the hip socket.  

The bone mineral density (BMD) is usually expressed as a “T” score which is a comparison of the patients BMD to a young person.  A “T” score that is - 1 or higher is considered normal.  A “T” score of - 2.5 or lower is considered osteoporosis.  A “T” score between – 1 and – 2.5 is called osteopenia – a softening of the bone.
Bone mineral density testing should be recommended to all postmenopausal women aged 65 years or older.  Bone mineral density testing may be recommended to postmenopausal women younger than 65 years who have 1 or more risk factors for osteoporosis (See Below). Bone mineral density testing should be performed on all postmenopausal women with fractures to confirm the diagnosis of osteoporosis and determine disease severity.

Risk Factors for Osteoporotic Fracture in Postmenopausal Women

•    History of prior fracture
•    Family history of osteoporosis
•    Caucasian race
•    Dementia
•    Poor nutrition
•    Smoking
•    Low weight and body mass index
•    Estrogen deficiency*
o       Early menopause (age younger than 45 years) or bilateral oophorectomy
o       Prolonged premenopausal amenorrhea (>1 year)
•    Long-term low calcium intake
•    Alcoholism
•    Impaired eyesight despite adequate correction
•    History of falls
•    Inadequate physical activity