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Osteoporosis -- or "porous bone" -- is a serious disease of the skeleton in which the bone becomes thin, brittle, fragile, and prone to fracture. The disease affects the entire skeleton to varying degrees. Older women are especially susceptible due to the reduced estrogen levels that follow menopause, but many men also develop osteoporosis.

This loss of bone volume and density can affect all racial, ethnic, and gender groups. In the U.S., an estimated 10 million people age 50 and older have osteoporosis, and about 2 million of these are men. The recommendation is that all women at age 65 and all men at age 70 get screened for this debilitating condition.

Osteoporosis has been dubbed "the silent thief" because it quietly whittles away bone tissue, often with no obvious symptoms – until a sudden fracture occurs. With bone strength diminished, fractures can result from such low-stress activities as bending over, coughing, or opening a window.

Apart from pain and reduced strength and mobility, fractures due to osteoporosis can require hospitalization, multiple surgeries, and sometimes nursing home care during recovery. Major surgery such as for hip fracture repair can increase the risk of death, especially in older people.

Biology of Bone Turnover

The process determining bone volume and density is a complicated one. It involves the interplay of many contributing factors – your individual genes, your age, your sex, your hormone levels, and family history, as well as your diet, physical activity patterns, and lifestyle. Even some of the medications you take for completely different medical conditions, as well as the conditions themselves, can promote osteoporosis.

Far from being a static and unmovable scaffold, your skeleton is a dynamic organ that is continually renewing itself. The main players orchestrating this tissue turnover are three types of bone cells:

• Osteoblasts, which make new bone

• Osteoclasts, which peck away at old worn-out bone in need of replacement, thereby making room for the deposit of new bone

• Osteocytes, long-living bone cells that make up about 90% of bone cells (whereas osteoblasts and osteoclasts make up less than 5%) and appear to be the major "orchestrator" of many of the functions of bone

When all is working well, osteoclasts and osteoblasts have a balanced partnership, in which the breakdown of old bone tissue is well matched by the formation of new replacement bone.

But as women and men age, an imbalance develops, causing more bone to be broken down than replaced. Older bodies are less able to absorb calcium, and have a reduction in the sex hormones that promote bone building. Older people also tend to have more fat cells in the bone marrow where the bone-building cells are made, and these fatty cells can crowd out the cells that turn into bone. Such changes all work to create a weaker and more brittle skeleton that becomes increasingly vulnerable to breaks.

Warning Symptoms

Often the first symptom of osteoporosis is a fracture, but sometimes the following problems provide an earlier clue:

• Unexplained back pain, which could result from osteoporosis-weakened or collapsed vertebrae in the spine

• Noticeable shrinking from your adult height

• Stooped posture – in extreme cases, the so-called "dowager's hump." In this condition an osteoporotic compression fracture in the spine results in a forward curve, which causes the head to drop forward, and in order for the eyes to see forward, the head has to be drawn back and upward

Risk Factors for Osteoporosis

Although some risk factors can be changed, others cannot.

Non-Changeable Risk Factors

• Age, sex, race, and time of menopause

• Race: Whites and Asians are at greater risk than Blacks

• Build: A naturally small frame with thinner bones predisposes individuals to osteoporosis, so women are naturally more at risk for low bone density than men are

• Genes and family medical history: Do you have relatives who have been diagnosed with osteoporosis or have a history of easy fracture?

• Failure to reach peak bone mass: Not maximizing your bone density by your teens or early adulthood can leave you vulnerable later

• Ovariectomy: Removal of the estrogen-producing ovaries, especially before age 40. The reproductive hormone estrogen promotes the activity of osteoblasts, and estrogen levels drop dramatically after surgical removal of the ovaries and after menopause. Sometimes estrogen replacement is prescribed, which protects the bones from thinning

• Other medical conditions: Kidney, liver, thyroid, celiac, or blood disorders, among others, can accelerate breakdown of the skeleton. Surgery for weight loss can result in bone loss and mineral imbalances that weaken bones

• Common drugs needed for other medical conditions: These include glucocorticoids and other anti-inflammatory agents for asthma, eczema, rheumatoid arthritis, and Crohn's disease, for example. Other culprits are anti-seizure and diabetes medications and anti-hormonal treatments for prostate and breast cancer

Changeable Risk Factors

• Diet: A diet low in bone-building calcium and vitamin D is risky. Eating disorders such as anorexia and bulimia that interfere with proper nutrition and suppress ovulation and the production of sex hormones can prevent you from achieving your protective maximum bone density

• Sunlight: Inadequate exposure to sunlight, which promotes the production of bone-building vitamin D

• Smoking and excess alcohol consumption are both associated with bone loss. The evidence for high caffeine consumption from coffee, tea, and cola is unclear, but caffeine may increase the amount of calcium excreted by the kidney

• Lack of physical activity: A sedentary lifestyle with low levels of weight-bearing, bone-building activity poses a risk. Injury-related immobility can promote bone loss and may require special physiotherapy

If you have a family history of fracture or osteoporosis, consult your doctor. And if you have been under treatment for a chronic medical condition, ask your doctor if the condition itself or the drugs you received for it are among those that promote bone loss. If yes, ask for a bone density test.

There are things you yourself can do to reduce your risk, and there are very effective treatments to reduce the risk of fractures.

MedPage Today's "Medical Journeys" is a set of clinical resources reviewed by doctors, meant for physicians and other healthcare professionals as well as the patients they serve. Each episode of this 12-part journey through a disease state contains both a physician guide and a downloadable/printable patient resource. "Medical Journeys" chart a path each step of the way for physicians and patients and provide continual resources and support, as the caregiver team navigates the course of a disease.