Q: I'm 68 years old. I used to be 5-foot-6-inches tall. I just visited my doctor and my height was just over 5-foot-4. Will I continue to shrink? What can I do to help stop it?

A: Starting at about age 40, people typically lose about half an inch each decade. And the decline usually speeds up after the 70th birthday. So now is a good time for you to be asking this question.

The two main causes of height loss are osteoporosis and bad posture.

As we get older, our bones become less dense and more brittle, and they're more likely to fracture because of osteoporosis.

When a hip or wrist bone weakened by osteoporosis breaks, it's usually the kind of crack we have in mind when we picture a broken bone. But fractures of the bones that make up our spine (vertebrae) are different. Osteoporotic vertebrae get easily crushed. It's like a cardboard box that has had too much weight put on it.

These compression fractures often occur without any known trauma or injury. And usually there are no symptoms. Sometimes there can be a more sudden vertebral collapse. This can be very painful.

All types of vertebral compression fractures result in loss of height. You can decrease the risk of losing more height the same way you prevent or treat osteoporosis. Eat a calcium rich diet and be sure to get enough vitamin D. I recommend 1,000 IU of vitamin D3 daily.

Healthy bones require daily exercise, such as walking and/or weight training. Putting "pressure" stimulates new bone growth and prevents bone loss.

If you haven't had a bone density test yet, it's time to get one. You may need drug therapy for osteoporosis.

Bad posture may be even more of a factor in height loss associated with age. Back muscles, like all muscles, get weaker with age. Ligaments get stretched and muscles get trained to be in a bent-over position. Flabby abdominal muscles don't help.

Some simple "core" exercises can help you stand up straighter. For example, lie on your stomach and then lift your head and shoulders for several seconds. You can either keep your arms by your side or stretch them out like "Superman." Let your head down and then repeat the lift.

Tighter "abs" can help your posture by counteracting tired back muscles. Modified sit-ups (crunches) are a good way to start. Lie flat on your back with knees bent. Support your head with your fingers and raise your head six inches off the floor, concentrating on pushing your lower spine down and tightening the abdominal muscles. Hold and repeat.

(Howard LeWine, M.D., is a practicing internist at Brigham and Women's Hospital, Boston, Mass., and Chief Medical Editor of Internet Publishing at Harvard Health Publications, Harvard Medical School.)

(For additional consumer health information, please visit http://www.health.harvard.edu.)