Can you slow down bone thinning, even if it's already started? Yes -- with weight-bearing exercise, the kind that puts modest stress on your bones. Though you can't replace lost bone, you can help preserve the bone health and bone density you already have.

You reach your peak bone mass in your early 20s, and it's natural to lose a little bone density as you age. But the brittle, porous bones caused by osteoporosis aren't natural at all. Neither are fractures caused by brittle bones. One factor that puts us at risk is lack of exercise.

Why Does Weight-Bearing Exercise Work?

Bones -- like muscle -- constantly adapt to the mechanical stress you put on them. In fact, it's the muscle action pulling on bone that stimulates new bone to grow.

"Just like any muscle, if you want to build bone you have to stress it," says Paul Mystkowski, MD, an endocrinologist at Virginia Mason Medical Center in Seattle and clinical faculty member of the University of Washington in Seattle. "Not to the point where you break it, but enough where it rebuilds and comes back stronger."

With weight-bearing exercise, you're carrying your body weight on your hips and legs, as you do when you're walking, running -- or doing a heart-thumping tango. Do the tango often enough -- or any other weight-bearing exercise -- and you can boost your bone density.

But with this caveat: Exercise only improves bone mass in the specific bones that you -- literally -- impact. So hiking can build bone health in your hips, a vulnerable site for fracture as you age, but won't do much for your arms or wrists. Since the big bones of your hips bear the stress of carrying your weight, that's where bone density is likely to make strides.

On the other hand, it's upper-body work that shapes up bone health in your arms. Case in point: most people have a 3% to 5% difference in bone mineral content between their dominant and non-dominant arm, just from everyday use. But in a study of women tennis and squash players in Finland, their dominant arm was a whopping 12% to 16% more mineral-rich than the other, from all that training.

Since your wrist is a frequent fracture site from osteoporosis, maybe it's time to dig that old racket out of the closet.

Weight-Bearing Exercise and Your Fracture Risk

Since you reached your peak bone mass back in your teenage years, the bone-building horse has pretty much left the barn after our 20s. So the name of the game is to preserve bone health as you age.

The good news? Many studies on exercise show a gain -- albeit modest -- in bone mineral density for adults of all ages. Bone mineral density (BMD) is also the "gold standard" test doctors use for measuring your fracture risk.

"Because we don't have a mechanism to stress the bone without breaking it to determine fracture risk," says Mystkowski, "we use bone mineral density as an indirect measure. In general, we say that improvements in bone mineral density improve fracture risk."

The Latest Weight-Bearing Workout Trends

What are the best ways to exercise and improve your bone health -- even when you have osteoporosis? Try weight-bearing workouts that stress bones and muscles more than your everyday life. Talk to your doctor and make sure the workout you choose is safe for you. Then give these latest trends a try!

1. Tai Chi

Tai chi -- a form of slow, graceful moves -- builds both coordination and strong bones. A study reported in Physician and Sportsmedicine found that tai chi could slow bone loss in postmenopausal women. The women, who did 45 minutes of tai chi a day, five days a week for a year, enjoyed a rate of bone loss up to three-and-a-half times slower than the non-tai-chi group. Their bone health gains showed up on bone mineral density tests.

2. Yoga

A study reported in Yoga Journal found an increase in bone mineral density in the spine for women who did yoga regularly. From the slow, precise Iyengar style to the athletic, vigorous ashtanga, yoga can build bone health in your hips, spine, and wrists -- the bones most vulnerable to fracture.

Standing poses like Warrior I and II work the large bones of the hips and legs, while poses like Downward Dog work the wrists, arms, and shoulders. Both the Cobra and Locust poses, which work the back muscles, may preserve the health of the spine. Yoga also sharpens your balance, coordination, concentration, and body awareness -- and thus helps prevent falls.

3. Brisk Walking

One fitness trend that never goes away, walking is still hugely popular among women -- and a great way to revamp your bone health. A study of nurses found that walking four hours a week gave them a 41% lower risk of hip fractures, compared to walking less than an hour a week. Brisk walking is best, but you can adapt your speed to your current fitness level. Walking is free, and you can do it anywhere, anytime, even when you're traveling.

4. Golf

Maybe you've always thought golf was for old folks -- people who could no longer do "real" sports. Think again. Shouldering that golf bag around 18 holes, and swinging the big clubs to drive the ball long, adds up to a lot of upper-body work. And all that walking, and chasing balls lost in the rough, means plenty of work for your hips and spine. Golf gives "weight-bearing exercise" a whole new name.

5. Dancing

OK, maybe you've got two left feet, or you were never the star in ballet. But we're not talking point shoes here; we're talking the hottest trends in salsa, samba, Lindy hop, rhumba, East coast swing, foxtrot, and tango. Use those hips to get your heart pumping in more ways than one, and build strong bones while you're at it.

Or try the newest aerobics, kickboxing, or step class at your health club or local Y. New classes emerge every few months to keep members motivated. A lot of them now combine strength training with dance or step moves -- and will perk up your balance as well.

6. Hiking

The work of weight-bearing -- and the impact as your feet hit the ground -- can increase bone density, especially in your hips. It's just like walking, and then some. You'll get even more impact on those bones if you're going uphill or downhill, and that can improve bone health even more. More impact on your feet and legs translates into more bone density, says the surgeon general.

And with hiking, boredom is rarely an issue. You're often socializing and meeting new people, as well as expanding your horizons as you see new landscapes.

7. Racquet Sports

Tennis, squash, and paddle tennis can rally your bone density. You're stressing your racquet arm, wrist, and shoulder every time you hit the ball, and working your hips and spine with all that running around -- and chasing down wild balls.

If you're going for racquet sports, go for singles. You'll get a lot more from your workout in terms of bone health, since you'll be running around more.

8. Strength Training

Lifting weights, using the weight machines at your health club, or doing calisthenics, are forms of strength or resistance training. You're working against some form of resistance -- whether it's a set of "free" weights, your own body weight, or weight machines -- to stress a sequence of muscles and bones. Strength training at least twice a week, says the surgeon general, is needed to stimulate bone growth.

Every gym has a trainer who can design a workout for your legs, back, shoulders, and arms -- one that's right for your fitness level and can rally your bone health.

Thin Bones Beware

Take a few exercise precautions if you already have bone thinning:

• Because your fracture risk is higher than normal, be cautious about trying any exercise with the potential for serious falls, like downhill skiing, ice skating, or roller-blading.
• If you have bone thinning in your spine, you may want to forgo any deep backbends in yoga.
• Again, check with your doctor before starting any new exercise program, especially if you're taking medications that slow your coordination or throw off your balance.

One final tip: Be patient. The bone-building phase in young adults -- at its speediest -- takes three to four months, and it may take a lot longer if you have osteoporosis or are older. So you won't be seeing big changes on any bone density tests after your first week of working out. Bones change slowly -- but they do change.