Exercise Has Its Limits in Fighting Osteoporosis : Health: Workouts can restore up to 2% of bone mass in postmenopausal women in year’s time, researchers find. Estrogen therapy can restore twice that much, and combining it with workouts can boost benefits.

Exercise builds stronger bones, but it’s a poor substitute for estrogen therapy in an older woman’s fight against osteoporosis, an expert says.

Studies have found that postmenopausal women, who are at greatest risk for the brittle-bone disease, can restore no more than 1% to 2% of bone mass in a year’s exercise program, said Barbara L. Drinkwater of Pacific Medical Center in Seattle. Estrogen therapy, on the other hand, can restore 3% to 4%, Drinkwater said.

Exercise is still good for women’s bones, but it’s best as part of an osteoporosis therapy that includes estrogen, Drinkwater said.

This idea runs counter to the belief held by many women that exercise plus extra calcium can substitute for estrogen in spurring the body to replace bone, Drinkwater said.

The reason can be found in the way bones grow and shrink, Drinkwater said. Bones are honeycomb-like structures made of walls of calcium. And bones constantly gain and lose calcium in a process called remodeling. In the young, gains are greater than losses, and bones grow. In the old, the losses are greater, and the holes in the honeycomb grow larger.

Exercise stresses bones, which respond to the load by adding calcium to the part of the skeleton that gets the stress. Estrogen, however, works by a different process, slowing the loss of calcium, which can swing the mineral balance toward a net gain.

But going from no exercise to exercise can magnify the effect of the gain, Drinkwater said. Women who don’t exercise may lose 1% to 2% of their bone mass per year, so starting workouts, by reversing the loss, creates an advantage of 3% to 4%, she said.

Research also indicates that some women benefit more than others; some show virtually no improvement while others show 8% to 10% gain in bone density, Drinkwater said.

But women should benefit from exercise in ways beyond bone strength, Drinkwater said: They should retain endurance, strength, coordination and balance, which can help them to stay active even into their 90s.

And exercise seems to have a special advantage for girls who are starting or going through adolescence, Drinkwater said. Research indicates that exercise can help the bones grow, so they become not just denser--with smaller spaces in the honeycombs--but bigger overall, she said.

This would give these early-starting athletes a larger bank of bone material from which to draw as they age, she said.

That’s one reason why experts think girls should work out. But they warn that teen-age girls and young women should not work so hard--and eat so little--that they disrupt or stop their monthly hormonal cycles. Some elite athletes, dancers and models do this to keep their weight low.

But Drinkwater and Dr. Michelle P. Warren of St. Luke’s-Roosevelt Hospital Center in New York say that this drops estrogen levels so low that bone density shrinks, as it does in postmenopausal women.

Estrogen therapy may not offset what bad diet and exercise habits take away, Warren reported at an Atlanta sports science meeting sponsored by the International Olympic Commission.

The study was of women who were not having their periods. The women got two years of estrogen therapy at dosage levels given to postmenopausal women, but they did not build bone mineral density.