Orthopedic surgeon Dr. Jan Grudziak likens the process of building bones strong enough to prevent osteoporosis to investing in a retirement fund. But in the case of bones, the fund is built from childhood on by eating calcium-rich foods and doing weight-bearing exercise.

His metaphor is particularly apt for women, who have lower peak bone mass than men, start to lose it much earlier and lose it at a slightly faster rate.

"The best picture is that it's an investment for the future," said Dr. Grudziak, of Children's Hospital of Pittsburgh. "With the retirement for bone, the age is 30 to 35. You have to drain the investments." Just like retirement funds, the earlier and bigger the investment, the more bone strength you have to lose.

For that reason, the U.S. Department of Health and Human Services' Office on Women's Health, the Centers for Disease Control and Prevention and the National Osteoporosis Foundation are cooperating on a campaign called "Powerful Bones, Powerful Girls" geared to 9- to 12-year-olds. Those so-called tween years, like those of teenagers, are critical times for building strong bones, as well as an age at which the good health habits encouraged by their parents and teachers are formed for life.

There's impetus for "Powerful Girls" in the 2004 Surgeon General's Report on Bone Health and Osteoporosis, which concluded that 10 million Americans over the age of 50 have osteoporosis and another 34 million are at risk. An estimated 1.5 million people suffer an osteoporosis-related bone fracture and, the report says, about 20 percent of those who break a hip will die within a year.

The report also estimates that by 2020, half of all Americans 50 and older will have or be at risk of developing osteoporosis, with women four times more likely to develop it than men.

Calculations for the bone health of tweens and teenagers aren't much better, but there's plenty of time to turn them around.

"It's been quoted that less than half [of the girls] get the calcium quantities they need," said registered dietitian Cindy Miller, who is part of the clinical nutrition staff of Children's Outpatient Nutrition Counseling Center. "For ages 9 to 12 it might be better. They're younger and parents might have a little more control over them than a teen who goes out to a restaurant and won't order milk. ... They say only 15 percent of teen girls get the required amount of calcium."

That amount is 1,300 milligrams, the high-calcium equivalent of four dairy servings. One serving is 8 ounces of milk or yogurt or 1¹/₂ ounces of hard cheese, Ms. Miller said. Other good sources include pudding; dark leafy vegetables, particularly collard and turnip greens and broccoli; calcium-fortified or calcium-set soy; dry beans; and calcium-fortified foods like orange juice, soy milk or rice milk.

But for the calcium to work at the optimal level, you have to get enough Vitamin D, too. "You need Vitamin D to process calcium," she said. "It helps the body absorb [calcium] better."

The daily amount needed is 200 IU, or international units. "You can get vitamin D from 15 or more minutes in the sun, but the problem is if you don't go outside or you're dark-skinned," Ms. Miller said. "Usually you need foods fortified with vitamin D."

Weight-bearing exercise also builds bone mass.

"Children as young as 7 and 8 can build stronger and denser bone," said Dr. Jill Landsbaugh, an exercise physiologist and wellness adviser with the Weight Management and Wellness Center at Children's Hospital.

"To do that they just need to do weight-bearing exercise, and they can work with resistance, too."

Weight-bearing exercises include walking, jogging, dancing, playing soccer or basketball or even simply skipping and jumping rope. Also good resistance exercises are pushups, squats, lunges and work with light weights or resistance bands, Dr. Landsbaugh added.

Two favorite exercises -- swimming and bicycling -- are not weight-bearing and therefore don't contribute to increased bone density, she said.

"[Calcium] is a product stimulated by changing the pressure and stress within the bone. Gravity actually gives you the stimulus to grow the bone mass," Dr. Grudziak said.

"If you live three or four months without gravity you lose 20 to 30 percent of bone mass."

While the National Bone Health Campaign is targeting 9- to 12-year-olds, the advice offered above is good for all ages.

"The strongest bones are when you're between the ages of 20 and 30," Dr. Landsbaugh said. "That's when you have the highest bone mineral density. ... After you're 20 or 30 it starts to decline, and after menopause it can decline really quickly."

But that decline in bone density can be slowed if you continue with a healthy diet and weight-bearing exercise.

"You are destined to lose it; you can only modify the pace at which you lose," Dr. Grudziak said. "For adults that means 30 minutes four or five times a week of exercise ... [and] absolutely no smoking and no excessive alcohol. Smoking is linked to decreased bone mass."

Another caveat: "You can overdose on calcium and vitamin D," he said. "If your intake of calcium is too high, it can lead to kidney stones."

Ms. Miller also cautions against taking calcium supplements totaling more than 500 milligrams at a time.

"It's best to take them at different times of the day because the body can only absorb about 500 milligrams at a time."