John Standard, a personal trainer at T Bar M Racquet Club, demonstrates a "leg curl" exercise, July 2, 2009 at the north Dallas, Texas gym, as a way to care for your bones through the ages.

DALLAS — Bones are the body's first lines of defense. They protect the brain, heart and lungs and anchor the muscles. They keep us mobile. And all they ask in return is our support to keep them strong: good nutrition, weight-bearing exercise, calcium and Vitamin D.

Bones' needs also vary with age. You accumulate an astonishing 90 percent of all your bone mass by the time you're 21, so it pays to bank that calcium in your early years. After that, it's all about maintenance — which is important if you don't want to be a senior citizen with broken hips or curved vertebrae, which can lead to hunched posture and difficulty breathing.

Here are tips on how to optimize bone health at the three major stages of life from doctors from Texas Health Presbyterian Hospital Dallas, Baylor Regional Medical Center at Plano and Texas Scottish Rite Hospital for Children:

Kids (birth to age 19)

This is when you need to think of drinking milk (or taking in other calcium-rich sources) as putting money in your bone bank as you strive to achieve your peak bone mass. Because once you hit adulthood, you are cruising on what you've accumulated in calcium and in Vitamin D, which helps you metabolize that calcium.

Healthy food intake, with lots of vitamins and minerals is key to maximizing your bone strength potential. Adolescent girls should be especially wary of fad diets and alert to the signs of anorexia and bulimia, as these can wreak havoc on bone health, said Dr. Karl Rathjen, a pediatric orthopedic surgeon at Texas Scottish Rite Hospital for Children.

Weight-bearing exercise of about 30 minutes most or all days of the week is important, but so is being careful to avoid major orthopedic injuries and overuses between childhood and adolescence, according to Dr. Amy Hayes, a pediatrician at Texas Health Presbyterian Hospital Dallas.

Adults (ages 20 to 70):

When young adults leave their parents' home, it's crucial that they don't leave good bone health habits behind. The time for building bone density has passed; now they must do their best to maintain and prevent bone loss.

Dr. Kim Allen, an internist at Texas Health Presbyterian Hospital Dallas, advises her adult patients to exercise at least 30 minutes five times a week, do weight-bearing exercise and take in at least two servings of calcium-rich foods, such as milk, yogurt, cheese and vegetables, daily.

It also helps the bones if you don't smoke, as that can inhibit the absorption of calcium and vitamin D. This is a particular issue for pregnant women because if they smoke, don't get enough calcium or vitamin D, or don't exercise, they can hurt their baby's bone-mineral acquisition in the womb, according to a 2008 MRC Epidemiology Resource Centre and Centre for Developmental Origins of Health and Adult Disease report.

Seniors (70+ years)

Seniors need to keep up the weight-bearing exercises and good nutrition they established as adults, increasing calcium intake to at least 1,200 mg.

It is particularly crucial that women have a bone density assessment after menopause, said Dr. Mitch Carroll, gerontologist and director of the Senior Health Clinic at Texas Health Presbyterian Hospital Dallas.

Men may need this assessment, too, but it's not always clear at what age they should get it. But men who have lost height or have been exposed to steroids should get them as soon as possible, he said.

The noninvasive bone scan, which feels like a slow-moving X-ray, is vital for women, Carroll said, because the loss of estrogen after menopause causes a thinning of the bones. That can lead to osteopenia, which can turn into osteoporosis, which can cause the bones to fracture spontaneously, leaving a bedridden patient subject to blood clots or pneumonia.

Early detection of bone loss can be mitigated by a variety of bone strengthening therapies to help prevent serious problems.

"Even if you followed all the advice for each stage of life, all the healthy lifestyle modifications may not work because of genetic predisposition," Dr. Carroll noted. "But fortunately there's still a lot we can do if they stay active, stay strong, get informed and take the medicines we tell them."

<B>HOW MUCH DO I NEED?</B>Normal daily recommended intakes in milligrams for calcium are:Person MilligramsInfants birth to 3 years 400 to 800Children 4 to 6 years 800Children 7 to 10 years 800Adolescent & adult males 800 to 1,200Adolescent & adult females 800 to 1,200Pregnant females 1,200 to 1,500Breast-feeding females 1,200 to 1,500Source: www.mayoclinic.com<B>BEYOND THE SUPPLEMENTS</B>You can also get calcium the old-fashioned way, from calcium-rich foods.<B>FOOD AND MILLIGRAMS</B>• Nonfat dry milk, reconstituted (1 cup) 375• Lowfat, skim, or whole milk (1 cup) 290 to 300• Yogurt (1 cup) 275 to 400• Sardines with bones (3 ounces) 370

John Standard, a personal trainer at T Bar M Racquet Club, demonstrates a "lat pull down" exercise, July 2, 2009 at the north Dallas, Texas gym, as a way to care for your bones through the ages.