Most Americans don't consume enough calcium to protect themselves from the bone-weakening disease osteoporosis, the National Academy of Science warned last summer when it increased the recommended intake of the mineral from 1,000 to 1,300 milligrams per day.
But calcium consumption is just part of the picture for preventing and treating this debilitating disease, which affects more than 28 million Americans, 80% of them women. Exercise also is essential for building bone in youth and preventing bone loss with age.
"You can eat lots of protein and not gain any muscle unless you strength-train," notes Tufts University physiologist Miriam Nelson. Similarly, "exercise will kick-start the calcium to make bones stronger and denser."
Vitamin D also is important because it aids the absorption of calcium, notes Nelson, who says good sources of vitamin D are fortified dairy products and 15 minutes of exposure to sunlight daily. However, the skin can't manufacture vitamin D in people wearing sunscreen or during winter months in latitudes above 42 degrees north, such as Boston or Milwaukee.
Two kinds of exercise are recommended for bone health: Weight-bearing activities are those in which you work your bones and muscles against gravity. Walking, stair climbing, dancing, running and racket sports are all weight-bearing activities with differing degrees of impact. In general, the higher the impact, the more the activity strengthens bones. This is why the bones in the racket arm of tennis players are denser than the bones in the other arm.
Resistance exercise, such as working out with free weights or weight machines, is a particularly effective bone strengthener. Nelson's study, published in the Journal of the American Medical Assn., showed that post-menopausal women who performed two 40-minute strength-training sessions each week for a year gained 1% in bone density, while women in a sedentary control group lost 2% to 2.5%.
"People who do weight-bearing activities have bone density that is up to 10% higher than people who don't exercise," notes Gail Dalsky, an assistant professor of medicine at the University of Connecticut Health Center. "People who do resistance exercise have bone density up to 30% higher."
If your goal is bone health, Dalsky says, "it's important to use a heavy weight that you can only lift between eight to 10 times. Doing lots of repetitions with a very light weight won't do much for your bones."
The exercise prescription for optimum bone health may differ depending on your life stage, says Felicia Cosman, a New York endocrinologist who serves as clinical director of the national Osteoporosis Foundation.
She offers these recommendations:
* Youth through age 30: Bone is built during this period, with peak bone mass achieved between ages 20 and 30. To help youngsters reach their full growth potential, they should get 30 to 60 minutes of physical activity most days of the week. For optimum bone health, choose sports that involve jumping and running, such as basketball, volleyball and soccer.
Activities that promote a "thin aesthetic," such as gymnastics and dance, also can be beneficial, except when a female athlete's menstrual periods become irregular. Unhealthy weight control practices used by some girls in these sports can lead to irregular periods that put them at increased risk of osteoporosis early in life.
* 30s and 40s: Bone loss may begin to occur in some parts of the skeleton at a rate of up to 1% per year. To help maintain bone, do at least 30 minutes of weight-bearing exercise three to five days a week and about 30 minutes of resistance exercise two to three days a week.
* Menopause (around ages 45 to 55) and up to eight years beyond: Bone loss is most rapid at this time, with women losing up to 20% of their bone mass during this time. Hormone replacement therapy or other "anti-resorptive" medications are particularly effective at this time in protecting against bone loss. To help make the decision about medication, women with osteoporosis risk factors (such as family history) should get a bone density test at menopause. While exercise is not a substitute for medications at this time, it is still helpful to continue with weight-bearing and resistance exercises.
* Age 60 and above: Men and women tend to lose bone mass at a similar rate during this period. People who have been sedentary should start exercising gradually with a walking program, working up to walking 30 minutes or more, most days of the week. Twice-weekly sessions of resistance exercise also may be beneficial.
In this age group, the combination of calcium and vitamin D is particularly effective at preventing bone loss, which is one reason the National Academy of Sciences just tripled the recommended intake of vitamin D for people over age 70 to 600 international units (IU) per day. The academy's calcium recommendation is 1,200 milligrams a day, while the Osteoporosis Foundation's is even higher: 1,500 milligrams daily. (One 8-ounce glass of milk provides about 300 milligrams of calcium and 100 IU of vitamin D.)
People with low bone density in the spine--which can be indicated by height loss or "Dowager's hump"--should avoid activities that involve bending forward and twisting, such as bowling or golf, says University of Connecticut's Dalsky. "These activities can put undue stress on the vertebrae," she says, "and literally be the straw that breaks their back."
* Fitness runs Monday in Health.