The ABC's of the RDA
Found in dairy products like milk, cheese, and yogurt, calcium is key to building healthy teeth and bones and averting osteoporosis, the debilitating bone-weakening disease that afflicts five to six million American women age 50 and older.
Scientists now know that girls acquire 90 to 99 percent of their bone mass by age 16, says Kimberly O'Brien, assistant professor of nutrition at Hopkins's School of Public Health. The rate at which calcium is deposited into bone peaks in young girls just before the onset of first menstruation, around age 12. But even adults need to continue getting calcium because everyone loses it in their wastes, urine, and sweat. When diets are calcium deficient, the body leeches the mineral from bone. As people age, they again need increasing amounts of calcium because absorption is less efficient.
In response to such findings, the National Academy of Science's Food and Nutrition Board recently revised the daily intake levels for calcium, significantly increasing these amounts. For example, women ages 25 and older should get 1,000 milligrams of calcium per day, compared to 800 mg/day as stated previously. (An eight-ounce glass of milk has about 300 milligrams.) For women age 51 and older, the adequate daily intake is 1,200 mg. Boys and girls, ages nine through 13, should have 1,300 mg/day of calcium.
What's New at Hopkins:
O'Brien is looking at the metabolic process of bone building, in an effort to maximize the process. Using stable, non-radioactive isotopes of calcium to trace its pathway, O'Brien recently examined what would happen when teenage girls whose mothers or grandmothers had osteoporosis ate a low-calcium diet for 10 days followed by a high-calcium diet for 10 days. Even though their high-calcium diet had more than the adequate level of calcium, these girls lost bone calcium at a much higher rate than did girls with no family history of osteoporosis.
In related work, O'Brien is tracking bone loss in a very different population: astronauts. They lose bone at a rate of 1 to 1.5 percent per month during space flight, she says. "Gravity helps you maintain bone, although it's not fully known why." Hormonal changes during space flight and a lack of sunlight may play a role, she says. (Ultraviolet light enables the body to synthesize vitamin D, which the body requires to absorb calcium.) Through a grant from NASA, O'Brien is tracking bone loss in astronauts and Mir cosmonauts, and is searching for ways to recoup that loss--a key issue for planners of the space station and other long-duration flights.
What We Can Do:
Parents should be sure that their children, especially girls whose families have a history of osteoporosis, take in enough calcium to maximize bone mass. Menopausal women should talk to their physicians about options for maintaining their existing bone mass. Dairy products are the best sources of calcium. Others include calcium-fortified orange juice, tofu, and broccoli. -- MH
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