At that continued rate, the researchers project, the girls who supplementtheir calcium could have 6 percent greater bone density by age 18, thereby reducing the risk of fractures in old age.
"What we've found is that the best time to concentrate on calcium intake and skeletal health is when people are younger, not older," said Tom Lloyd, professor of obstetrics, gynecology and pharmacology at Penn State's College of Medicine and head of the research team.
"The most cost-effective treatment for osteoporosis is prevention," he said. He said that medical costs connected with it now ran about $12 billion a year.
The teenage years, Lloyd said, are particularly critical because the hormones released during puberty increase the rate at which bones are built. About 95 percent of the skeleton's maximum strength is achieved between the ages of 2 and 18, he said.
The 18-month study, funded by the National Institutes of Health, is the first long-term look at bone building during adolescence, Lloyd said.
For the study, half of a group of 94 girls received an average of 350 milligrams of extra calcium (equal to one calcium-rich food serving) per day, bringing their average daily intake to about 1,400 milligrams.
The other half received placebos, but their dietary calcium intake was close to the 1,200-milligram recommended dietary allowance (RDA) for this age level. Quantitative skeletal measurements were made with a bone densitometer.
According to the researchers, one extra calcium serving per day is especially important for the majority of youth - 87 percent of teenage girls and 70 percent of teenage boys - who fall short of meeting even the RDA.
The supplement used in the study was calcium citrate malate, a soluble and readily absorbed form of calcium used to fortify some fruit juice drinks.