Exercise, not calcium, may have the strongest effect on growing bones in adolescent girls, researchers report.

In a decade-long study, calcium intake was not related to bone growth or bone strength in young women. But physical exercise was related to stronger bones, according to a report in the June issue of the Journal of Pediatrics.

“Exercise is the most important modifiable determinant of bone development for young women,” Dr. Tom Lloyd of Penn State University College of Medicine in Hershey told Reuters Health. “At least 15 percent of the variation in bone strength seen among healthy women can be attributed to exercise.”

Exercise is probably the most important influence on bone development in young men, too, Lloyd said, but the study included only young women.

Despite the apparent importance of exercise, teens should not skimp on calcium-rich foods, according to Lloyd.

“I believe that parents should continue to encourage teens to consume low-fat dairy products since they are convenient and are wonderful sources of macronutrients, minerals and vitamins,” Lloyd said.

The study included 80 young white women who were followed for 10 years. When the study began, the girls were around 12 years old, and none had begun menstruating. One of the study’s goals was to measure the effects of calcium intake, exercise and oral contraceptives on bone health.

“This study is unique in that the participants have been followed for over a decade, with measurements made every 6 months for the first 4 years and yearly thereafter,” Lloyd said.

What the study found was that calcium intake, which ranged from 500 milligrams (mg) to 1,900 mg per day, was not significantly associated with bone growth or bone density.

“We have the most comprehensive dataset of calcium intake throughout adolescence of teen women extant and are therefore confident about noting that over the calcium intake ranges of our cohort, there was no significant relationship to bone development,” Lloyd said.

The study also showed that the use of oral contraceptives was not related to bone health. Despite some research that has found that women on oral contraceptives have thinner bones, young women who had taken the Pill had just as healthy bones as those who had not taken oral contraceptives.

But exercise was related to bone health. The more a teen girl participated in sports, the stronger were her bones, according to the report. The results of the study support other research showing that exercise during the teen years can have a significant effect on bones.

“Regular load-bearing exercise, like walking 30 minutes a day, has a positive effect on bone development,” Lloyd said.

Although the brittle-bone disease osteoporosis does not usually strike until late in life, the teen years are an essential time for bone development. In fact, girls form 40 percent of their bone mass during adolescence.

SOURCE: Journal of Pediatrics, June 2004.