WOMEN'S HEALTH Study: Bones need exercise

Bones, like muscles, must be used, said one expert.
STATE COLLEGE, Pa. (AP) -- Got exercise?
A study published in the latest issue of Journal of Pediatrics indicates that exercise is more important than calcium intake in developing strong bones in girls and young women.
Researchers at Penn State University and Johns Hopkins University found that even among girls whose calcium intake was far below the recommended daily allowance, calcium did not significantly affect bone strength, said Tom Lloyd, professor of health evaluation sciences at Penn State's College of Medicine at the Milton S. Hershey Medical Center.
However, when the girls were asked about their exercise habits, a strong correlation was found between exercise and bone strength.
Lloyd said in tests conducted over the course of a decade, 17 percent of bone strength could be attributed to exercise habits. What's more, girls with better muscle development also had stronger bones.
"When we looked at their lean mass, what we saw was that a 1 kilogram increase in lean mass was associated with a 2 1/2 percent increase in their bone strength," said Moira Petit, assistant professor of health evaluation sciences at Penn State. Lean body mass is the mass of the body minus the fat.
About the study
The Penn State Young Women's Health Study began in 1990 with 112 12-year-old girls from central Pennsylvania. The ongoing study has tracked the cardiovascular, reproductive and bone health of the subjects, now in their mid-20s. The paper in the Journal of Pediatrics used data generated between 1990 and 2000.
Studies have shown that women build most of their bone mass in their early and mid-teens. That bone mass then slowly erodes as women age. Building good bone mass in adolescence, then, is thought to be the best way to prevent osteoporosis in old age.
In the Penn State study, calcium intake among the girls varied widely -- from 500 milligrams per day to 1,800 milligrams per day. Most recommendations call for teenage girls to consume about 1,300 milligrams per day.
But even at the lower levels, Lloyd said, calcium intake seemed to have little effect on bone strength.
"Now a caveat to this study ... was that the lowest intake in our population was about 500 milligrams per day. Twenty-five percent of teen women in the United States get less than 500 milligrams per day," Lloyd said. "For those kids, having additional calcium to bring them up to at least 500 milligrams per day may be an important determinant of bone health."
More important
But above that baseline, Lloyd said, exercise appeared to be the most important factor in developing bone strength.
John Patnott, professor of kinesiology at Hope College in Holland, Mich., said he was not surprised by the findings.
Bones "are very similar to muscles -- you have to use them to develop strength," Patnott said. "I think that calcium in the diet is very important, ... but calcium by itself won't accomplish what is necessary without bone stress."
The Journal of Pediatrics is published in Pennsylvania, and is not the official journal of the Academy of Pediatrics in Chicago. That journal is called Pediatrics.