A recent study indicates that exercise is more important than calcium in developing strong bones in girls and young women.
Researchers at Penn State University and Johns Hopkins University found that even when girls took in far less calcium than the recommended daily allowance, bone strength was not significantly affected, said Tom Lloyd of 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.
No one is encouraging teens to stop drinking milk though. The researchers noted that the young women studied were still getting more calcium than many female American teens.
Lloyd said that in tests over 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 2½ percent increase in their bone strength," said Moira Petit, another Penn State researcher. Lean body mass is the mass of the body minus the fat.
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 study was published in the June issue of the Pennsylvania-based Journal of Pediatrics, a lesser known publication than the official journal of the Academy of Pediatrics in Chicago, which is simply called Pediatrics.
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.
Dr. Thomas P. Olenginski, who works with osteoporosis patients at Geisinger Medical Center in Danville, Pa., praised the study for its detailed look at bone strength, but warned that no one should ignore calcium entirely.
"There is a concern that kids might think, 'I can still drink nothing but sodas as long as I'm working out,"' Olenginski said. "It's the whole package that's still important."
Lloyd agreed. 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."
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."
By Dan Lewerenz