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Release: Oct. 16, 2001

UI study shows kids need more activity for healthy bones

IOWA CITY, Iowa -- Anyone who has ever watched five-year-olds zipping around a playground will be shocked to learn that even this most active segment of the population isn't getting enough of the right activity to develop strong bones.

That is the result of a study by a multi-disciplinary team at the University of Iowa, led by Kathleen Janz, associate professor of health, leisure, and sport studies in the College of Liberal Arts and Sciences. The group studied 470 children between ages four and six and monitored the amount and intensity of their activity with a device worn at the hip that recorded minute-by-minute movement information for four days.

When the study began, researchers measured the children's bone density and bone mineral content in order to compare it with activity measures. The children who were most active, especially those doing vigorous activity, were much more likely to have more bone mineral content and denser bones than children who did little vigorous activity. Janz and her team theorize that many children are not involved in the right kinds of activity to promote the development of stronger bones.

"Kids need to be jumping, tumbling, hopping, or otherwise engaged in activities that strain the bone with impact," Janz said. "That's what causes the bone to grow denser, just as working muscles by lifting heavy loads in strength training enhances muscle development."

The amount of activity is not as important as the type, she said. The research suggests that kids can increase their bone density by up to 3 percent with just 15 additional minutes of the right types of activity per day.

Janz said it is crucial for children to start early in developing strong, dense bones because there is a limited window for bone development. In most cases, bone growth peaks during late adolescence or early adulthood, so by the time people reach their early 30s they have all the bone they ever will have and actually begin losing it gradually. Once bone loss begins, those with lower bone density will experience negative effects like osteoporosis much sooner than those with greater bone density. "It's like backing people up from a starting point and expecting them to run the same race," Janz said.

Even more disturbing, Janz said, is that the girls in the study were less active than the boys, which leaves them lagging behind boys in developing healthy bones from a very young age. Since women already do not develop the bone strength that men do because of smaller mass and hormonal differences, Janz said it is even more critical for young girls to become involved in activities that promote bone development.

"We need to get girls involved in sport and give them the opportunity to do other types of vigorous activity that they enjoy, like dance and jumping rope," she said. "It's not a stretch to speculate that we're creating the next generation of osteoporotic women due to inactive lifestyle."

This research is part of the Iowa Bone Development Study, a longitudinal study of Iowa children. The Bone Development Study grew out of the Iowa Fluoride Study, which began 10 years ago and continues to measure the effects of fluoride exposure and ingestion in 800 Iowa children. Steven M. Levy, professor of preventive and community dentistry, is the primary investigator for the fluoride study. The team includes: Trudy Burns, professor of biostatistics, Teresa Marshall, visiting assistant professor of dentistry, James Torner, professor and head of epidemiology, John Warren, assistant professor of preventive and community dentistry and Marcia Willing, associate professor of pediatrics.