Young people who are more active growing up tend to end up with stronger bones, but many older teenagers don't get enough exercise to see those benefits, a recent study found.
The good news, researchers said, is that lots of physical activity during childhood seems to set up young adults for years of strong bones, even if they don't exercise much during their teen years.
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"What parents do to make sure kids are active today matters down the road," said Kathleen Janz, the study's lead author from the University of Iowa in Iowa City.
"When you accumulate physical activity as a child, you end up with what looks like better bone as an adolescent," she told Reuters Health.
Participants in the research were part of the Iowa Bone Development Study, an ongoing study of bone health during childhood and young adulthood. The children had been recruited for that study between 1998 and 2002 when they were about five years old.
At ages five, eight, 11, 13, 15 and 17 years old, the 530 participants wore a device called an accelerometer for four or five consecutive days, including one weekend day, to measure their physical activity whenever they were awake.
When the participants were 17 years old, researchers used bone scans to measure the density, strength and brittleness of their bones. They also used pictures from the scans to estimate the precise geometry of the teenagers' bone shape, a crucial factor in bone strength.
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The authors found that during childhood, less than 6% of girls were highly active, and by their late teens, almost all had become inactive.
Better bone shape
Male participants also were less active as they got older, but tended to get more exercise each day than females, according to the findings published in the British Journal of Sports Medicine.
On average, girls went from being active for 46 to 48 minutes a day in early childhood to being active for just 24 minutes a day as 17-year-olds.
Among boys, activity levels fell from 60 to 65 minutes a day at the beginning of the study to an average of 36 minutes a day by the end.
At age 17, both boys and girls who had been the most active throughout their lives had denser bones and better bone shape than other participants their age who had been less active.
The results show that despite the importance of exercise and its many benefits, very few adolescent girls get enough, Janz said.
Running and jumping
"In an ideal world, children are active and maintain their activity into retirement, but this activity declines dramatically during adolescence, which is ironically a time when bone is most responsive to activity," she said.
Because it may be tough to cajole adolescents into being active, parents should maximize kids' chances of having strong bones later on by encouraging lots of activity during childhood.
"Even once kids became less active, those who had been active had better bones," said Janz.
"It is not all that difficult for kids to be active, whereas sometimes getting adolescents to be active can be more difficult. They have different ideas as to how to spend their leisure time," she said.
The best activities for kids and teens are those that involve running and jumping, Janz said. But for girls in particular, any exercise is better than none, which is what most study participants were getting.
"Girls as a population need to be more active," said Janz. "Physical inactivity in adolescent girls is endemic."
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