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 Childhood and the teen years are critical periods for growing bones
and establishing a bone density level that can affect osteoporosis
risk much later in life.
“We found a relationship between higher screen time and lower bone
mineral density in boys,” said Anne Winther, a physiotherapist at
University Hospital of North Norway in Tromso and the study’s first
author. “We are not able to detect causality with this study design,
but it is likely that screen time is an indicator of a lifestyle
that has negative impact on bone mass acquisition.”
Among the 316 boys and 372 girls aged 15 to 19 years old, those who
spent two to four hours, or more than six hours, in front of the
screen every day tended to be slightly heavier than their peers who
spent less time in front of screens. And boys overall spent more
time in front of the computer and television than girls (five hours
a day versus four).
But the boys with heavy screen time also had lower bone mineral
density (BMD) levels - the amount of mineral per square centimeter
of bone - while the girls’ BMD was higher with heavier screen time.
Winther, who is also a doctoral student at UiT The Arctic University
of Norway, and her team note in the journal BMJ Open that decreased
lean mass (muscle) and increased fat mass could be more harmful to
boys than girls and might actually protect female bones.
For the study, the youngsters reported how many hours per day they
spent in front of the computer or watching television or DVDs on
weekends, as well as how much time they were sedentary, walked,
cycled and participated in recreational sports weekly.
Bone density scanning was used to measure their bone mineral density
at the hip, the top of thigh bone, and other body areas, which are
“strong indicators of fracture risk,” Winther noted.
“The most important finding was that the detrimental relationship
between this screen-based sedentary behavior and bone mass density
in boys persisted two years later,” she said.
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The American College of Sports Medicine recommends 10 to 20 minutes
of gymnastics or running or jumping, or other weight-bearing
exercise at least three days weekly for children and adolescents.
“I think you can never say too often what the authors were saying,”
said Dr. Laura Bachrach, a pediatric endocrinologist at Stanford
University Medical School in California. “We’re really worried about
this because there’s sort of this critical time between being born
and reaching the early 20s when you’re setting up the scaffolding of
life (in terms of the geometry and density of the bone),” she told
Reuters Health by email.
“You sort of max out in your early 20s and there is real concern
that the lifestyle of young people nowadays versus 40 or 50 years
ago is setting people up to be more at risk as adults for not having
a very robust bone bank as they age,” Bachrach said.
The study focused on older teens, although sedentary time and
exercise would have the most bone impact on nine to 15-year-olds,
Bachrach pointed out.
“The horse may have been a little bit out of the barn here in terms
of what they’re looking at,” Bachrach said. “Girls tend to mature
earlier . . . the girls were even more fixed in their position in
the skeletal world by the time they started, whereas the boys were
perhaps a little more malleable.”
SOURCE: http://bit.ly/1er92SH BMJ Open, online April 22, 2015.
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