By Janice Neumann

(Reuters Health) - Teenaged boys who spend too many hours in front of the computer or television without participating in enough weight-bearing exercise could develop weaker bones as they age, a small Norwegian study suggests.

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, 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 colleagues note in BMJ Open that decreased lean mass 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.

"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," Winther said.

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 2015.