Education: Study links screen-based media use to poorer bone health in teen boys
Those of you who have a teenage son will know first hand the hard work involved in luring them away from TV and computer games. But a new study suggests yet another good reason to; the amount of time teenage boys spend in front of a TV or computer screen may affect their bone health.
Researchers have found a link between the amount of time teenage boys spend using screen-based media and reduced bone mineral density.
Anne Winther, of UiT The Arctic University of Norway in Tromsø, and colleagues publish their findings in the journal BMJ Open.
The American Academy of Pediatrics (AAP) recommend that screen time for children and teenagers - including the use of TVs, computers, smartphones and other screen-based electronic devices - should not exceed 2 hours a day, while use of such devices should be avoided for children aged 2 and under.
For this latest study, Winther and colleagues set out to investigate how use of screen-based media may impact the bone health of teenagers - a topic they say few studies have investigated, with those that have, producing conflicting results.
In 2010-11, the team surveyed 961 teenagers from Norway aged 15-17 years who were part of the Tromsø Fit Futures Study. In 2012-13, 688 of these teenagers were surveyed again.
For both surveys, teenagers were asked how much time they spent using their computers or watching TV and movies at the weekends and outside of school hours during the week.
Their tobacco and alcohol intake was also assessed, and they completed a food frequency questionnaire detailing their soft drink and calcium intake - factors known to affect bone health.
Information on the average weekly physical activity of all participants over the previous year was gathered and categorized to one of four groups: sedentary, a minimum of 4 hours of walking, cycling or formal exercise each week, at least 4 hours of recreational sports a week, or engagement in competitive sports/hard training several times a week.
Using X-ray absorptiometry, the researchers analyzed the teenagers' bone mineral density in the whole skeleton, as well as in the hip and top of the thigh bone (femoral neck). Body mass index (BMI) was assessed, as were the teenagers' vitamin D levels.
Weekend screen time linked to lower bone mineral density in all body sites for teen boys
The team found that boys had more screen time than girls during weekdays and weekends. Boys spent 5 hours a day in front of a screen at weekends and almost 4 hours during weekdays, while girls spent 4 hours a day in front of a screen at weekends and just over 3 hours during weekdays.
The more time boys and girls spent using screen-based media during weekends, the lower their physical activity. Though the team notes that 1 in 5 girls and 1 in 4 boys who spent more than 4 hours in front of a screen at weekends took part in more than 4 hours of hard training or competitive sport on weekdays.
On assessing the link between screen time and bone mineral density, the team found weekend screen time was linked to lower bone mineral density at all body sites - but only in boys. Weekend screen time was only marginally linked to lower bone mineral density in the femoral neck of girls.
The researchers say differences in body fat distribution and hormones between teenage boys and girls may explain why girls' bone density appears to be less affected by screen time.
Compared with boys who had less than 2 hours daily screen time at weekends, boys who spent 2-4 hours or more than 6 hours in front of a screen each day at weekends had much lower bone mineral density in the femoral neck.
Boys who spent 4-6 hours in front of a screen on Saturdays and Sundays, however, were found to have higher-than-expected bone mineral density, while girls who had 4-6 hours daily screen time during the week had higher bone mineral density - despite girls reporting lower physical activity levels than girls who had less screen time during the week.
The team notes that the relationship between screen time and reduced bone mineral density was present from analysis of both the surveys conducted 2 years apart.
While the team says their study is purely observational and is unable to establish a cause-and-effect link between screen time and bone health, they may have implications for public health:
"To our knowledge, this is the first study to present associations between bone mass and screen-based sedentary behavior with repeated measurements.
Our study suggests persisting associations of screen-based sedentary activities on bone health in adolescence. This detrimental association should therefore be regarded as of public health importance and followed closely, since improvement of peak bone mass is possible."
Dr. Benjamin Jacobs, of the Royal College of Paediatrics and Child Health in the UK, says the team's findings are a cause for concern and emphasize the need for adolescents to adopt healthy lifestyles.
"There are a number of ways they can do this - adequate exercise, screen time limited to less than 4 hours a day and appropriate vitamin D supplements to improve the bone health and general health of children [...]," he adds.
In February, MNT reported on another study published in BMJ Open, which found spending more than 4 hours a day using electronic devices may lead to poorer sleep in adolescents.