June 17, 2015
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Increased screen time linked to poorer bone health in teenage boys

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Adolescent boys who reported spending more sedentary time in front of a TV or computer screen had a lower bone mineral density than boys who were more physically active, according to research in BMJ Open.

In a cross-sectional, population-based study of Norwegian boys and girls, researchers also found that the screen-time behavior of adolescent girls had no effect on their BMD.

“These conflicting results may be related to different factors, as the relationship between fat and bone varies with age and hormones,” the researchers wrote.

Anne Winther

Anne Winther

Anne Winther, MSc, a PhD student at UiT The Arctic University of Norway, and colleagues analyzed data from 961 students aged 15 to 17 years (492 boys) participating in Fit Futures, an expansion of the population-based Tromso Study. Researchers used DXA to measure BMD at the total hip, femoral neck and total body, and also measured height and weight to calculate BMI and vitamin D levels.

Students completed surveys asking how many hours per day were spent in front of a computer or TV outside of school on both weekdays and weekends, with answers ranging from “none” to “10 hours or more.” Students also rated their time spent in physical activity in an average week during the past year and completed a food frequency questionnaire on daily calcium intake and soft drink consumption.

In Fit Futures 2, a follow-up to the original study completed 2 years later, 688 of the participating students (372 girls) returned and provided repeat BMD measurements.

On average, boys reported spending more sedentary time in front of a TV or computer than girls, with an average of 5.1 hours spent on weekends and 3.8 hours spent on weekdays vs. 4 hours spent on weekends and 3.2 hours spent on weekdays for girls (P < .001).

Researchers found weekend screen time was linked to a lower BMD in all areas among boys, but not in girls. The association strengthened after adjustments for age, sexual maturity and weekday screen time. In addition, boys who spent either 2 to 4 hours or more than 6 hours watching TV on weekends had a lower BMD at femoral sites, with a reduction of approximately 0.06 g/cm² (P ≤ .035) compared with boys who had less than 2 hours of weekend screen time. Boys who reported an average of 4 to 6 hours of weekend screen time tended to have higher BMD levels than expected, which researchers attributed to higher BMI levels combined with more intense physical activity in this group.

“In follow-up analysis of BMD levels measured at [Fit Futures 2], the consistent patterns of associations persisted, although the associations were weaker and only statistically significant at the total hip in boys,” the researchers wrote.

Researchers noted that it remains unclear why there was no link between increased screen time and lower BMD in girls, and further studies with objectively measured data on sedentary behavior and physical activity are needed. 

“Our study suggests persisting associations of screen-based sedentary activities on bone health in adolescence,” the researchers wrote. “This detrimental association should therefore be regarded as of public health importance.” – by Regina Schaffer

Disclosure: The researchers report no relevant financial disclosures.