Watching too much TV is a curse for boys.

The amount of time teenage boys spend in front of a TV or computer may affect their bone health, according to a new study. Anne Winther, of UiT The Arctic University of Norway in Tromso, set out to investigate how use of screen-based media may impact the bone health of teenagers. The topic has been investigated by few studies but they have produced conflicting results, researchers said. In 2010-11, the team surveyed 961 teenagers from Norway aged 1-17 years who were part of the Tromso Fit Futures Study. In 2012-13, 688 of these teenagers were surveyed again. In 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, 'Medical News Today' reported. Information on the average weekly physical activity of all participants over the previous year was gathered. 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. The team found that 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. 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, the study found. The researchers said 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. Researchers cautioned that the study is purely observational and is unable to establish a cause-and-effect link between screen time and bone health. The study is published in the journal BMJ .