Teenage boys who spend a lot of time at the weekend looking at various screens have poorer bone health, a new study has found.

This finding does not seem to apply to teenage girls however.

Researchers in Norway looked at almost 1,000 teenagers aged between 15 and 17. All of the participants were assessed in 2010 and 2011, and almost 700 of these were then followed up in 2012 and 2013.

During both periods, the young people were asked about their lifestyles, including how much time they spent looking at screens, such as computers and television.

They were also asked about diet, exercise, smoking and alcohol. Each participant also had their height, weight, vitamin D levels and bone mineral density measured.

Bone mineral density refers to the amount of minerals, such as calcium, in the bones. It determines how strong your bones are. People with a low bone mineral density are at an increased risk of fractures and conditions such as osteoporosis.

The study noted that overall, boys spent more time in front of screens than girls. They spent an average of five hours per day in front of screens at the weekend and four hours per day during the week. Girls spent an average of four hours per day at the weekend and three hours per day during the week.

The researchers found that increased screen time at the weekend was linked to lower bone mineral density, but only among boys. Furthermore, after taking into account possibly influential factors such as age and sexual maturity, this link actually strengthened in boys.

Boys who spent between two and four hours or more than six hours, per day in front of a screen at the weekend had significant reductions in bone mineral density compared to those who spent less than two hours per day.

Although the study found that boys who spent between four and six hours in front of a screen each day at the weekend had higher than expected bone density levels.

The researchers suggested that the findings may be related to differences in body fat distribution.

"These conflicting results may be related to different factors, as the relationship between fat and bone varies with age and hormones. But 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," they insisted.

Details of these findings are published in the journal, BMJ Open.