Teenage boys who spend lots of time in front of the TV or computer screen are at risk of poorer bone health, research has found.
A study carried out in Norway found the same did not apply to girls, which it suggested could be explained by their different body fat distribution.
Researchers looked at how much time 15 to 18-year-olds said they spent sitting in front of screens at the weekend, with between four and six hours being the most common amount of time in the 1,000 students questioned.
This was followed by more than six hours in boys, but between two and four hours in girls.
The school pupils' bone mineral density was analysed along with their height, weight and details of their lifestyles.
The team found that while girls in the four to six hour category had the highest bone mineral density, the amount of time spent in front of screens had a negative effect on the bone mineral density of boys.
"Our study suggests persisting associations of screen-based sedentary activities on bone health in adolescence," the study authors said.
"This detrimental association should therefore be regarded as of public health importance and followed closely, since improvement of peak bone mass is possible."
While time spent in front of the TV and computer games was linked to lower levels of physical activity, one in five (20%) of the girls and a quarter (26%) of the boys who said they spent more than four hours a day in front of a screen at the weekend also spent more than fours hours a week doing sport.
Researchers added that it was possible students either under or over-reported their screen times due to social pressures or gender differences.
They said girls tend to perform several activities at once - spending time in front of the TV or computer screen as well as on their phones or carrying out other tasks, such as hobbies or crafts.
Meanwhile male adolescents are less likely to report spending lots of time on the phone, texting and instant messaging, and it might therefore have been easier for boys to give a precise amount of time spent in front of a screen.
The study, which was led by the UiT The Arctic University of Norway in Tromso, is published in the online journal BMJ Open.
Dr Benjamin Jacobs, of the Royal College of Paediatrics and Child Health (RCPCH), said: "This is an important study showing that teenagers in Norway spend far more time in front of screens than the two hour recommended maximum, especially boys who had an average of four hours on weekdays and five hours over weekends.
"The teenage boys with more screen time had significantly lower bone mineral density, lower vitamin D levels and higher BMI. This effect was found again when the study was repeated two years later. This relationship was not found in girls for reasons that are not yet clear. However the low bone density in the boys who spend more than six hours a day in front of screens on the weekend is a major concern.
"This large important study adds further weight to the RCPCH's drive to support paediatricians and others, to promote healthy lifestyles for young people in the UK.
"There are a number of ways they can do this – adequate exercise, screen time limited to less than four hours a day and appropriate vitamin D supplements to improve the bone health and general health of children in the UK."