Kids who watch a lot of television may build less bone during critical years, and be more vulnerable to osteoporosis and bone breaks later in life as a result, a new study suggests.

However, the act of watching TV is not necessarily the cause of weak bone mineral content, but the harming factor may be sitting for long periods of time. Several studies have shown a link between the time spent sitting and bone health.

"Sitting watching television does two things, it takes away from being active, therefore we do not get the benefit of physical activity and second it immobilises us for prolonged period of time which we know from bed rest studies triggers physiological response that change the balance in our body chemistry that keeps our bone strong," said Dr Sebastien Chastin of Glasgow Caledonian University in the UK, who was not part of the new study.

There are practical ways to break up periods of seated screen time, like getting up during ads.- Natalie Pearson

The new study showed that the children and teens followed until age 20, when bone mass is peaking, had lower bone mass at that age the more hours they had spent watching TV in childhood, researchers reported online July 4th in the Journal of Bone and Mineral Research.

The researchers accounted for height, body mass, physical activity, calcium intake, vitamin D levels, alcohol, and smoking at age 20, and still found that kids who were consistently high-level TV watchers at younger ages had lower bone mineral content than others as adults.

"Poor bone health ultimately can lead to osteoporosis, which affects over 200 million women worldwide," Chastin said. "You can imagine that a fall on a brittle hip is more likely to result in fracture."

“Our bodies reach peak bone density around age 22, after which time bone density decreases over time, though we can slow the decrease by maintaining an active and healthy lifestyle,” he added.

According to Chastin, impact sports, such as parkour and running, are the most beneficial for bone health, especially for muscle strength, balance and coordination.

"It is often very difficult to make parents and doctors aware of the very long term health implications of sitting (at a screen or other sitting occasions such as school time, work time, travel etc.) for long periods, as in today's society we are very interested in the immediate responses to our actions and not what will happen 20 years down the line," said Natalie Pearson of the School of Sport, Exercise and Health Sciences at Loughborough University in the UK, who was not part of the study.

There are practical ways to break up periods of seated screen time, like getting up during ads or while working on a computer getting up to answer the phone instead of emailing a colleague or friend, she said.