A Bone to Pick with Sitting Still

Sedentary teens have less dense bone

(RxWiki News) Being active has always been good for the bones of the young and old alike. But how does sitting still affect bones in kiddos? Teens are more likely to have lower bone mineral levels in parts of the body where they sit sedentary for long periods of time, a new study has found.

"It is already well known that an inactive lifestyle has implications for young people, such as obesity and heart diseases," researchers said in a press release.

"Combined with that, our findings emphasize the need for exercise. We hope it will give some focus for young people and their parents to ward off any health problems later in life."

"Be physically active for at least 30 minutes a day."

Exercise, especially those that are high impact or bear a lot of weight on the person, has been known to help strengthen and grow bone. The aim of the study, led by Luis Gracia-Marco, PhD, lecturer in the School of Sport and Health Sciences at the University of Exeter in the UK, aimed to see how sedentary activities affected bone strength and its mineral content.

Researchers looked at the mineral levels inside the bones of 359 Spanish teenagers between October 2006 and December 2007. The kids were between 12 and 18 years of age and a little more than half were girls. The height, weight and body mass index of each child was measured. To figure out the kids' bone density, researchers also measured their fat and muscle mass and subtracted it from their overall body density.

Schools where the students came from were randomly selected by district. Researchers made sure to select up to three classes from two grades at each school to make sure they got a good representation of the population.

Researchers surveyed students on how they spent their time during school and the time spent in front of the TV, playing video or computer games and Internet usage for studying or other reasons. They also asked how much time they spent studying outside of school and the minutes being sedentary each day.

The kids were grouped as either active or non-active based on whether they exercised more than 60 minutes a week or if they did more than three hours of sports.

After taking muscle and physical activity into account, researchers found that boys' recreational use of the Internet is linked with lower bone mineral levels throughout the whole body.

As for girls, those who spent time studying had lower bone mineral content in their femur, the biggest bone in the leg that's often a point where osteoporosis is diagnosed. "Clearly, we are not telling girls not to study," Dr. Gracia said in a press release.

"It is a fact of modern life that teenagers spend more time engaged in desk-bound or sitting activities, but our research is one of the first to identify a connection between this behavior in adolescents and low levels of bone mass in key regions of the body."

Between boys and girls, there was no difference in their age and bone mass in the lumbar spine in their lower back. They also spent equal time watching TV and being sedentary.

"More research is needed to establish exactly why there are differences between the two genders, and why these types of activity are particularly damaging to teenage boys and girls, but we can speculate that it is linked to how long they remain in the same position," Dr. Gracia-Marco said.

"Our findings indicate that activities such as studying, where you spend a long time sitting down without getting to your feet, could be detrimental to bone health."

Kids who engage in at least three hours of sports and physical activity can help counteract the bone loss among girls. Though bone mass is linked to the amount of exercise kids do, the authors note it doesn't mean lack of exercise is the cause of the decrease in mineral content.

The study was published online November 13 in BMC Public Health. The authors have no competing interests to declare.

Review Date: 
November 20, 2012