Too much television isn't just bad for kids' heads, it appears to weaken their bones as well, Australian researchers found.
Long periods of television watching could have the same negative effects on kids' bone growth as total bed rest, an Australian study found.
Australian research presented in the Journal of Bone and Mineral Research shows decreased bone mass in young adults who spent more of their childhood hours in front of a television set.
Of more than1,000 young men and women scanned at age 20 those who had watched 14 or more hours of TV each week as children and adolescents had less bone mineral content than their peers.
Joanne A McVeigh of Western Australia’s Curtin University and colleagues drew on data from the Western Australian Pregnancy Cohort Study.
The longitudinal Raine study followed 2,900 prenatal clinic patients enrolled by their mothers in utero and assessed at birth and at ages 1, 2, 3, 5, 8, 10, 14, 17, and 20 years. This latest bone mass study included the 1,181 participants in the 20-year assessment for whom complete data for TV watching, bone mass, physical activity, serum 25-hydroxyvitamin D, dietary calcium intake, smoking habits, and alcohol consumption were available.
TV-watching data were parent-reported at ages 5, 8, and 10 years becoming self-reported at 14, 17 and 20.
Based on Australian and US recommendations against more than 2 hours of screen time per day for children TV watching groups were identified as “low”, consistently 14 hours per week or less “increasing”, changing from 14 or less to 14 or more during adolescence or “high” consistently 14 hours or more.
Bone mineral content (BMC) and bone mineral density (BMD)were assessed by Norland XR-36 densitometer whole body DXA scan.
Even after adjusting for height, body mass, physical activity, calcium intake, serum 25-hydroxyvitamin D levels, alcohol and smoking ,bone mineral content and density were markedly lower in the high TV watcher group.
BMC in low TV watcher males was on average 227 grams more than the average 3111 grams found in males who had watched more than 14 hours per week during the crucial growing stages.
BMD in low TV watchers was 1.137 g/cm2 while high TV watchers were only 1.120 g/cm2. In women BMC was 2711 grams for the average low TV watcher and 2610 for high.
The difference in BMD for women was less dramatic yet still analogous at 1.019 g/cm2 for low TV watchers and 1.015 g/cm2 for high.
McVeigh cited known factors that could play an as yet unknown role in the relationship between healthy bone growth and TV watching.
One indirect effect is a decrease in time available for osteogenic or “bone healthy” activity.
High fat and sugar foods that may threaten healthy bone growth often accompany TV watching, the team noted
A more alarming possibility, they wrote, is that spending long periods of time essentially motionless could produce the systemic changes in bone metabolism seen in bed rest studies.