Decrease in physical activity may not be a factor in increased obesity rates among adolescents

Decreased physical activity may have little to do with the recent spike in obesity rates among U.S. adolescents, according to researchers at the Johns Hopkins Bloomberg School of Public Health. Prompted by growing concern that the increase was due to decreased physical activity associated with increased TV viewing time and other sedentary behaviours, researchers examined the patterns and time trends in physical activity and sedentary behaviours among U.S. adolescents based on nationally representative data collected since 1991. The review found signs indicating that the physical activity among adolescents increased while TV viewing decreased in recent years. The results are featured in the October 30 online issue of Obesity Reviews.

'Although only one third of U.S. adolescents met the recommended levels of physical activity, there is no clear evidence they had become less active over the past decade while the prevalence of obesity continued to rise,' said Youfa Wang, MD, PhD, MS, senior author of the study and an associate professor with the Bloomberg School's Centre for Human Nutrition and the Department of International Health. 'During the recent decade, U.S. adolescents had greater access to TV, but significantly fewer of them watched TV for three or more hours per day. In addition, daily physical education attendance rates improved along with the use of physical education class in engaging in physical activity. However, there are considerable differences in the patterns by age, sex and ethnicity.'

Wang, along with co-authors Shiru Li, MD, MS, former visiting scholar with the Bloomberg School's Centre for Human Nutrition, and Margarita Treuth, PhD, adjunct associate professor with the Bloomberg School's Centre for Human Nutrition and a professor with the University of Maryland East Shore, examined findings from the nationally representative Youth Risk Behaviour Surveillance (YRBS) surveys from 1991 to 2007. The survey included U.S. high school students in grades 9 through 12 and provided information about their physical activities including enrolment and participation in physical education in school and sedentary behaviours including screen time. Based on these surveys, researchers examined the patterns and time trends and compared the observed physical activity patterns with the national goals set in Healthy People 2010, a comprehensive agenda for improving the health of all Americans. They found that minority students were less likely to be physically active and more likely to engage in sedentary behaviours than white students. Girls were less active than boys and decreased physical activity was related to an increase in age.

'The large gaps between the 2007 achievement and the 2010 targets indicate that the goals are unlikely to be achieved by 2010,' said Wang. 'Our study suggests that more vigourous efforts are needed to help young Americans engage in adequate regular physical activity and reduce sedentary behaviours, which will help promote good health. In addition, these findings may suggest factors other than physical activity, and sedentary behaviours such as unhealthy eating may play a more important role to help explain the recent increase in obesity.'