MedWire News: Many US children do not meet recommended physical activity guidelines by the time they reach the age of 15 years, study results show.

"This decrease augurs poorly for levels of physical activity in US adults and potentially for health over the course of a lifetime," comment lead researcher Dr Philip Nader, from the University of California in San Diego, USA, and team.

Physical inactivity significantly increases the risk of obesity and associated conditions, such as high blood pressure and heart disease. Young people are therefore recommended to engage in a minimum of 60 minutes of moderate-to-vigorous physical activity a day. However, it is not known how many children meet this recommendation, says the team.

To investigate, the researchers monitored the physical activity levels of 1032 children between the ages of 9 and 15 years. All the children wore a minute-by-minute movement monitor for 1 week a year at the ages of 9, 11, 12 and 15 years.

The researchers found that average levels of moderate-to-vigorous physical activity fell significantly as the children increased in age.

At the age of 9 years, children engaged in moderate-to-vigorous physical activity approximately 3 hours a day on both weekdays and weekends. By the age of 15 years, the participants were only engaging in moderate-to-vigorous physical activity for 49 minutes on a weekday and 35 minutes on a weekend day.

The estimated age at which girls fell below the recommended 60 minutes of moderate-to-vigorous physical activity was around 13.1 years, compared with 14.7 years in boys.

"More research is... needed to understand the reasons for such substantial decreases in youth activity," write Dr Nader and team in the Journal of the American medical Association.

Nevertheless, they conclude: "There is a need for programmes and policy action as early as possible at the family, community, school, healthcare, and governmental levels to address the problem of decreasing physical activity with increasing age."