Decline in Kids' Activity with Age Documented: New Study

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Concern about the increase in obesity in children and adolescents has spurred research into their activity levels. Data garnered from girls' reports of their activity, for example, indicates that as girls mature, their participation in leisure-time and total exercise appears to decrease. Because these studies are based on participants' reports of their activities, it is hard to know how accurate they are.

A new study, published in the July 16 issue of the Journal of the American Medical Association (JAMA 2008; 300:295-305) confirms and extends the earlier reports with a more quantitatively reliable method. The researchers, led by Dr. P.R. Nader from UC San Diego, had children of both genders (1,032 -- 50% boys and 77% white at the start of the study) wear accelerometers -- devices that measure movement, not just steps as pedometers do -- and their activity levels were monitored for seven days at nine, eleven, twelve, and fifteen years of age.

Initially, at nine years of age, almost all the children engaged in well over the sixty minutes per day of moderate to vigorous physical activity (MVPA) recommended by the Department of Agriculture -- on average, 182 minutes on weekdays and 179 on weekends. These respective averages decreased to:

•124 and 120 minutes at eleven years
•ninety-six and eighty-four minutes at twelve years
•and forty-nine and thirty-five minutes at fifteen years of age.

At all ages, boys were more active than girls, but boys' and girls' decreases in MVPA occurred at about the same rate. Girls first decreased below the recommended sixty minutes of activity per weekday at about thirteen years of age, while boys hit that mark at about fifteen years.

The authors pointed out that it is not clear whether such decreases in MVPA are a part of normal maturation or whether, since the study was performed during a period (2000-2006) when obesity was increasing, activity levels were affected by non-biological factors.

Whichever of these explanations is true, understanding the typical pattern of activity decline among young people should give some impetus to parents and others concerned with the health of the young to encourage them to maintain healthy activities well into their teens. Hopefully, once such patterns have been set, activity levels will be maintained at a higher level into adulthood.

Ruth Kava, Ph.D., R.D., is Director of Nutrition at the American Council on Science and Health (,