NEW YORK (Reuters Health) - Children who exercise at school don't make up for the extra effort by being less active at home, according to a new U.S. study that used accelerometers to track kids' activity levels.
"What this argues for is we should be increasing activity in schools," said Michael Long, the lead author of the new study and a post-doctoral research fellow at the Harvard School of Public Health in Boston, Massachusetts.
The Centers for Disease Control and Prevention recommend that kids get at least one hour of physical exercise during the day, but less than a third of high school students meet that goal.
Physical education at schools has also taken a back seat in recent years to academic preparation.
In May, the U.S. Institute of Medicine urged education leaders to encourage physical activity in schools, recommending that younger kids get at least 30 minutes a day of exercise and older students at least 45 minutes a day.
Some previous research has suggested, however, that kids have a built-in "activitystat" that regulates the total amount of energy they expend (see Reuters Health story of May 7, 2009 here: reut.rs/iGDVdO).
That would imply that raising kids' activity levels at school will not raise their overall activity, just shift it to school hours, and will therefore do little to combat obesity and poor fitness.
For instance, Terence Wilkin, a professor at the University of Exeter in the UK, and his colleagues have found that kids who exercise a lot at school will exercise less at home, and kids who are relatively inactive at school will spend more time physically active outside of school.
His studies compared the activity levels of children at different schools.
To see whether the activitystat idea holds up for the same child on different days, Long and his colleagues analyzed data from a large U.S. survey on health and lifestyle.
As part of that study, children wore an accelerometer on their waists to measure the amount of time they spent moving around and also the intensity of their activity.
The 2,548 kids participating wore the monitor for about six days.
On average each day, the younger kids, aged six to 11, spent 86 minutes moderately or vigorously active, and 37 of those minutes were at school, according to the results published in the American Journal of Preventive Medicine.
Kids aged 12 to 19 spent 41 minutes moderately or vigorously exercising, and 18 of those minutes occurred during the school day.
Long's group found that for each extra minute that kids were exercising at school, they had slightly more than an extra minute of total exercise for the day.
In other words, they did not reduce vigorous activity outside of school to offset the "extra" minutes of exercise in school.
"We didn't find any evidence youth were compensating for higher levels of activity during school hours (with less) activity outside of the school period," Long told Reuters Health.
Wilkin does not think the new results invalidate the activitystat concept.
For one, Wilkin said, children don't necessarily compensate for the extra energy on the same day, and his evidence for compensation was seen over the course of a week.
He noted as well that the study did not take into account the season or the weather, and only examined time spent moderately or vigorously active, rather than total activity.
"However unpalatable and counter-intuitive, we believe that the experimental data is clear - an activitystat defends the physical activity of a child against more physical activity than is necessary for survival," Wilkin said in an email to Reuters Health.
While more experimental research is needed to confirm whether or not children compensate for exercising at school, there is little argument about the benefits of physical activity for kids.
Long said that it's important not only for their health, but it may also improve children's mood and their ability to concentrate.
"But the question remains - how do you get children who do less to do more?" Wilkin said.
SOURCE: bit.ly/1b18CLk American Journal of Preventive Medicine, August 2013.