Even a Little Extra Exercise Lowers Kids' Risk of Obesity

BRISTOL, England, March 21 -- Even an extra 15 minutes a day of moderate to vigorous running, jumping, or kicking a ball can dramatically cut the risk of childhood obesity, according to researchers here.

In a cross-sectional study of more than 5,000 12-year-olds, a daily quarter-hour of moderate to vigorous exercise was associated with a 50% drop in the risk of obesity for boys, found Andy Ness, Ph.D., of the University of Bristol.

In girls, the decline in risk was lower, at nearly 40%, Dr. Ness and colleagues reported online in the open-access journal PLoS Medicine.

"The association between physical activity and obesity we observed was strong," Dr. Ness said. "These associations suggest that modest increases in physical activity could lead to important reductions in childhood obesity."

The study is among the first to use objective measures both of physical activity and lean and fat mass, the researchers said.

The study was part of the long-running prospective Avon Longitudinal Study of Parents and Children, which enrolled more than 14,000 pregnant women in England in 1991 and 1992 and has followed the mothers and children in detail since.

For this analysis, the researchers asked children in the larger study to wear an Actigraph, a device containing an accelerometer, for a week, taking it off only to bathe, to swim, or when cycling (because the instrument is not accurate for cycling).

The device registers vertical motion as counts per minute and has been validated as a measure of energy expenditure, Dr. Ness and colleagues said. For this study, the researchers calculated both total physical activity and moderate to vigorous activity, defined as an Actigraph output of more than 3,600 counts per minute.

The researchers calculated the body-mass index of the children using standard tools and also measured fat and lean mass using dual energy x-ray absorptiometry scanning.

Of the 6,622 children who agreed to wear an Actigraph, 5,595 wore it at least 10 hours a day for at least three days and were included in the analysis. Absorptiometry scanning was available for 5,500 of them.

Analysis found "a strong negative dose-response association between objectively measured physical activity and DXA-derived measures of fat mass and obesity," the researchers said.

Specifically:

• In boys, a difference of 15 minutes of moderate to vigorous activity was associated with between a 55% and 70% reduction in the risk of obesity, depending on which multivariate model was used. Both risk reductions were highly significant at P

The authors pointed out several important limitations of the study which prevent determination that the lack of exercise caused obesity.

"First, our study is cross-sectional and we cannot therefore rule out the possibility that these associations represent reverse causality, and that obesity leads to a reduction in physical activity."

"Second, these data are observational, and it is possible that confounding could explain our results."

For these reasons, they concluded "prospective studies are required to confirm these associations and to describe how physical activity-obesity associations vary over time."