Tackling a chicken-and-egg-type question that researchers are divided on, a study that first appeared June 23 in the online version of Archives of Disease in Childhood has concluded that fatness leads to inactivity, but inactivity does not lead to fatness in children.

Since it is widely believed from cross-sectional studies that childhood obesity is caused by inactivity, the findings could have profound implications for where and how childhood obesity resources are targeted, as well as how programs combating the problem are designed. Study authors B.S. Metcalf and colleagues concluded that “physical inactivity appears to be the result of fatness rather than its cause. This reverse causality may explain why attempts to tackle childhood obesity by promoting physical activity (PA) have been largely unsuccessful.”

An 11-year study of 202 children from 40 schools in Plymouth, England, examined participants annually from 7 to 10 years of age as part of the Peninsula Medical School’s EarlyBird Diabetes Study. Physical activity was measured using Actigraph accelerometers, which the children wore for 7 consecutive days at each annual time point. Two components of PA were analyzed: total volume of PA and time spent at moderate and vigorous intensities. Body fat percent (BF%) was measured annually by dual energy X-ray absorptiometry. BF% was predictive of changes in PA over the 3 years, but PA levels were not predictive of subsequent changes in BF% over the same follow-up period. Accordingly, a 10% higher BF% at age 7 years predicted a relative decrease in daily moderate and vigorous intensities of 4 minutes from age 7 to 10 years (r = –0.17, p = 0.02), yet more PA at 7 years did not predict a relative decrease in BF% between 7 and 10 years ((r = –0.01, p = 0.8).

“The image of the ‘couch-potato’ child who is obese because he is sedentary runs deep in the Western consciousness,” the authors stated. “However the possibility that the reverse obtains—that his fatness is the cause rather than the result of his inactivity—has far reaching implications. . . . Our findings suggest that rather than giving children ever-increasing doses of physical activity, we should first question the basic paradigm that more physical activity leads to less fat. If childhood fatness is not the result of physical inactivity, the implication may be that excess energy intake underlies fatness and inactivity.”

Although the researchers admit that it is not clear what type of intervention is most likely to succeed in preventing childhood obesity, they suggest that future studies should perhaps focus more on reducing energy intake than on increasing energy expenditure.