Effective strategies to fight the epidemic of childhood and adolescent obesity — one-third of kids under 20 weigh more than they should — have been elusive. A new study suggests a simple step that might help cut the problem down to size: start school sooner.

“Early admission to a school environment might have a long-term protective effect in terms of adolescent girls’ propensity to obesity,” says the study appearing online in the Journal of Adolescent Health.

The researchers analyzed data on nearly 6,000 girls from the National Longitudinal Survey of Youth, which included when they started school and their body mass index (BMI) from ages 12 to 18.

They found that girls who were born a month or less before the cutoff date for school enrollment — and so started school when younger than most of their classmates — were significantly less likely to be overweight during adolescence than those who were born during the month after cutoff.

Children who enrolled in school a year later than when they first became eligible similarly were more apt to be overweight than those who started on time.

The researchers did not find a comparable effect for adolescent boys.

Why school entrance age should influence weight 10 years later is not clear from the data, said lead study author Ning Zhang, Ph.D., of University of Rochester School of Medicine. One possibility is peer effect: “Within any grade, younger girls may be exposed to relatively older friends, who are more careful about their weight and physical appearance,” she said.

Physical and health education curricula in the United States are “grade specific,” Zhang added, with more detailed and sophisticated health and diet instruction offered in higher grades. Girls who are young for their grade have earlier exposure to this information and participate in more advanced physical exercise regimens as well.

Matt Longjohn, M.D., is a fellow with the Altarum Institute, a non-profit health systems research organization. Longjohn said that the study findings might reflect the cumulative impact of early childhood events years later and added that considerable research has shown that “changes in just a few small behaviors can have large and lasting effects on small bodies.”

TERMS OF USE: This story is protected by copyright. When reproducing any material, including interview excerpts, attribution to the Health Behavior News Service, part of the Center for Advancing Health, is required.

Journal of Adolescent Health: visit http://www.jahonline.org

Zhang N, Zhang Q. Does early school entry prevent obesity among adolescent girls? J Adol Health online 2010.

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Source: Health Behavior News Service