updated 12/14/2010 9:40:45 AM ET 2010-12-14T14:40:45

Girls who start school early for their age are less likely than others to be obese as teens, according to new research.

The study, published today (Dec. 14) in the Journal of Adolescent Health, found that girls who started school early for their age had lower body mass indexes (BMI), a measure of fatness, as teens. The reason why starting school younger affects weight later isn't known, said study researcher Ning Zhang of the University of Rochester School of Medicine.

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"Within any grade, younger girls may be exposed to relatively older friends, who are more careful about their weight and physical appearance," Zhang said in a statement.

Zhang and her colleagues analyzed data on almost 6,000 teenage girls from the National Longitudinal Survey of Youth, which is an annual survey of a nationally representative group of adolescents born in the early 1980s. Using data collected between 1997 and 2004, the researchers took advantage of school enrollment cut-offs. Children whose birthdays fall just after the cut-off date are required to delay starting school until the next year, making them old for their grade. Children whose birthdays are just before the cut-off date are young for their grade.

Cut-off dates create two groups of kids at the same developmental stage with one year's difference in their schooling, the researchers wrote. Comparing the two groups, the researchers found that among girls whose birthdays were within a month of the cut-off date, those who started early for their age (that is, almost a full year earlier than their oldest classmates) were more likely to be normal weight. Those who started late for their age were more likely to be overweight or obese.

The results held after the researchers controlled for age, race, level of maternal education and mother's body weight. In boys, there was no relationship between school start time and weight, according to the researchers.

Schooling might help lower weight because of physical education classes, Zhang said. Girls who start school earlier are exposed to more sophisticated health classes sooner and may participate in more advanced physical exercise, she said.

The findings may also represent the cumulative effect of early childhood, said Matt Longjohn, a physician and fellow with the non-profit Altarum Institute. Longjohn, who was not involved in the study, said in a statement that "changes in just a few small behaviors can have large and lasting effects on small bodies."

Previous research has found big impacts from early education. For example, kindergarten grades are tied to adult income, while elementary school bad behavior can compromise education and lead to career problems.

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You can followLiveScienceSenior Writer Stephanie Pappas on Twitter @sipappas.

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