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updated 12/15/2010 1:48:26 PM ET 2010-12-15T18:48:26

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.

"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 an interview with the Health Behavior News Service.

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 effect was significant, Zhang told LiveScience: Girls who start school earlier are 30 percent less likely to be obese and nearly 20 percent less likely to be overweight than girls who start later.

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.

"The definition of beauty is pretty different for girls and for boys," Zhang told LiveScience. "For girls, 'slim' and 'beauty' are very related, but for boys, researchers have not found a connection... We think that this could be one reason why younger girls are probably more willing to keep slim."

The findings may 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, told the Health Behvaior News Service 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.

You can followLiveScienceSenior Writer Stephanie Pappas on Twitter @sipappas.

© 2012 LiveScience.com. All rights reserved.

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