Family Matters

Why Do Kids Gain So Much Weight Between First and Third Grade?

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What happens to kids between first and third grades? They discover chapter books. They tackle multidigit subtraction. And they gain more weight than at any other point in elementary or middle school.

Research published Monday in the journal Pediatrics finds that the largest weight gains are occurring at what researchers call a “critical time” in early elementary school, with the most significant leap — a 5.8% increase in body-mass index (BMI) — taking place between first and third grades.

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To account for the scale-tipping, kids at this age must either be eating more or exercising less — or some combination of the two. As mom to both a first- and third-grader, I can speculate that it’s probably not coincidence that even children who were previously of normal weight start packing on the pounds once they start formal schooling. Preschool is all about play, with young children constantly on the move. But starting in kindergarten, kids are expected to sit still and learn — effectively burning fewer calories.

To construct BMI percentiles, researchers looked at data from about 4,200 white, nearly 700 black, and more than 1,000 Hispanic kindergarteners, tracking their height and weight for nine years. “We wanted to look at how BMI changes over time, not just for kids who were overweight or obese but for all children,” says study author Ashlesha Datar, a childhood obesity researcher and health economist at the RAND Corporation in Santa Monica, Calif.

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For sure, weight problems didn’t begin in first grade. About 40% of the children studied had landed in the top 25% of pediatric growth charts by the time they entered kindergarten. That proportion rose to 45% by the end of third grade and to 48% by the time they were ready to start middle school. Researchers found no significant weight changes in middle school, but over the course of the study period, Hispanic kids and black girls notched the greatest increases in BMI percentiles.

Datar and her colleagues didn’t examine the reasons behind the weight gains, but there’s existing evidence that physical activity, particularly among girls, tends to drop as children age. Now that a danger zone has been identified, public-health experts can start to think about interventions that can be put into place to stave off the excess pounds. “This can be a springboard to say, What is it about this period that’s causing children to gain weight?” says Datar.

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The potential pitfalls are many. Physical education classes may not be scheduled frequently enough, and vending machines in schools tempt with calorie-packed snacks and drinks. After school, kids may veg in front of the television with a bag of chips instead of getting exercise.

There’s lots of talk about overscheduled kids, but until we figure out a way for them to get adequate exercise during the school day, shuttling children after school to basketball/soccer/gymnastics seems like one of the only ways to make sure their BMI doesn’t shoot up even more. Sigh.

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Bonnie Rochman is a reporter at TIME. Find her on Twitter at @brochman. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.

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