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Why do children and teens become overweight? The answer to that question is not as straightforward as counting calories. NPR's Allison Aubrey has this story about a study published today in the journal Pediatrics. It suggests that the eating habits of overweight teens are not all that different from their healthy-weight peers.

ALLISON AUBREY, BYLINE: Physical activity comes easiest to the youngest of children. From soccer tots, to elementary school baseball teams, lots of kids are excited about sports at this age. And parents are often invested, too.

(SOUNDBITE OF CHILDREN PLAYING SPORTS)

AUBREY: But fast forward to the tween years, and chances are that many of these little athletes will have given up organized sports for more sedentary pursuits such as video gaming.

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AUBREY: About the only thing getting a workout here are the fingers. It may not seem like a big deal to replace an hour of soccer practice with an hour of video gaming, but small changes in physical activity lead to fewer burned calories. And researcher Ashley Skinner, of the University of North Carolina, was curious. Was this difference alone enough to make a kid overweight? What about the role of diet?

ASHLEY SKINNER: So the question was, well, I wonder what these kids are actually eating. Are they actually eating that much differently compared to healthy-weight kids?

AUBREY: To examine this, she and her colleagues studied the weight and diet of about 19,000 children from toddlers to teens, using information from a large federal survey. Skinner says, what she found was surprising. Up to the age of eight or nine, overweight kids were eating more calories than their healthy-weight peers. But as the tween years set in, the picture changed.

SKINNER: At about age nine or 10, we start to see essentially a flip. That the overweight kids are actually eating less than the healthy-weight ones.

AUBREY: The overweight kids are actually eating less?

SKINNER: Right. So they're actually taking in fewer calories than the healthy-weight kids.

AUBREY: Now, the researchers could not independently verify the amount kids said they were eating. So it's possible some were not accurate. But the findings were consistent. On average, heavy kids were eating about 100 fewer calories a day compared to their healthy-weight peers. Obesity expert Matthew Gilman of Harvard medical school says this isn't the first study to suggest that heavy teenagers don't necessarily eat more calories. And it supports the idea that small differences in physical activity do seem to be key.

MATTHEW GILMAN: When you're less physically active, you actually need fewer calories to maintain your weight.

AUBREY: And, on top of that...

GILMAN: You actually need fewer calories to increase your weight.

AUBREY: Skinner's findings also hint at the significance of being overweight at such a young age in the early elementary years. Sophia Yen is with Stanford medical school.

SOPHIA YEN: Once you've become overweight there are changes in your body that make you different from someone who's not. So you have extra fat cells, you have different insulin levels.

AUBREY: Which can make it feel like you're eating less than you are. And these changes, she explains, are very persistent.

YEN: And once the effects have taken place, you know, the fat deposition, or the insulin changes in your body, then it's a lot harder to reverse.

AUBREY: Yen says, the important message is that early life experiences can shape the trajectory for weight throughout life. Take for instance, fat cells. Once the body creates a fat cell, it never goes away.

YEN: You can slim down that fat cell, but that fat cell will always be there waiting to get larger if you give it any extra calories.

AUBREY: So Yen says, her advice to parents of young children is: if you see signs of a weight problem, it's better to take action sooner rather than later. Allison Aubrey, NPR News.

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