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LINDA WERTHEIMER, HOST:

This is MORNING EDITION from NPR News. I'm Linda Wertheimer.

STEVE INSKEEP, HOST:

And I'm Steve Inskeep. Today in Your Health, preventing weight gain. The usual suspects in America's obesity epidemic are eating too much and not getting enough exercise. Now a new study suggests that Americans actually are not eating more than we did 20 years ago. It's the amount of physical activity that's changed. NPR's Patti Neighmond reports.

PATTI NEIGHMOND, BYLINE: Dr. Uri Ladabaum wanted to understand more about the reasons for the obesity epidemic. He decided to dig through 20 years' worth of health information. Ladabaum's a gastroenterologist at Stanford University. He looked at government surveys that asked people how much they ate and how active they were. He compared what people said in 1988 to what they said in 2010.

URI LADABAUM: In the early time periods, approximately 10 to 20 percent of people reported no leisure time physical activity, meaning that 80 to 90 percent reported doing at least some physical activity in their leisure time, whereas in more recent time periods, it was up to half who said they weren't doing any leisure time physical activity.

NEIGHMOND: And he says that's likely an underestimate because people tend to overestimate how active they are. When it came to food, Ladabaum expected to find people eating more over the past two decades. Instead, he found the daily calorie counts stayed about the same.

LADABAUM: Which may seem a little surprising given the trends in portion size and so on and the popular belief that we're eating more and more. So it may well be that we're eating too much, but it doesn't seem that we are accelerating our caloric intake.

NEIGHMOND: Ladabaum says it's difficult to figure out how much of the epidemic comes from eating too much and how much from not getting enough exercise.

LADABAUM: This is something that some of my colleagues have framed as, is it gluttony or sloth? Those are not nice words, but those are the terms in which some people think about this debate. And I think it's clear that there are contributions to this both from the energy-intake side, how much we're eating, and the energy-expenditure side. So anyone looking for a simple explanation, it's either A or B; we're not going to find that. It's clearly both.

NEIGHMOND: And when it comes to activity, our lives have literally done a 180, not only during leisure time but also on the job. Dr. Tim Church is an obesity specialist at the Pennington Biomedical Research Center

TIM CHURCH: In 1960, 1 of 2 Americans had a job which required a lot of physical activity. They essentially exercised at work. By the year 2008, very few Americans - actually we, you know, do work that doesn't involve sitting around all day.

NEIGHMOND: Church recently looked at federal job surveys over 40 years.

CHURCH: And we have these great old pictures of these cars being built in the '60s, and these men were physically picking up a bumper and putting it onto the car. And then you get these pictures of these cars being built, you know. Now there's not even a human being in the room. There's these computers that grab the bumpers and put it onto the car, and then it's bolted on by another big, electronic arm.

NEIGHMOND: Church wondered how today's lack of movement translates into fewer calories burned and more weight gained. He compared food and activity levels in the 1960s to today and calculated that men now burn 140 fewer calories a day and women 120.

CHURCH: That doesn't sound like much, but when it's day after day after day after day, it adds up.

NEIGHMOND: And often means weight gain over months and years. But Church says there are simple ways to combat this. It takes commitment, and - you probably know what it is - moderate exercise for 30 minutes at least five days a week. Patti Neighmond, NPR News.

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