AUDIE CORNISH, HOST:
Ultra-processed foods dominate the American diet - chips, soda and packaged snack bars but also breakfast cereals, energy bars and ready-to-eat meals. The rise of these highly processed foods has coincided with growing rates of obesity. That's led many to suspect these foods have played a big role in our growing waistlines. But is it something about the highly processed nature of these foods that drives people to overeat? A new study suggests the answer is yes. NPR's Maria Godoy reports.
MARIA GODOY, BYLINE: The study comes from researchers at the National Institutes of Health. They recruited 20 healthy adult men and women to live at NIH for a whole month.
KEVIN HALL: You know, we actually have to house people and measure every morsel of food that they eat because we can't rely on self-report.
GODOY: That's NIH researcher Kevin Hall. He led the study. One group of people was randomly assigned to eat an ultra-processed diet. So think foods like packaged white bread, eggs made with liquid mix and peaches canned and heavy syrup. The other group ate unprocessed foods. After two weeks on one diet, the groups switched to the opposite diet. Both groups were allowed to eat as much or as little as they wanted, and meals in both diets contained the same number of calories, carbs, protein and other nutrients.
HALL: This is, I think, critically important - a diet that's matched for a lot of the usual suspects that people think of when they think of ultra-processed foods.
GODOY: Specifically fat, salt and sugar. And sugar is known to drive hunger.
HALL: Our study didn't allow the diets to be different in that regard. And yet we still saw a big effect of the ultra-processed diet.
GODOY: How big an effect? People on the highly processed diet ate about 500 calories more per day, and they gained an average of 2 pounds over two weeks. People on the unprocessed diet - they ended up losing an average of 2 pounds over the same period.
DARIUSH MOZAFFARIAN: You know, these are landmark findings that the processing of the foods makes a huge difference in how much a person eats.
GODOY: That's Dariush Mozaffarian. He's the dean of the Tufts Friedman School of Nutrition Science and Policy. He says while the study was small, it's significant because it was so highly controlled. Unlike prior observational studies that linked ultra-processed foods to weight gain, this study can show cause and effect.
MOZAFFARIAN: These findings are really consistent with a lot of evidence that, you know, we can't judge a food by its calories.
GODOY: In other words, for healthy weight management, eat real foods like fruits and vegetables, whole grains, fish and nuts. Maria Godoy, NPR News.
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