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The experiment
The experiment ran in an enclosed, semi-prison setting. Everyone ate matched meals for two weeks: one group's were homemade, the other's ultra-processed, and the calories and macros were the same. The ultra-processed group kept coming back to the buffet for more.
Why people ate more
Nobody knows for sure why. The chemicals in ultra-processed food could affect the gut microbes, which then send signals to the brain to eat more, as if something strange were going on in the body. The structure of the food, its matrix, could also play a part.
Another possibility is speed. Ultra-processed food can be absorbed much faster than food in its natural form, which brings a sugar rush and floods the body with nutrients before the brain has time to register being full. That fullness signal normally takes around 20 minutes to arrive.
Why does eating speed change how full people feel?
Fullness signals come from hormones such as cholecystokinin, peptide YY, and GLP-1, which the gut releases as food reaches the small intestine. These signals build over many minutes, so a meal eaten quickly can be finished before they register. In a 2019 trial at the US National Institutes of Health led by Kevin Hall and published in Cell Metabolism, volunteers ate ultra-processed meals noticeably faster than unprocessed ones. Soft, energy-dense food needs less chewing, which raises how many calories a person takes in per minute.
How does food structure affect absorbed calories?
Plant foods hold their fat and starch inside cell walls, and digestion has to break those walls open before the nutrients can be absorbed. Grinding or refining removes that barrier. A 2012 USDA study by Rachel Novotny and colleagues in the American Journal of Clinical Nutrition found that whole almonds delivered about 32 percent fewer calories than standard calculations predicted. Part of the fat stayed locked inside intact cells and left the body undigested.
How do gut microbes influence appetite?
Gut bacteria ferment fiber into short-chain fatty acids such as acetate, propionate, and butyrate. These acids prompt gut cells to release the fullness hormones GLP-1 and PYY. In a 2014 paper in Nature Communications, Gary Frost's group at Imperial College London reported that acetate made by gut bacteria reaches the brain and reduces appetite in mice.
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