At the start of his latest clinical trial in 2018, National Institutes of Health researcher Kevin Hall was sure he wouldn’t see a difference.

His study, intended to monitor caloric intake and weight gain, offered its participants one of two nearly identical menus. Both contained the same number of calories, and comparable amounts of carbohydrates, fats, and proteins. Even the diets’ fiber, sugar, and sodium contents were matched. Nutrient-wise, they were about as similar as two meal plans could get.

But as the days ticked by, Hall quickly began to see how wrong his initial hunch had been. Despite the superficial similarities, one group was eating much more of the food they were offered. And by the end of two weeks, the members of that same group had gained an average of two pounds, while their counterparts had lost two pounds.

The only explanation was the one factor Hall had thought would have no effect at all: While one menu was made up mostly of whole, unprocessed foods, the other—the one tied to weight gain—was composed almost entirely of ultra-processed foods.

Compared to unprocessed foods like fresh fruits and nuts, ultra-processed foods like cookies and chips tend to have more calories, sugar, fat, and salt, all of which have been linked to putting on weight. But the findings from Hall’s team, published today in the journal Cell Metabolism, are the first to show there’s something inherent to ultra-processed foods, independent of nutritional makeup, that seems to encourage overeating.

“This is really important work,” says Dana Small, a psychologist and neuroscientist studying food choice at Yale University who was not involved in the study. “This study produces a definitive answer to a question we did not have a definitive answer to.”

link to study

https://www.ncbi.nlm.nih.gov/pubmed/31269427