"Though we examined a small group, results from this tightly controlled experiment showed a clear and consistent difference between the two diets. This is the first study to demonstrate causality that ultra-processed foods cause people to eat too many calories and gain weight," wrote the study's lead author Kevin D. Hall in the study published in the Journal of Cell Metabolism.
For the study, researchers admitted 20 healthy adult volunteers, 10 male and 10 female, for one continuous month and, in random order for two weeks on each diet, providing them with meals made up of ultra-processed foods or meals of minimally processed foods.
For example, an ultra-processed breakfast might consist of a bagel with cream cheese and turkey bacon, while the unprocessed breakfast was oatmeal with bananas, walnuts, and skim milk.
The ultra-processed and unprocessed meals had the same amounts of calories, sugars, fiber, fat, and carbohydrates, and participants could eat as much or as little as they wanted.
"We need to figure out what specific aspect of the ultra-processed foods affected people's eating behavior and led them to gain weight. The next step was to design similar studies with a reformulated ultra-processed diet to see if the changes can make the diet effect on calorie intake and body weight disappear," Hall said.
"Over time, extra calories add up, and that extra weight can lead to serious health conditions. Research like this is an important part of understanding the role of nutrition in health and may also help people identify foods that are both nutritious and accessible helping people stay healthy for the long term," said one of the researchers, Griffin P. Rodgers
While the study reinforces the benefits of unprocessed foods, researchers note that ultra-processed foods can be difficult to restrict.
"We have to be mindful that it takes more time and more money to prepare less-processed foods. Just telling people to eat healthier may not be effective for some people without improved access to healthy foods", explained Hall.