A small but tightly controlled study has become the first to demonstrate a clear link between eating ultra-processed foods and the buildup of more calories and increased weight in people compared to eating a minimally processed diet.

The study by the National Institutes of Health (NIH) is the first randomized controlled trial examining the effects of ultra-processed foods as defined by the NOVA classification system. NOVA defines ultra-processed as those with ingredients mostly found in industrial food manufacturing.

These ingredients include hydrogenated oils, high-fructose corn syrup (HFSC), flavoring agents and emulsifiers. The study was published in the journal Cell Metabolism.

This small-scale study of 20 adult volunteers was conducted by researchers at the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK),

NIH said previous observational studies looking at large groups of people revealed associations between diets high in processed foods and health problems. But since none of these studies randomly assigned people to eat specific foods and then measured the results, scientists couldn’t say for sure if the processed foods were a problem on their own, or if people eating them had health problems or other reasons.

“Though we examined a small group, results from this tightly controlled experiment showed a clear and consistent difference between the two diets,” Dr. Kevin D. Hall, Ph.D., the study’s lead author, said. “This is the first study to demonstrate causality -- that ultra-processed foods cause people to eat too many calories and gain weight.”

Twenty healthy adult volunteers (10 male and 10 female) were provided with meals made up of ultra-processed foods or meals of minimally processed foods in random order for two weeks on each diet. The ultra-processed and unprocessed meals had the same amounts of calories, sugars, fiber, fat and carbohydrates. Participants could eat as much or as little as they wanted.

Hall said they needed 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’s effect on calorie intake and body weight disappear.

“Over time, extra calories add up, and that extra weight can lead to serious health conditions,” said NIDDK director Griffin P. Rodgers, M.D. “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.”