Ultra-processed foods cause overeating, weight gain, NIH study says

Photo Credit: Jennifer Rymaruk, NIDDK 

Ultra-processed foods lead to higher calorie intake and more weight gain than a minimally-processed diet, according to a new National Institutes of Health (NIH) study published in the journal Cell Metabolism.

The small-scale study of 20 adult volunteers, conducted by researchers at the NIH's National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), admitted 10 men and 10 women to the NIH Clinical Center for one continuous month and, in random order for two weeks on each diet, provided them with meals made up of ultra-processed foods or meals of minimally processed foods.

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, according to the study abstract. 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, the study said.

On the ultra-processed diet, people ate about 500 calories more per day than they did on the unprocessed diet. They also ate faster on the ultra-processed diet and gained weight, whereas they lost weight on the unprocessed diet. Participants, on average, gained 0.9 kilograms, or 2 pounds, while they were on the ultra-processed diet and lost an equivalent amount on the unprocessed diet. Slight differences in protein levels between the ultra-processed and unprocessed diets in this study could potentially explain as much as half the difference in calorie intake, researchers said.

The study is the first randomized controlled trial examining the effects of ultra-processed foods as defined by the NOVA classification system. This system considers foods "ultra-processed" if they have ingredients predominantly found in industrial food manufacturing, such as hydrogenated oils, high-fructose corn syrup, flavoring agents, and emulsifiers.

Previous observational studies looking at large groups of people had shown associations between diets high in processed foods and health problems. But, because none of the past studies randomly assigned people to eat specific foods and then measured the results, scientists could not say for sure whether the processed foods were a problem on their own, or whether people eating them had health problems for other reasons, such as a lack of access to fresh foods, according to Kevin Hall, PhD, NIDDK senior investigator and the study's lead author. 

"We need to figure out what specific aspect of the ultra-processed foods affected people's eating behavior and led them to gain weight," Hall said in a statement. "The next step is 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."

While the study reinforces the benefits of unprocessed foods, researchers said 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," Hall said. "Just telling people to eat healthier may not be effective for some people without improved access to healthy foods."