BETHESDA, MD. — Results from a National Institutes of Health study found people eating ultra-processed diets consumed more calories and gained more weight than when they ate a minimally processed diet.

The study involving 20 adults appeared online May 16 in Cell Metabolism. Researchers considered foods to be ultra-processed if they had ingredients predominantly found in industrial food manufacturing, including hydrogenated oils, high-fructose corn syrup, flavoring agents and emulsifiers.

The ultra-processed meals and the minimally processed meals contained the same amounts of calories, sugar, fiber, fat and carbohydrates. People could eat as much as they wanted. People on the ultra-processed diet on average ate 508 more calories per day than people on the minimally processed diet. People on the ultra-processed diet on average gained 0.9 kilograms (2 lbs). People on the minimally processed diet on average lost 0.9 kilograms.

“Though we examined a small group, results from this tightly controlled experiment showed a clear and consistent difference between the two diets,” said Kevin D. Hall, Ph.D., the study’s lead author and a senior investigator at the National Institutes of Health’s National Institute of Diabetes and Digestive and Kidney Diseases. “This is the first study to demonstrate causality — that ultra-processed foods cause people to eat too many calories and gain weight.”

Ten men and 10 women lived at the National Institutes of Health’s Clinical Center in Bethesda for a month. Their average age was 31. Their average body mass index was 27.

They spent two weeks on an ultra-processed diet and then two weeks on a minimally processed diet. An example of an ultra-processed breakfast was a bagel with cream cheese and turkey bacon. An example of a minimally processed breakfast was oatmeal with bananas, walnuts and skim milk.

Support for the study primarily came from the National Institute of Diabetes and Digestive and Kidney Diseases.

The researchers noted 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,” Dr. Hall said. “Just telling people to eat healthier may not be effective for some people without improved access to healthy results.”

Researchers estimated the weekly cost for ingredients to prepare ultra-processed meals with a total daily calorie count of 2,000 was $106, which compared to $151 to prepare minimally processed meals.

Researchers also noted differences in carbohydrate intake and fat intake. People on the ultra-processed diet on average consumed 280 more calories per day and 230 more calories of fat per day than people on the minimally processed diet.

Researchers noted the ultra-processed meals and minimally processed meals differed in proportions of added sugar to total sugar (54% for ultra-processed versus 1% for minimally processed), insoluble fiber to total fiber (77% versus 16%) and saturated fat to total fat (34% versus 19%). The ratio of omega-6 fatty acids to omega-3 fatty acids was 11:1 in the ultra-processed meals and 5:1 in the minimally processed meals.