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Processed foods are usually more convenient and cost less, so you’ll eat more and gain weight more quickly. Getty Images

If you eat a diet high in ultra-processed foods, odds are you’re eating more than you would with an unprocessed diet.

Researchers have long suspected a link between ultra-processed foods and the obesity epidemic.

Now, in a report published in the journal Cell Metabolism, experts have undertaken the first randomized controlled trial comparing differences in calorie consumption between unprocessed and ultra-processed diets.

Even when the two diets in the trial were matched for things such as fat content, participants on the ultra-processed diet still ate more food and gained more weight.

“Because the meals were designed to be matched for carbohydrates, fat, sugar, salt, and calories, I suspected we would find little difference between them in terms of overall calorie intake or weight change,” Kevin Hall, PhD, lead author of the study and a section chief in the Laboratory of Biological Modeling at the National Institute of Diabetes and Digestive and Kidney Diseases within the National Institutes of Health (NIH), told Healthline.

However, Hall said researchers observed that the ultra-processed diet increased calorie intake by about 500 calories per day.

“This was a surprise,” Hall said.

He said the consumption of ultra-processed foods has been associated with obesity and poor health in observational studies, but there’s never been a randomized controlled trial testing whether processed foods actually cause overeating or weight gain.

“Most nutrition studies focus on the nutrient content of foods, so I was skeptical about whether the processing of foods has any importance apart from their nutrient content,” Hall said. “So we thought it was important to conduct the first [randomized controlled trial] where subjects were exposed to ultra-processed diets versus unprocessed diets matched for a variety of nutrients to see if ultra-processed foods caused overeating and weight gain.”

In undertaking the research, Hall and his team enlisted 20 healthy volunteers.

They were admitted to the NIH’s Metabolic Clinical Research Unit for one month.

The participants were allocated either an ultra-processed diet or unprocessed diet for two weeks, then switched.

They ate three meals a day and were given bottled water and snacks that were either ultra-processed or unprocessed. Participants could eat as much as they wanted and the quantities of all that they ate were measured.

The NOVA food classification system, which categorizes foods based on how much they’ve been processed, was used by the researchers to choose foods for the participants.

On the ultra-processed diet, participants were given Honey Nut Cheerios, whole milk with added fiber, and a blueberry muffin with margarine for breakfast.

The participants who ate an unprocessed diet were given a parfait with strawberries, banana, walnuts, salt, olive oil, Greek yogurt, and apple slices with a squeeze of lemon.

After two weeks eating an ultra-processed diet, participants gained an average of two pounds. Those on the unprocessed diet had an average weight loss of two pounds.

Dana Hunnes, PhD, a senior dietitian at the University of California Los Angeles Medical Center, said the research results aren’t surprising.

“Ultra-processed diets tend to have foods in them that are more calorically dense and with less water content, making each individual food less satiating and satisfying,” she told Healthline. “To achieve the same satiation in the stomach — or sense of fullness — which may have more to do with volume than calorie intake, it would make sense that more of the calorically dense foods would be eaten (and therefore more calories) than when eating an unprocessed diet.”

The researchers hypothesized a few reasons why the participants on an ultra-processed diet ate more food.

One reason is the speed they ate at.

“People ate the ultra-processed food faster, and this may have contributed to overeating,” Hall said.

Kristin Kirkpatrick, MS, a licensed, registered dietitian and manager of wellness nutrition services at the Cleveland Clinic Wellness Institute in Ohio, says people need to be mindful of how quickly they eat their food.

“Many studies cite chewing longer and mindfulness as effective tactics for weight loss,” she told Healthline. “Eating fast obviously means eating more — more food, more calories, and too much fuel at the end of the day that can’t be burned off, which results in abundant energy still there before you go to bed.”

Researchers acknowledge that a notable limitation to the study was that all food given to participants was prepared by someone else. This doesn’t consider convenience or cost of foods eaten, which are common reasons a person may choose something ultra-processed.

“Processed food is quick, and in these busy times people often go for fast and easy,” Lauri Wright, PhD, an assistant professor in public health at the University of South Florida, told Healthline.

“Ultra-processed foods often have high amounts of sugar, sodium, and fat. Examples of these foods to avoid include chips, hot dogs, instant soups, soft drinks, and packaged baked goods,” she said.

Not all processed foods are bad.

Wright says some minimally processed foods can be helpful for busy people.

“Any time we cook, bake, or prepare food, we’re processing food,” she said. “Minimally processed food can actually help you eat more nutrient-dense foods. Milk and juices are sometimes fortified with calcium and vitamin D, and breakfast cereals may have added fiber. Canned fruit is a good option when fresh fruit isn’t available. Some minimally processed food such as pre-cut vegetables and pre-washed spinach are quality convenience foods for busy people.”

If you want to minimize your intake of processed food, aim to do more food prep and cooking at home. “Base your meals on whole foods, including vegetables, beans, and whole grains,” Wright said. “Eating processed food in moderation is fine, but avoid those with a lot of added sugar, fat, and sodium.”