(CNN) Less-expensive, easier-to-prepare ultraprocessed foods can make you fat, a new study says.
People limited to a diet of primarily highly processed foods ate more calories and gained more weight than when their diet mostly consisted of minimally processed foods, finds the study, published Thursday in the journal Cell Metabolism.
This small-scale study is the first randomized controlled trial -- considered the gold standard in science -- examining the effects of ultraprocessed foods. They are defined as containing industrial ingredients, such as hydrogenated oils, high-fructose corn syrup, flavoring agents and emulsifiers.
During a one-month study, 20 healthy adult volunteers stayed at the National Institutes of Health Clinical Center, where all their meals were provided to them. For 14 days in turn, they were limited to each diet and told they could eat as much or as little as they liked.
The two versions of meals had the same amounts of calories, sugars, fiber, fat and carbohydrates. For example, the unprocessed breakfast might be oatmeal with bananas, walnuts and skim milk, while the other consisted of a bagel with cream cheese and turkey bacon.
Participants exercised about the same amount each day throughout the study.
On the ultraprocessed diet, people ate faster while consuming about 500 calories more per day (by taking extra helpings) than they did while on the unprocessed diet; this increase in calories was due to higher quantities of carbohydrates and fat but not protein. As a result, they gained weight -- on average, about 0.9 kilograms or 2 pounds. While on the diet of unprocessed foods, they lost an equal amount of weight.
The gender of the participants, the order of their diet assignment and their body mass index did not influence the varying calories each participant ate on each diet, according to the study authors.
The ultraprocessed foods caused people to eat too many calories and gain weight, they concluded.
Engineered and heavily processed foods can be difficult to restrict due to reasons beyond taste, the researchers noted. For example, the weekly cost of a 2,000-calorie-per-day diet of processed meals was estimated to be $106, versus $151 for the more natural meals.
It also takes more time to prepare less-processed foods, they said.
Ultraprocessed foods are mostly consumed as ready-to-eat meals, as well as snacks and desserts. People have been eating more of them over the past several decades. In the United States, 61% of adults' total diet comes from ultraprocessed foods; in Canada, it is 62%; and in the UK, it is 63%, a recent unrelated study found.
Research also shows that industry engineered foods can lead to obesity, high blood pressure and cancer.
"We are living in a fast world, and people are looking for convenient solutions," Nurgul Fitzgerald, an associate professor in the Department of Nutritional Sciences at Rutgers University, said of a previous study on processed foods.
Fitzgerald recommends reviewing the back of a package of ready-made food. "Look at the ingredients list. Do you understand all those ingredients that go into your foods?" she asked. Buy only those products "with the least number of ingredients and with ingredients you understand."
The study authors suggest that consumer confusion is another aspect of this problem.
"The perpetual diet wars between factions promoting low-carbohydrate, keto, paleo, high-protein, low-fat, plant-based, vegan, and a seemingly endless list of other diets have led to substantial public confusion and mistrust in nutrition science," they wrote. "Limiting consumption of ultra-processed foods may be an effective strategy for obesity prevention and treatment."