A National Institutes of Health (NIH) study has become the first to establish causality between the consumption of ultra-processed foods and overeating and weight gain. The findings compared the health outcomes of one group eating an ultra-processed diet, with those following a minimally processed diet – even though meals provided in both diets had the same number of calories and macronutrients. The results were published in Cell Metabolism. The researchers note that while the study reinforces the benefits of unprocessed foods, ultra-processed foods can be difficult to restrict in terms of economic income. Therefore, access to healthy foods for all in the US must be improved.

The results come ahead of European Obesity day, on May 18. By 2030, it is expected that over half of Europe will have obesity, with up to 89 percent in some countries, while 39 percent of adults aged 18 years and over were overweight in 2016 worldwide. European Obesity Day aims to raise awareness and increase knowledge about obesity and the diseases on which it impacts, as well as remind society that obesity is preventable with a combination of exercise and decreased calorie intake.

For the study, researchers admitted 20 healthy adult volunteers, ten male and ten female, to the NIH Clinical Center for one month. In random order, for two weeks, subjects were provided with meals comprising either ultra-processed foods or minimally processed foods. For example, an ultra-processed breakfast might consist of a bagel with cream cheese and turkey bacon, while an unprocessed breakfast might serve oatmeal with bananas, walnuts and skim milk.

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.

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 two pounds, while they were on the ultra-processed diet and lost an equivalent amount on the unprocessed diet.

“We need to figure out what specific aspect of the ultra-processed foods affected people’s eating behavior and led them to gain weight,” says Hall. “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.”

For example, 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.

“Over time, extra calories add up, and that extra weight can lead to serious health conditions,” says NIDDK Director Griffin P. Rodgers, MD. “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.”

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