A preliminary study reports that eating ultra-processed leads to eating more calories and weight gain.

Instant Noodles.

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People who eat ultra-processed foods have a higher calorie intake and gain more weight compared to those who eat a minimally-processed diet, a new study from the National Institutes of Health reports. This difference, the team explains, was seen even when participants in the ultra-processed and minimally-processed diet groups received the same number of calories and macronutrients in their food.

Processed

“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., an NIDDK senior investigator and the study’s lead author.

“This is the first study to demonstrate causality — that ultra-processed foods cause people to eat too many calories and gain weight.”

The study, at the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), was small-scale — it only included 20 adult volunteers. The authors report that it is the first randomized trial meant to look into the effects of ultra-processed foods as defined by the NOVA classification system. Previous observational studies, they explain, worked with large groups of people and have uncovered an association between diets with high amounts of processed food and health complications. However, these efforts had been randomized, so they can’t be used to establish a clear link between the two (people might have experienced health complications due to other factors, such as lack of access to fresh food, not necessarily from ultra-processed ones).

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Under the NOVA system, foods that have ingredients predominantly found in industrial food manufacturing, such as hydrogenated oils, high-fructose corn syrup, flavoring agents, and emulsifiers, are considered to be “ultra-processed”.

“Results from this tightly controlled experiment showed a clear and consistent difference between the two diets,” said Kevin D. Hall, Ph.D., an NIDDK senior investigator and the study’s lead author. “This is the first study to demonstrate causality — that ultra-processed foods cause people to eat too many calories and gain weight.”

The participants, 10 male and 10 female, were admitted to the NIH Clinical Center for one continuous month. They were placed on each diet for two weeks (in random order), the team providing them with meals consisting of either ultra-processed or minimally processed foods. An ultra-processed breakfast, for example, might consist of a bagel, cream cheese, and turkey bacon, while the unprocessed breakfast was oatmeal with bananas, walnuts, and skim milk. Meals in both courses were controlled to have the same amounts of calories, sugars, fiber, fat, and carbohydrates. Participants were allowed to eat as much as they wanted.

People on the ultra-processed diet ate about 500 calories more per day than those on the unprocessed one. They also ate faster and gained weight, whereas their counterparts lost weight. On average, participants in the ultra-processed group gained 2 pounds (0.9 kilograms) 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,” Hall admits. For example, the team says that slight differences in protein levels between the ultra-processed and unprocessed diets used in the study could explain up to half of the difference in caloric intake between the two groups.

“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,” Hall explains.

“Over time, extra calories add up, and that extra weight can lead to serious health conditions,” said NIDDK Director Griffin P. Rodgers, M.D. “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 said. “Just telling people to eat healthier may not be effective for some people without improved access to healthy foods.”

The paper “Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake” has been published in the journal Cell Metabolism.