In the first study of its kind, scientists have shown that eating ultra-processed foods leads to weight gain in human volunteers in as little as 2 weeks.

There are plenty of studies in mice linking processed foods to problems such as obesity and intestinal inflammation. But mice are not people, as critics of such studies are quick to point out. In humans, researchers have reported associations between processed foods and health outcomes, such as an increased risk of developing obesity, cancer, autoimmune conditions, and even death.

Yet, ultra-processed foods make up a staggering 57.9% of energy intake in the United States.

According to the NOVA food classification system, ultra-processed foods include soft drinks, packaged snacks, meat nuggets, frozen meals, and foods high in additives and low in unprocessed ingredients.

"Previous studies have found correlations between ultra-processed food consumption and obesity," Kevin D. Hall, from the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, MD, which is part of the National Institutes of Health (NIH), explained to Medical News Today.

Hall and his colleagues now present the results of a controlled clinical trial, comparing the effects of unprocessed vs ultra-processed foods on humans in the journal Cell Metabolism.

'Surprised by the findings'

The research team recruited 10 male and 10 female volunteers who stayed at the NIH Clinical Center for 28 days.

Half of the participants ate ultra-processed food for the first 2 weeks while the others received unprocessed foods. After the 2-week period, the groups switched, allowing each participant to eat both the ultra-processed food and the unprocessed food for 2 weeks.

The volunteers ate three meals per day, and the researchers asked them to eat as much or as little as they wanted. They also had access to snacks and bottled water all day.

"We hypothesized that ultra-processed foods might lead to increased calorie intake because they are often high in sugar, fat, and salt while being low in fiber," Hall told MNT. "Therefore, when we matched the ultra-processed and unprocessed diets for these nutrients, we expected the ultra-processed diet to result in similar calorie intake and little differences in body weight."

When the volunteers were on the ultra-processed diet, however, they ate an average of 508 calories more each day than when they were on the unprocessed diet. As a result, they put on an average of 2 pounds (0.9 kilograms) during this time, mostly in the form of body fat.

"I was surprised by the findings from this study because I thought that if we matched the two diets for components like sugars, fat, carbohydrates, protein, and sodium, there wouldn't be anything magical about the ultra-processed food that would cause people to eat more."

—Kevin D. Hall

Participants in the unprocessed food group lost an average of 0.9 kg during the 2-week study period. This group also saw increases in the gut hormone peptide YY, which suppresses hunger, and decreases in the hunger hormone ghrelin.

Speed might be the problem

There are several reasons that Hall and his colleagues think may have led the volunteers in the ultra-processed study group to put on weight. Although the study participants rated the pleasantness and familiarity of the diets as equal, they ate significantly faster in the ultra-processed group.

In fact, they consumed an extra 17 calories, or 7.4 grams of food per minute, than their counterparts in the unprocessed food group.

"There may be something about the textural or sensory properties of the food that made them eat more quickly," Hall comments. "If you're eating very quickly, perhaps you're not giving your gastrointestinal tract enough time to signal to your brain that you're full. When this happens, you might easily overeat."

Despite a close match in the macronutrient composition of both diets, the unprocessed diet contained slightly more protein. "It could be that people ate more because they were trying to reach certain protein targets," Hall comments.

Yet the team found that the ultra-processed food group actually consumed more carbohydrates and fat than the unprocessed food group, but not protein.

Finally, the meals in the ultra-processed group had a higher energy density than in the unprocessed group, which Hall proposes "likely contributed to the observed excess energy intake."

Are ultra-processed foods a social problem?

The authors identify several limitations in their study, which include that "the inpatient environment of the metabolic ward makes it difficult to generalize our results to free-living conditions." They also acknowledge that they did not take into consideration how cost, convenience, and skill influence consumers to choose ultra-processed over unprocessed foods.

"Ultra-processed foods contribute to more than half the calories consumed in the United States, and they are cheap and convenient options," Hall commented to MNT. "So, I think it may be difficult to substantially reduce consumption of ultra-processed foods," he continued, "especially for people in lower socioeconomic brackets who may not have the time, skill, equipment, or resources to purchase and safely store unprocessed food ingredients and then plan and safely prepare tasty, unprocessed meals."

In the paper, Hall concludes, "However, policies that discourage consumption of ultra-processed foods should be sensitive to the time, skill, expense, and effort required to prepare meals from minimally processed foods—resources that are often in short supply for those who are not members of the upper socioeconomic classes."

He is not the first to suggest a connection between socioeconomics and food choices. A recent, large-scale study in the journal Nature suggests that in high-income countries, such as the United States, rural populations are putting on weight faster than their city counterparts.

The authors in that study comment that this may, in part, be due to "economic and social disadvantage, including lower education and income, lower availability, and higher price of health[ful] and fresh foods."