Ultra-processed foods make it difficult to lose weight, even if you follow a healthy diet.

Sodas, pastries, but also cereal bars and spaghetti. Ultra-processed foods are industrial preparations that artificially stimulate the appetite . If you're looking at a packaged product with a long list of ingredients and convoluted names, it's probably one of them. The problem is that they taste good (too good, perhaps) and some can cause something close to addiction . Their consumption is so widespread that it reaches the palates of millions of people. The cost of this popularity is high, as it has been shown to be associated with an increased risk of obesity and cardiometabolic diseases.

Now, a new finding has put them back in the spotlight. A group of researchers from Global University London (United Kingdom) demonstrated for the first time how this type of diet, even if it follows a healthy pattern, is less effective for weight loss. The study, led by Samuel Dicken of the Obesity Research Centre in London, analyzed the behavior of 55 adults divided into two groups for eight weeks. One group began the trial with a low-ultraprocessed food (LFP) diet, such as overnight oats or homemade spaghetti Bolognese.

After a four-week rest period, during which the participants returned to their usual diet, they switched to a diet containing ultra-processed foods (UPF), such as oatmeal bars for breakfast or prepared lasagna. The second group followed the reverse order. Both diets adhered nutritionally to the recommendations of the Eatwell Guide , the official U.S. dietary guidelines for a healthy and balanced diet.

After the trial period, the results of the analysis published in the journal Nature showed that all participants lost weight, likely as a result of a better diet compared to their usual diet. Despite this, the effect was greater—a 2.06% reduction—with the low-ultraprocessed food diet than with the UPF diet, which saw a 1.05% reduction. “Although the diets were nutritionally balanced, the form and level of food processing appeared to influence the amount they ate and their feelings of satiety,” Dicken said at a press conference on Monday.

Participants were given more calories than they needed and instructed to eat as much or as little as they wanted. The lead author argues that if these results are extended over a year, there could be a 13% weight loss in men and a 9% weight loss in women on the minimally processed diet, but only a 4% weight loss in men and a 5% weight loss in women on the ultra-processed diet. This, over time, would make "a big difference." This, according to the authors, remains speculative and would require long-term confirmation.

One of the most curious observations of the research is that during the period in which participants consumed minimally processed foods, several subjects were lost to follow-up. One person did not comply with the protocol, and five dropped out of the study during that phase. Furthermore, two other subjects were also lost due to the same type of diet. Jordi Salas-Savadó , professor and director of the Human Nutrition Unit at Rovira i Virgili University, believes that although it is "a fairly comprehensive study," there is "a gray area" in this regard.

“Supplementary information could shed more light on these losses. In total, 10 of the 50 participants were lost, which represents 20% of the sample,” says Salas-Savadó, adding, “This is a considerable proportion. It's not clear what happened to these people.”

In this regard, the lead author assured EL PAÍS that the study included all participants who were part of the trial from the beginning. "We evaluated the overall effect, regardless of whether they were able to continue," he explains. And that although they tried to motivate the groups to continue, sometimes it wasn't possible because many participants' personal circumstances weighed more heavily.

“Even with these limitations, it's the best we currently have and is in line with epidemiological studies that have shown that high consumption of ultra-processed foods is associated with an increased risk of disease and mortality,” says Salas-Savadó.

Ultra-processed foods are cheap

The authors gave participants the same official UK dietary recommendations, but at different times. One phase included ultra-processed foods and the other minimally processed foods. Samuel Dicken and co-author Adrian Brown published another study in 2024 on whether nutritional labeling helps people make healthier dietary choices.

What they found was that, overall, UPFs generally had worse scores for calories, saturated fat, sugar, and salt. Despite this, when analyzing the red traffic light labels—the system used to identify unhealthy products—a significant number of UPFs lacked the labels. “This highlights that UPFs are a heterogeneous category. Not all are nutrient-poor, and nutrition labels alone don’t capture the level of processing,” Brown adds.

With all the evidence now emerging, it's not enough to simply eat a balanced diet; quality must also be considered. In Spain, according to a 2018 study , 20.3% of the foods Spaniards consume are ultra-processed. There's a need to shift the political focus from individual responsibility to the environmental factors that drive obesity, such as the influence of multinational corporations or the creation of unhealthy food environments.

According to Jordi Salas-Savadó, this type of analysis needs to continue, as in the long run it will be important to "incorporate this dimension into dietary guidelines." Foods have multiple variables: nutritional value, degree of processing, presence of potentially toxic additives, and even their environmental impact.

This professor and his colleagues hope to launch a similar study in the country next year. They are currently in the drafting phase of the project. However, Salas-Savadó reiterates that if more evidence is generated that ultra-processed foods have negative effects on health, it will be necessary to consider how to intervene at the government level.

"This could translate into stricter recommendations from institutions or even fiscal policies," concludes this professor.