New research suggests that people can lose more weight by avoiding ultraprocessed foods, even those that are typically considered healthy.
The study, published today in the journal Nature Medicine, is the largest and longest clinical trial yet to examine the effects of ultraprocessed foods on weight. Participants lost twice as much weight when they followed diets made up of minimally processed foods, like pasta, chicken, fruits and vegetables, as they did when they followed diets with ultraprocessed foods that met nutrition standards, such as ready-to-heat frozen meals, breakfast cereals, protein bars and shakes.
Federal officials have been sounding the alarm about ultraprocessed foods, which account for about 70 percent of the food supply in the United States. Robert F. Kennedy Jr., the health secretary, said that ultraprocessed foods were “poisoning” Americans, and called them a primary culprit of high rates of obesity and chronic diseases.
Although numerous studies have linked ultraprocessed foods to obesity, most have been observational, meaning they can’t prove that the foods directly cause weight gain.
Two previous trials found that adults consumed about 500 to 800 more calories per day when their diets were made up of ultraprocessed foods than when they were made up of minimally processed foods. But those studies were small and short; the larger of the two, conducted at the National Institutes of Health, included 20 participants who followed each diet for just two weeks. Critics have argued that the results might have been different if the trials were longer, or if they included healthier ultraprocessed foods.
The new study, though still small, was designed to address some of those concerns, said Samuel Dicken, a research fellow at University College London, and the lead author of the study.
Losing weight and body fat
Dr. Dicken and his colleagues recruited 55 participants, most of whom were women, ranging in age from their early 20s to their mid-60s, Dr. Dicken said. All had body mass indexes in the overweight or obesity ranges, and before the study, about two-thirds of their calories came from ultraprocessed foods — more than the average for adults in Britain.
The researchers designed two diets and provided the meals. Both diets met U.K. nutrition guidelines, with limited sugars, saturated fats and sodium, but one was composed mostly of minimally processed foods, like overnight oats, plain yogurt and scratch-made spaghetti Bolognese; the other was mostly made up of ultraprocessed foods considered healthy, like whole grain breakfast cereals, plant-based milk, flavored yogurt and frozen lasagna.
Half the participants followed the minimally processed diet for two months, after which they returned to their normal diets for one month. Then they followed the ultraprocessed diet for two months. The other half followed the diets in the opposite order. All could eat as much or as little as they liked.
This kind of “crossover” study design is strong because it can show how each diet affected each participant, rather than averaging the responses across a group, said Brenda Davy, a professor of nutrition at Virginia Tech, who was not involved in the study.
Most of the participants lost weight on both diets. But on average, they lost more weight during the two months on the minimally processed diet — about four pounds compared with just two pounds on the ultraprocessed diet. Dr. Dicken and his colleagues estimated that if the weight loss had continued over one year, even as it naturally slows with time, it could have added up to nine to 13 percent of body weight on the minimally processed diet, compared with just four to five percent on the ultraprocessed diet.
The participants also lost more than twice as much body fat on the minimally processed diet than they did on the ultraprocessed diet.
Fewer calories per bite, and fewer food cravings
It was somewhat surprising — and encouraging — that people lost weight on the ultraprocessed diet, said Kevin Hall, a nutrition scientist and a coauthor of the study. This was likely because the study’s ultraprocessed diet was more nutritious than the typical diets of the participants, he said. But participants still lost more weight on the minimally processed diet — a finding that aligns with those of previous studies.
That may be because minimally processed foods tend to have fewer calories per bite, said Filippa Juul, a nutritional epidemiologist at SUNY Downstate Health Sciences University who was not involved in the study. And those foods generally have a harder texture that requires more chewing, so people may eat more slowly and consume fewer calories before feeling full, she said.
The participants also reported feeling like they had better control of food cravings on the minimally processed diet. That’s surprising, Dr. Dicken said, because “when people lose weight, they tend to want to eat more.” Better craving control may help them keep the weight off longer, Dr. Dicken added.
Dr. Juul speculated that perhaps following a diet of mostly minimally processed foods may “reset cravings” and reduce “food noise,” helping people eat only when they are hungry.
A growing body of evidence
The study was relatively short, Walter C. Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health who was not involved in the study, wrote in an email. “We need about a year, at minimum, for serious weight loss studies because early changes often reverse or don’t continue,” he added.
The researchers could not measure exactly what and when people ate, or how much they consumed, and the participants reported eating some foods besides those provided. And most of the participants were women; men or children may have responded differently, Dr. Davy said.
Still, the study suggests that even if a person follows a healthy diet, there’s an added benefit to choosing minimally processed foods over ultraprocessed ones, Dr. Juul said. Evidence consistently suggests that diets high in ultraprocessed foods can make it harder for people to stay at a healthy weight, she added.
Avoiding ultraprocessed foods can be a challenge, though, since they are so ubiquitous and tend to be cheaper than minimally processed foods, Dr. Juul said. She advises choosing foods with recognizable ingredients.
Food manufacturers could help by making more minimally processed products that are convenient, affordable and appealing, she added. “It’s a shift in food culture that’s needed.”
You don’t have to cut out ultraprocessed foods entirely, Dr. Davy said. But try to cook at home as much as you can, focusing on fresh fruits and vegetables, lean proteins and whole grains — “those things that we know are good for us,” she said.
Alice Callahan is a Times reporter covering nutrition and health. She has a Ph.D. in nutrition from the University of California, Davis.
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