In a closely controlled trial, adults ate more when offered an ultraprocessed foods diet and allowed to choose how much to eat. The result suggests that the foods and menus in that experiment made overeating easier. It does not show that every packaged food has the same effect or establish what happens to health over the long term.
What the ultraprocessed foods trial actually matched
In a 2019 randomized trial published in Cell Metabolism, Kevin D. Hall and colleagues studied 20 adults living at the National Institutes of Health Clinical Center. Participants, whose average age was about 31, spent 28 days there. They ate an ultra-processed or an unprocessed menu for two weeks, then switched to the other menu. They could eat as much or as little as they wanted.
The researchers designed the menus to be similar in calories offered, energy density, and overall amounts of carbohydrate, fat, protein, sugar, sodium, and fiber. That matters: this was not simply a comparison between a high-calorie menu and a low-calorie one. Participants were offered ample food and could decide their own intake.
But “matched” did not mean the meals were identical apart from processing. The proportions of added sugar and different types of fiber and fat differed between menus. The foods also differed in their ingredients and physical form. So the trial tested two menus that differed in processing category and in other characteristics—not processing as a single isolated variable.
Participants ate more—and their weight changed
On the ultra-processed menu, participants consumed an average of about 508 kilocalories (2,125 kilojoules) more per day. Over that two-week period, their weight increased by about 0.9 kg (2 lb) on average. During the unprocessed-menu period, they ate less and lost about the same amount of weight. The changes in weight tracked closely with differences in calorie intake.
Because the researchers controlled what food was available and measured what participants ate, this trial provides direct evidence that these particular ultra-processed meals led people to consume more in that setting. It does not reveal exactly why. The trial cannot separate the effect of processing itself from differences such as food texture, ingredients, or the balance of sugar and fiber types.
The study was also small and short, and participants lived in a research setting with meals and snacks supplied. Everyday eating involves different choices, routines, budgets, and food access. The results are therefore a meaningful signal, not a precise forecast of how much more any individual would eat at home.
What a two-week result cannot tell you
The observed weight change shows that greater intake and short-term weight change occurred together under the trial conditions. It does not establish that the same pattern continues for months or years, or that the study measured long-term outcomes such as diabetes or heart disease. It also does not show that a processing label alone predicts a person’s health.
That distinction matters when you hear broad claims about the dangers of ultra-processed foods. A short feeding trial can test eating behavior and near-term weight change; it cannot settle the long-term health effects of every food in a broad category. The authors’ findings support concern about these menus and overeating, but they are not proof that all foods classified as ultra-processed have identical effects.
A processing category is not a nutrition label
Ultra-processed is a category based on how a food is formulated and processed. It is not a summary of every nutrient in the food, and “packaged” does not automatically mean “ultra-processed.” Packaged foods can differ substantially in ingredients and nutritional composition. Likewise, two foods in the same processing category need not be nutritionally equivalent.
If you are comparing products, look at the details that matter to you—such as fiber, sodium, added sugars, and serving size—rather than treating the category as a complete verdict. The trial gives a reason to pay attention to how a menu affects your intake, but it does not make the processing label a substitute for considering the food itself.
Bottom line: In one carefully controlled, short-term trial, adults ate more on an ultra-processed menu than on a minimally processed one, despite broad matching of several nutritional measures. That is evidence about those menus and that setting—not a guarantee about every product or a conclusion about long-term health. If you have health concerns or need help making dietary changes, a qualified clinician or dietitian can help you consider your individual needs.