Does Ultra-Processed Food Cause Weight Gain?

Two controlled trials say processing changes how much you eat#

Does ultra-processed food make you gain weight? The best evidence says it makes you eat more, and higher intake appears to explain much of the weight difference. Two randomized feeding trials, one on a locked hospital ward and one in people's own homes, gave adults diets that looked similar on a nutrition label but differed in how processed the food was. In both, people ate more on the ultra-processed version when allowed to eat freely. That is a stronger claim than "ultra-processed eaters are heavier," which is what most of the headlines rest on, and a narrower one than "processing is toxic."

The dispute is about the category they tested. Critics argue the label "ultra-processed" lumps together foods that have little in common, so a result for the category may not transfer to any particular food on your plate. Both things can be true at once, and for anyone keeping a food log the practical consequence is the same: the clearest pathway in the trials is extra calories, and extra calories are something a log records whether or not you know which foods count as ultra-processed. The broader macro picture lives in macronutrients explained; this article is about the processing question on top of it.

Inside the ward: 508 more calories a day on menus matched on paper#

The first trial ran at the NIH Clinical Center. Twenty weight-stable adults (average age about 31, average BMI about 27) lived on a metabolic ward and ate an ultra-processed diet for two weeks and an unprocessed diet for two weeks, in random order. The menus were designed to be matched for presented calories, energy density, macronutrients, sugar, sodium and fiber, and participants were told to eat as much or as little as they wanted¹.

On the ultra-processed diet, energy intake was 508 ± 106 kcal a day higher. The extra came from carbohydrate (280 kcal a day) and fat (230 kcal a day), not protein, which barely moved. Weight tracked intake closely (r = 0.8): participants gained 0.9 kg on the ultra-processed diet and lost 0.9 kg on the unprocessed one, both within two weeks¹. The flat protein line is the hook for a separate theory, covered in the protein leverage hypothesis.

Two details in the full paper matter more than the headline. People ate the ultra-processed meals faster, by about 17 kcal per minute. And the "matched energy density" held only when drinks were counted: the non-beverage foods on the ultra-processed menu were roughly 85% more energy dense, partly because fiber was added to the drinks to hit the target². Participants did not rate the meals as more pleasant, and reported hunger and fullness did not differ between the diets.

On a ward where every bite was weighed, people ate about 500 calories a day more of the ultra-processed menu, and did not rate it as tastier.

So the trial shows that this ultra-processed menu led to overeating. It does not isolate why. Eating speed, the energy density of the solid food, or something else about the products could each carry part of the effect, and the authors listed exactly that among their limitations: the design did not identify which features of processing drove the excess intake.

At home for eight weeks: weight fell on both diets, twice as much on one#

The obvious objection to a two-week ward study is that nobody lives on a ward. The UPDATE trial, published in 2025, moved the question into daily life. Researchers at University College London randomized 55 adults in England, mostly women and all hospital staff, with overweight or obesity (BMI 25 to under 40) who normally got at least half their calories from ultra-processed food. The plan was two eight-week diets, one minimally processed and one ultra-processed, in random order, with a washout between; 50 contributed primary-outcome data and 43 completed both periods. Both diets were built to follow the UK's Eatwell Guide, the national healthy-eating template, and people ate as much as they liked³.

Here the result has a different shape from the ward study, and it is easy to misreport. Average weight fell on both diets. Participants lost weight on both diets, which makes sense for people switching from their usual diet to a guideline-following one. The minimally processed diet simply produced more of it:

UPDATE trial, 8 weeks	Minimally processed	Ultra-processed
Weight change	−2.06%	−1.05%
Weight change (kg, approx.)	−1.84 kg	−0.88 kg
Fat mass change	−1.59 kg	−0.61 kg
Energy density of the diet	1.25 kcal/g	1.60 kcal/g

The difference (minimally processed minus ultra-processed) was −1.01 percentage points of body weight (95% CI −1.87 to −0.14, p = 0.024), a moderate effect by the authors' own statistics³. Self-reported intake was about 327 kcal a day lower on the minimally processed diet, and food cravings improved more on it as well⁴.

Hold the numbers loosely in both directions. The calorie gap was self-reported, which is the weakest measure in the trial; the weight and fat-mass differences were measured and are the stronger evidence. Among participants who returned food diaries (32 minimally processed and 35 ultra-processed periods), adherence was high (84.5% and 91.2%). The trial was supported by the NIHR UCLH Biomedical Research Centre and the Rosetrees Trust. Several authors disclosed ties that readers may weigh, including book royalties (the first author for a book that mentions ultra-processed food, a coauthor for one about it) and payments from pharmaceutical companies that sell obesity drugs (others). Kevin Hall, who led the ward study, is a coauthor.

Put together, the trials point in the same direction in different settings. The ward study shows the effect under tight control over two weeks; the home study shows a smaller but real version of it over two months, inside diets that were "healthy" by nutrient standards.

The definition fight: what counts as ultra-processed?#

"Ultra-processed" is not a nutrient; it is a category in the NOVA classification, developed by Carlos Monteiro's group in Brazil. NOVA sorts foods into four groups by the extent and purpose of industrial processing. Its authors define ultra-processed products by how they are made (fractioning whole foods into substances, modifying them chemically, reassembling them, and adding cosmetic additives) and suggest a practical test: check whether the ingredient list contains at least one item rarely used in home kitchens, such as high-fructose corn syrup, hydrolysed proteins or emulsifiers⁵.

The critique comes from a different camp and makes different points. Michael Gibney and colleagues argued in 2017 that the category is too broad to be turned into dietary guidelines and that NOVA offers little advantage over the conventional approach of linking nutrients to disease⁶. In a 2019 follow-up, Gibney documented that the definition and its lists of example foods had shifted considerably since NOVA was introduced, and noted that national surveys in the UK and France did not show a link between body mass index and ultra-processed intake⁷. One of the 2017 critique's coauthors, Ciarán Forde, later coauthored the NIH ward trial, which is a useful reminder that the two sides are not cleanly separate camps.

The sharpest test of the definition is an empirical one. French researchers asked food and nutrition specialists to assign foods to NOVA groups. Agreement between evaluators was low: Fleiss' kappa of 0.32 for 120 branded products with ingredient lists and 0.34 for 111 generic foods (1.0 would be perfect agreement). Most branded products that experts called ultra-processed were agreed on; a cluster of 28 generic foods was scattered across all four groups⁸. Several of those authors disclosed food-industry grants or consultancy work outside the study.

What this dispute does and does not change:

It does mean "ultra-processed" is a fuzzy label at the edges. A packaged wholegrain bread and a soft drink can share the group.
It does not undo the trials. Hall and Dicken did not rely on anyone's judgment call about an edge case; they compared two concrete menus that sat clearly on either side of the line.
It does limit what the trials can tell you about any single food. The finding is about diets built mostly from one end of the spectrum or the other.

What this means for a food log#

Higher calorie intake appears to explain much of the weight difference; the trials did not isolate every mechanism. In the ward study, weight change followed energy intake with a correlation of 0.8. In the home study, the diets differed in energy density and in how much people ate. The leading candidate mechanisms, eating faster and eating more energy-dense solid food, are ways of taking in more calories before fullness catches up. That idea is developed in calorie density and estimation.

This is the part that matters if you track. A calorie total does not care whether the 500 extra calories came from an emulsifier or from a second helping; it records them either way. What processing plausibly changes is how easy it is to reach that total without noticing. So the useful question for your log is not "is this food ultra-processed?" but "which foods reliably push my day over?" A few weeks of entries can help you spot foods and meals associated with higher-intake days; how to review your food log covers the method.

If you do not want to count, the trials still suggest a lever. Swapping toward minimally processed staples lowered intake in free-living adults even when the comparison diet met the same nutrient guidelines, which fits the approaches in losing weight without counting calories. If you do count, the values for the staples themselves are gathered in calories in common foods.

FAQ#

Did people gain weight on ultra-processed food in the 2025 UPDATE trial?#

No. On average, people lost weight on both diets: about 1.05% of body weight on the ultra-processed diet and 2.06% on the minimally processed one over eight weeks. The finding is that the minimally processed diet produced roughly twice the loss, not that the ultra-processed diet caused gain³.

How much more did people eat on the ultra-processed diet in the NIH ward study?#

About 508 kcal a day more, measured directly on a metabolic ward, with a weight gain of 0.9 kg in two weeks versus a 0.9 kg loss on the unprocessed diet¹.

Why do some scientists reject the ultra-processed label?#

Critics argue the category is too broad and has been defined inconsistently over time, and a survey of French specialists found low agreement (kappa about 0.32 to 0.34) when they classified the same foods (Gibney, 2019; Braesco et al., 2022).

Can I still lose weight while eating some ultra-processed food?#

The UPDATE trial suggests yes: its ultra-processed arm still lost about 1% of body weight in eight weeks on a guideline-based diet. The loss was smaller than on the minimally processed arm, so the processing level appears to change how easy the deficit is, not whether it is possible.

Sources#

Source: BurnWeek — "Does ultra-processed food make you gain weight? What the two trials show", https://burnweek.fit/blog/ultra-processed-food-and-weight/. Licensed CC BY 4.0: free to quote or reuse with a link to this page.

This article was researched and drafted with AI assistance and reviewed for accuracy by the BurnWeek team. It is general information, not medical advice. How we research and correct our articles →