Ultra-processed foods make people eat more not because they taste better, but because they pack more calories into less food and get eaten faster. In a tightly controlled 2019 NIH trial, adults who spent two weeks eating freely from an ultra-processed menu consumed about 508 more calories a day than when they spent two weeks eating freely from an unprocessed menu matched for calories, sugar, fat, sodium, and fiber — and gained nearly a pound in that time, while losing almost a pound on the unprocessed diet.
In brief:
- Adults ate about 508 more calories a day on an ultra-processed diet than an unprocessed diet built to match it on calories, sugar, fat, sodium, and fiber.
- The ultra-processed foods people actually ate were roughly 85% denser in calories per gram, and were eaten faster, measured in both calories and grams per minute.
- Pleasantness and familiarity scores were about the same between the two diets — the extra intake wasn't explained by the ultra-processed food being rated more enjoyable.
What counts as "ultra-processed," exactly?
Researchers commonly sort food using the NOVA classification system, which groups items by how much industrial processing and reformulation they've undergone rather than by their nutrient content alone: unprocessed or minimally processed foods, processed culinary ingredients (oils, butter, sugar), processed foods (canned vegetables, simple breads, cheese), and ultra-processed foods — industrial formulations typically made with ingredients rarely used in a home kitchen, such as protein isolates, emulsifiers, and flavor compounds, and designed to be convenient, long-lasting, and easy to eat.² NOVA has its critics, who argue the category is too broad and imprecise to drive specific dietary advice on its own. It's a classification system, not a nutrient panel — and that distinction turns out to matter for what the trial below actually tested.
What did the study actually test?
A team led by Kevin Hall at the National Institutes of Health admitted 20 weight-stable adults to the NIH Clinical Center for four weeks. Each person ate two weeks of meals built almost entirely from ultra-processed foods and two weeks built almost entirely from unprocessed or minimally processed foods, in random order, with a two-week washout in between. The two menus were deliberately engineered to match on paper — the same presented calories, energy density, protein, fat, carbohydrate, sugar, sodium, and fiber. Participants were simply told to eat as much or as little as they wanted at each meal, and staff weighed what came back to calculate exactly how much they ate.
That design is what makes the result notable. This wasn't a comparison of a calorie-dense junk-food diet against a calorie-sparse "healthy" one — the two diets were built to look nutritionally equivalent going in. The only thing that differed by design was how processed the food was.
So why did people eat more, if the diets were matched?
Despite the matched nutrition targets, two measurable differences showed up once people started eating freely:
- The food itself was denser in calories. Setting aside beverages (used mainly as a vehicle for a fiber supplement on the ultra-processed menu), the solid and semi-solid foods participants actually ate were roughly 85% more calorie-dense on the ultra-processed days than on the unprocessed days — more calories packed into the same weight of food.
- People ate it faster. Meal eating rate was significantly higher on the ultra-processed diet, both in calories consumed per minute and in grams consumed per minute. Faster eating gives the body's fullness signals, which take time to catch up with what's already been swallowed, less of a chance to catch up before the meal is over.
What the data ruled out is just as telling. Participants rated the ultra-processed and unprocessed meals about the same for pleasantness and for familiarity — neither difference was statistically significant. Hunger, fullness, and satisfaction scores after eating were also similar between the two diets. The common assumption — that ultra-processed food wins because it simply tastes better — isn't what this trial found. The researchers' own explanation points instead to the food's physical properties: softer, easier-to-chew textures that can be eaten and swallowed quickly, combined with more calories packed into each bite.
What the research says
Hall and colleagues found that energy intake was 508±106 kcal/day higher during the ultra-processed diet than the unprocessed diet (p=0.0001), and that weight change tracked closely with how much extra people ate (r=0.8) — participants gained about 0.9 kg over two weeks on the ultra-processed diet and lost about 0.9 kg over two weeks on the unprocessed diet.¹ Eating rate was significantly faster on the ultra-processed diet by both calorie and weight measures, while the foods consumed (excluding beverages) carried substantially more calories per gram.¹
Two limits matter here. First, this was a small trial — 20 adults — conducted over two weeks per diet in a supervised inpatient setting where every gram of food was weighed; a larger or longer trial, or one in a free-living setting where people shop and cook for themselves, could find a different-sized effect. Second, because it's one randomized trial rather than a body of replicated studies, the exact 500-calorie figure is this cohort's result, not a number guaranteed to apply to any individual. What the controlled design does support more confidently is the direction of the finding: when nutrient content is held equal on paper, higher processing was followed by higher calorie intake in this trial, and the data point to density and eating speed — not taste — as the likely reasons why.
What you can do this week
- Build meals around less-processed foods when you can. The trial's own design suggests that swapping a highly processed version of a meal for a less-processed one with similar ingredients — not just "eating less" — is what moved the calorie count in this study.
- Slow down on purpose at meals that are soft and easy to eat quickly. Since eating rate was one of the two measured mechanisms, a deliberate pause — chewing more, putting utensils down between bites — works directly against it.
- If diet and activity changes aren't enough to reach a healthy weight, that's a conversation worth having with a clinician. Pallas prescribes compounded semaglutide and tirzepatide when a US-licensed clinician decides it's appropriate.
None of this requires eliminating every processed food from your kitchen — the trial compared two full diets, not one food choice. The direction that moved the needle in this study is the one worth starting with.
