People on an ultra-processed diet ate about 500 more calories a day than people eating whole foods, even when both diets offered the same calories on the plate.
Quick Take
- A National Institutes of Health (NIH) trial locked 20 adults in a research hospital and fed them two different diets for two weeks each.
- On the ultra-processed diet, people ate 508 more calories a day on average and gained nearly two pounds.
- On the whole-food diet, the same people lost almost two pounds without trying.
- Researchers say eating speed, not protein content, appears to drive the extra calories.
The Experiment That Put Processed Food To The Test
Kevin D. Hall led a team at the National Institutes of Health Clinical Center that ran one of the first controlled trials testing how food processing affects eating habits. Ten men and ten women, all healthy and weight-stable, moved into the facility for 28 days. Each person ate an ultra-processed diet for two weeks and a minimally processed diet for two weeks, in random order, with every meal offered in unlimited amounts.
Both diets were built to match on calories, sugar, fat, fiber, and salt as closely as possible. The only intended difference was how much the food had been industrially altered before it reached the plate. Participants could eat as much or as little as they wanted at every meal, a method scientists call ad libitum feeding.
The Numbers Behind The Calorie Gap
During the ultra-processed phase, people ate 508 more calories per day than during the whole-food phase, a gap so large the odds of it happening by chance were about one in ten thousand. The extra calories came almost entirely from carbs and fat, 280 calories a day from carbs and 230 from fat. Protein intake barely moved, changing by just two calories a day, which tells researchers this was not a protein story.
Weight changes tracked the calorie gap closely. Participants gained about 0.9 kilograms, roughly two pounds, during the ultra-processed stretch and lost about the same amount during the whole-food stretch. The link between how much people ate and how much weight they gained was strong, with a correlation of 0.8 out of a possible 1.0.
Why Eating Speed May Be The Hidden Culprit
NIH researchers noticed something simple during the trial: people ate faster when the food was ultra-processed. That single behavior change, combined with roughly 500 extra calories a day, produced the weight gain seen in just two weeks. It suggests the texture, softness, and easy chewability of many packaged foods may let people take in more calories before their body signals them to stop.
The design matters here. Because this was a randomized, controlled trial conducted inside a research hospital, researchers could rule out most outside factors like grocery budgets, cooking skill, or food access. Everyone ate exactly what was placed in front of them, under the same roof, during both halves of the study.
What This Study Does And Does Not Prove
This trial involved only 20 people over two weeks per diet, so it cannot tell us whether the pattern holds for months or years, or across every age group and health condition. It also tested one specific menu built by researchers, not every food that falls under the ultra-processed label. Scientists describe it as strong early evidence of a real effect, not the final word on the subject.
A broader look at the research field backs up the direction of this finding. A review of ten controlled feeding trials found that diets higher in ultra-processed food consistently led to higher calorie intake compared with less processed options. Separately, a review of long-term population studies has linked heavy ultra-processed food consumption with higher rates of obesity, diabetes, and heart disease, even though those studies cannot prove cause and effect the way a controlled trial can.
A Practical Read For Everyday Eaters
The takeaway for anyone standing in a grocery aisle is straightforward. Two meals can carry identical calorie counts on the label and still produce very different results in the body, depending on how heavily the food was processed before it got there. Slower eating, whole ingredients, and foods that require more chewing appear to help people stop eating sooner, long before counting a single calorie.
None of this means every packaged food is dangerous or every whole food is automatically better for a given person’s goals. But the NIH trial gives families a concrete, tested reason to look past the nutrition label and think about how their food was made, not just what it contains.
Sources:
irp.nih.gov, nih.gov, cc.nih.gov, npr.org, pubmed.ncbi.nlm.nih.gov, cell.com
