Eating more food does not necessarily mean consuming more calories. A new analysis suggests that when people eat minimally processed foods, they may naturally eat larger volumes while still taking in fewer calories overall.
Researchers at the University of Bristol reanalyzed data from a well-known randomized controlled feeding trial that compared diets made up entirely of ultraprocessed foods with diets composed of unprocessed or minimally processed foods. Their findings, published in The American Journal of Clinical Nutrition, shed new light on how people respond to different food environments.
In the original trial, participants eating an ultraprocessed diet consumed significantly more calories and gained weight, while those eating an unprocessed diet consumed fewer calories and lost weight. The new analysis explored why those differences emerged.
The researchers found that participants eating unprocessed foods consumed about 57% more food by weight than those eating ultraprocessed foods yet still averaged roughly 330 fewer calories per day. Much of that extra volume came from fruits and vegetables, which are low in energy density but rich in essential nutrients.
“It’s exciting to see when people are offered unprocessed options they tend to select foods that balance enjoyment, nutrition and fullness,” said Jeff Brunstrom, professor of experimental psychology at the University of Bristol and lead author of the analysis. “Our dietary choices aren’t random — in fact we seem to make much smarter decisions than previously assumed when foods are presented in their natural state.”
Rather than filling plates with higher-calorie whole foods such as meat, pasta, rice or cream, participants eating the unprocessed diet consistently chose large amounts of fruits and vegetables, sometimes consuming several hundred grams per meal. This allowed them to meet micronutrient needs while keeping calorie intake lower.
The analysis also examined nutrient adequacy. The researchers found that if participants on the unprocessed diet had relied mainly on higher-calorie whole foods, they would have fallen short on several vitamins and minerals. Those gaps were filled by lower-calorie, micronutrient-rich foods like fruits and vegetables.
Study co-author Mark Schatzker said the findings point to a trade-off between calories and micronutrients that seems to guide food choices when diets are minimally processed. The researchers describe this phenomenon as “micronutrient deleveraging,” meaning people prioritize foods rich in vitamins and minerals even if those foods provide fewer calories.
Ultraprocessed foods showed a different pattern. While often described as providing “empty calories,” many ultraprocessed foods in the study met micronutrient requirements through fortification. For example, some of the richest sources of vitamin A in the ultraprocessed diet were calorie-dense foods such as French toast sticks and pancakes.
“This raises the possibility that ultraprocessed foods deliver both high energy and micronutrients in one hit,” said Annika Flynn, senior research associate at the University of Bristol, adding that the combination “could result in calorie overload, because they effectively kill the beneficial trade-off between calories and micronutrients.”
The authors emphasize that the findings do not suggest people consciously overeat or lack self-control. Instead, food composition and structure appear to shape choices in subtle ways, nudging people toward higher- or lower-calorie patterns depending on what is available.
Importantly, the researchers note several limitations. The analysis was post hoc, meaning it examined existing data rather than testing a new hypothesis in a fresh trial. The diets were also extreme and tightly controlled, designed to isolate the effects of food processing rather than reflect typical eating patterns.
Even so, the findings help clarify why minimally processed diets are often associated with lower calorie intake and weight loss, without requiring smaller portions or intentional restriction.
“Overeating isn’t necessarily the core problem,” Brunstrom said. “Our research clearly demonstrated consumers on a wholefood diet actually ate far more than those on a processed food one. But the nutritional make-up of food is influencing choices and it seems that UPFs are nudging people towards higher calorie options, which even in much lower quantities are likely to result in excess energy intake and in turn fuel obesity.”
The research was supported by the National Institute for Health and Care Research Bristol Biomedical Research Centre.