A Bristol-led team found that when people eat only unprocessed foods, they naturally pile their plates with fruits and vegetables, ending up better nourished while taking in fewer calories. The work suggests our bodies may be wired with a kind of nutritional intelligence that today’s ultra-processed foods can disrupt.
If your New Year’s resolution was to cut back on processed snacks and ready meals, new research suggests your body may reward you in a surprising way: you could end up eating more food, feeling full and nourished, yet still taking in fewer calories.
A team led by scientists at the University of Bristol has found that when people are offered only unprocessed, “whole” foods, they naturally gravitate toward fruits and vegetables and away from richer options like meat, pasta and butter. As a result, they ate more than 50% more food by weight than people eating only ultra-processed foods, but still consumed, on average, hundreds fewer calories per day.
The findings, published in The American Journal of Clinical Nutrition, add to growing evidence that humans may have an inbuilt “nutritional intelligence” that helps guide us toward a balanced diet when we eat foods in their natural form.
This pattern challenges the idea that people’s food choices are chaotic or purely driven by taste, according to lead author Jeff Brunstrom, a professor of experimental psychology at the University of Bristol.
“It’s exciting to see when people are offered unprocessed options they intuitively select foods that balance enjoyment, nutrition, and a sense of fullness, while still reducing overall energy intake. 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,” he said in a news release.
The new analysis builds on a widely cited clinical trial led by Kevin Hall at the U.S. National Institutes of Health, which originally showed that eating an all–ultra-processed diet leads to overeating and weight gain compared with an all–unprocessed diet. In that trial, participants lived in a controlled setting and were given either only ultra-processed foods (UPFs) or only unprocessed foods for set periods, with instructions to eat as much or as little as they wanted.
The Bristol team went back to those data to ask a more detailed question: not just how many calories people ate, but which specific foods they chose within each diet, and how those choices affected their intake of vitamins and minerals.
When participants were on the unprocessed diet, they consistently loaded their plates with fruits and vegetables, sometimes eating several hundred grams per meal. They tended to pass up more calorie-dense options that were also available, such as steak, pasta and cream. Overall, this led to a 57% increase in the total weight of food eaten compared with the ultra-processed phase, but with a substantially lower calorie intake.
The researchers then examined how nutritious those unprocessed meals were. They found that the variety and volume of fruits and vegetables supplied essential vitamins and minerals that would have been missing if participants had focused only on the richer whole foods.
Co-author Mark Schatzker, a writer in residence at the Modern Diet and Physiology Research Center, which is affiliated with McGill University, noted that the data highlight how important those lower-calorie plant foods were.
“Had participants eaten only the calorie-rich foods, our findings showed they would have fallen short on several essential vitamins and minerals and eventually developed micronutrient insufficiencies. Those micronutrient gaps were filled by lower calorie fruits and vegetables,” Schatzker, who authored The Dorrito Effect and The End of Craving, said in the news release.
The team describes this pattern as a form of “micronutrient deleveraging,” a term they use for the idea that people, when given the chance, prioritize foods rich in vitamins and minerals even if those foods are lower in calories. In other words, when the food environment is made up of whole foods, people seem to trade some energy density for micronutrient density, piling on produce to meet their bodies’ needs.
That is not the case with ultra-processed foods. Contrary to the popular belief that they only provide “empty calories,” the analysis suggests many UPFs actually deliver plenty of vitamins and minerals, often because they are fortified. For example, some of the foods richest in vitamin A in the trial were calorie-heavy items like French toast sticks and pancakes.
On paper, that might sound like a good thing. But the Bristol-led researchers argue it could be part of the problem.
Co-author Annika Flynn, a senior research associate at the University of Bristol, warned that combining high energy and added nutrients in the same products may short-circuit the body’s usual balancing act.
“This raises the alarming possibility that UPFs deliver both high energy and micronutrients in one hit, which could result in calorie overload, because they effectively kill the beneficial trade-off between calories and micronutrients,” she said.
In whole-food diets, that trade-off creates what the team calls “healthy competition” between foods. To get enough vitamins and minerals, people are nudged toward fruits and vegetables and away from large portions of pasta, meat and other calorie-dense items. That dynamic appears to work in favor of both health and weight control.
“Conversely, this healthy competition is promoted by wholefoods and therefore encourages people eating them to favour micronutrient powerhouses, such as fruit and veggies, over high-energy options like pasta and meat,” added Flynn.
In the ultra-processed environment, that competition is blunted. Because fortified UPFs can satisfy micronutrient needs while also being packed with calories, there is less incentive for the body to drive people toward bulky, lower-calorie plant foods. That may help explain why people in the original trial ate fewer grams of food but more total calories when their diets were made up of ultra-processed products.
This means the usual focus on “overeating” may be missing the point, according to Brunstrom.
“Overeating is not necessarily the core problem. Indeed, our research clearly demonstrated consumers on a wholefood diet actually ate far more than those on a processed food one,” he said. “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 study adds to a growing body of work suggesting that the structure and composition of modern food environments shape not just how much we eat, but what we crave and choose. Ultra-processed foods now make up a large share of diets in many countries, especially among young people and those with limited time or money for cooking.
Public health researchers are increasingly interested in small, practical changes that can harness people’s underlying nutritional intelligence instead of fighting it. In related work, also led by the University of Bristol, simply changing the order of dishes on a weekly menu nudged more diners to pick healthier, more environmentally friendly options. That “dish swap” study, like the new analysis, points to the power of subtle design choices in cafeterias, restaurants and home kitchens.
The team’s latest findings do not mean everyone must immediately switch to a perfectly unprocessed diet, which can be difficult and expensive. But they do suggest that making room for more whole fruits and vegetables, and relying less on fortified ultra-processed products, may allow the body’s own balancing mechanisms to work better.
Future research will likely explore how these patterns play out in more diverse, real-world settings and over longer periods of time. For now, the message is both simple and encouraging: when our plates are filled with foods closer to their natural state, our instincts about what and how much to eat may be smarter than we think.
Source: University of Bristol