A study led by researchers at the University of Bristol, with contributions from leading US nutrition experts, found that participants who ate only unprocessed foods consumed more than 50 percent more food by weight than those eating only UPFs (ultra-processed food). Even so, their daily calorie intake was about 330 calories lower on average.

A Built-In Ability to Balance Nutrition and Energy

Published in The American Journal of Clinical Nutrition, the findings offer new insight into how people make food decisions. The results support the idea that humans may possess a built-in “nutritional intelligence” that helps guide balanced eating. This instinct appears to function best when foods are eaten in their natural form and may be disrupted by modern fast food environments.

Lead author Jeff Brunstrom, Professor of Experimental Psychology at the University of Bristol, said: “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.”

Reexamining a Landmark Processed Food Trial

The research involved a fresh analysis of data from a landmark clinical trial led by Dr. Kevin Hall, a longtime researcher at the US National Institutes of Health. That original study showed that diets made up entirely of ultra-processed foods lead to overeating and weight gain. The new analysis took a closer look at why people eating only whole foods consumed much larger portions of certain foods while still taking in fewer total calories.

Participants on the unprocessed diet consistently filled their meals with fruits and vegetables, sometimes eating several hundred grams at a time. They tended to avoid more calorie-dense choices such as steak, pasta, and cream. As a result, people eating whole foods consumed 57 percent more food by weight overall.

Fruits and Vegetables Fill Nutrient Gaps

Researchers also evaluated how nutritious the diets were. They found that the variety and quantity of fruits and vegetables provided essential vitamins and minerals that would have been missing if participants had relied only on higher calorie whole foods.

Study co-author Mark Schatzker, author of The Dorrito Effect and The End of Craving, explained: “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.”

The researchers believe this behavior reflects a process they call “micronutrient deleveraging.” In simple terms, people appear to prioritize foods rich in vitamins and minerals, such as fruits and vegetables, even if that means eating fewer energy-dense options.

Why Ultra-Processed Foods Change the Equation

Ultra-processed foods produced a very different outcome. While they are often described as providing “empty calories,” the study found they can meet micronutrient needs, largely because of vitamin fortification. For example, calorie-rich foods like French toast sticks and pancakes turned out to be among the top sources of vitamin A. On the unprocessed diet, vitamin A mostly came from carrots and spinach, which provide far fewer calories.

Study co-author Dr. Annika Flynn, Senior Research Associate at the University of Bristol, said: “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 added that whole foods restore that balance by encouraging competition between nutrient-rich, lower calorie foods and higher energy options. This helps steer people toward fruits and vegetables rather than foods like pasta and meat.

Processed Foods and Modern Eating Behavior

The findings offer further insight into how widespread consumption of highly processed foods may influence behavior and decision making. According to the researchers, overeating itself may not be the main problem.

Prof Brunstrom said: “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. 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.”

Small Changes Can Shape Healthier Choices

Related research from the University of Bristol has shown that even small adjustments can influence decisions. In a separate study, simply changing the order of healthier, more environmentally friendly meals on a weekly menu led more diners to choose them.

The research was supported by the National Institute for Health and Care Research (NIHR) Bristol Biomedical Research Centre (Bristol BRC).