If you pledged to follow an unprocessed food diet as a New Year’s resolution, research shows you’ll naturally choose to eat a lot more fruit and veggies than higher calorie wholefood options – like rice, meat and butter – which could help with losing weight.
The research, led by scientists at the University of Bristol and co-authored by top US nutritional experts, revealed that people following a completely unprocessed diet ate more than 50% greater amounts of food than those eating just UPFs (ultra-processed food), yet still consumed on average 330 fewer calories from food daily.
The findings, published today in The American Journal of Clinical Nutrition, shed exciting new light on how humans make food choices and give further weight to the idea we have an inbuilt ‘nutritional intelligence’ to achieve a well-balanced diet, which kicks in when eating food in its natural form but is impaired by today’s fast food culture.
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.”
The research reanalysed data from a seminal clinical trial1, led by US National Institutes of Health veteran Dr. Kevin Hall, which first exposed how eating exclusively UPFs results in excessive calorie consumption and weight gain. But a closer second look has uncovered something much more intriguing at play, which helps explain exactly why people eating solely wholefoods ate much greater quantities of certain types of food, while maintaining a significantly lower overall calorie intake.
All participants eating only wholefoods chose to load up on fruit and veggies, sometimes consuming several hundreds of grams per meal, instead of more calorific options like steak, pasta, and cream. Overall, this resulted in those on the unprocessed diet eating 57% greater amounts, measured by weight, of food.
The team at Bristol went on to examine how nutritious the participants’ diets were. This uncovered that the range and volume of fruit and veggies contained essential vitamins and minerals, which wouldn’t have been gained if participants ate just higher calorie wholefoods on offer.
Study co-author Mark Schatzker, acclaimed 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 scientists reckon this reflects a form of nutritional intelligence, which they call “micronutrient deleveraging”, whereby humans prioritise micronutrient-rich foods – in other words fruit and veggies – at the expense of energy-dense options.
The ultra-processed foods told a very different story. Contrary to the common belief they deliver “empty calories,” findings demonstrated they likely met micronutrient requirements, often through vitamin fortification. For example, the foods richest in vitamin A were calorie-rich French toast sticks and pancakes. By contrast on the unprocessed diet, the majority of vitamin A was obtained from carrots and spinach, which are much lower in calories.
“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.”Dr Annika Flynn, Senior Research Associate at the University of Bristol
“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.”
The research provides further insight into the effects of highly processed food, which is increasingly prevalent in today’s society, on human behaviour and decision making.
Prof Brunstrom added: “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.”
Recent research, also led by the University of Bristol, similarly showed that by simply switching the order of healthier, more environmentally-friendly dishes on a weekly menu prompts more diners to select them.
The dish swap study2 is another example of how promoting “healthy competition” prompts people to make greener, leaner food choices.
The research is supported by the National Institute for Health and Care Research (NIHR) Bristol Biomedical Research Centre (Bristol BRC).
References
1) https://pubmed.ncbi.nlm.nih.gov/31105044/
2) https://www.bristol.ac.uk/news/2025/september/dish-swap-research.html
(Newswise/HG)