A new study led by researchers at the University of Bristol, with help from top U.S. nutrition experts, has found that people who eat only whole, unprocessed foods actually consume more food by weight—but fewer calories—than those eating only ultra-processed foods (UPFs).
On average, those on the unprocessed diet ate 57% more food by weight but still took in 330 fewer calories each day. The findings were published in the American Journal of Clinical Nutrition.
Ultra-processed foods are everywhere in today’s world. These are packaged items like chips, soda, ready meals, and sugary breakfast cereals.
They’re often high in sugar, fat, and salt, and they’re made in ways that make them easier to eat quickly and in large amounts. While it’s long been known that UPFs are linked to overeating and weight gain, this study goes deeper to understand why.
The research team, led by Professor Jeff Brunstrom from the University of Bristol, revisited data from a key clinical trial by U.S. scientist Dr. Kevin Hall. That earlier study had shown that people gained weight when they ate only ultra-processed foods.
But this new analysis found something even more surprising—when people ate only whole foods, they not only chose larger portions, they also picked healthier options like fruits and vegetables, which are lower in calories and rich in vitamins and minerals.
Participants eating only unprocessed foods often loaded up on large servings of fruits and vegetables instead of high-calorie items like meat, pasta, or creamy dishes. This naturally led them to eat a more nutritious, lower-calorie diet, even though they were eating much more food overall.
This surprising behavior reflects what the researchers call “nutritional intelligence”—an instinctive ability to seek out foods that meet the body’s nutrient needs, especially when those foods are presented in their natural state.
Co-author Mark Schatzker, author of *The Dorito Effect*, explained that if people had only eaten the high-calorie whole foods offered, they would have missed out on important nutrients. But thanks to fruits and vegetables, they naturally filled those nutritional gaps without needing to think about it.
The story was quite different with ultra-processed foods. While often thought of as “empty calories,” many UPFs are actually fortified with vitamins. For instance, French toast sticks and pancakes provided large amounts of vitamin A.
In contrast, on the whole food diet, vitamin A came from carrots and spinach—foods that are lower in calories but naturally rich in nutrients.
Dr. Annika Flynn from the University of Bristol warned that UPFs may pack too much into a single bite: both high calories and nutrients. This combination might fool the body’s natural balance system and lead to overeating.
Whole foods, on the other hand, promote a healthy trade-off—people have to eat more low-calorie, nutrient-rich foods like vegetables to get the vitamins they need, helping to naturally limit their calorie intake.
This research adds to growing concerns about how UPFs may influence our eating habits in harmful ways. According to Professor Brunstrom, the problem may not be overeating itself, but the way processed foods are designed to encourage high-calorie consumption in small amounts.
When people eat unprocessed foods, they seem to make smarter choices, guided by their body’s natural signals.
The study also ties into other research from the University of Bristol showing that simply changing the order of dishes on a menu can lead people to choose healthier, more sustainable meals.
All of this points to a powerful message: by changing the food environment and promoting whole foods, we may be able to help people make better, healthier decisions without strict dieting or complex rules.
If you care about nutrition, please read studies about why vitamin K is so important for older people, and this snack food may harm your heart rhythm.
For more health information, please see recent studies about vitamin that may protect you from type 2 diabetes, and results showing this common chemical in food may harm your blood pressure.
The study is published in The American Journal of Clinical Nutrition.
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