What Happened When People Could Only Eat Whole Foods For Two Weeks

For years, nutrition research has focused on how much people eat—calories, portions, macros. But occasionally a study shifts the conversation in a more interesting direction: why we choose certain foods in the first place.

Researchers at the University of Bristol1 recently revisited data from a tightly controlled nutrition trial, not to test a new diet, but to examine the small, everyday decisions people made when their food options were stripped back to the basics. No labels. No calorie counts. Just unprocessed ingredients and the freedom to eat as much as they wanted.

Rather than launching a new trial, researchers took a deeper dive into data from a well-known NIH-controlled feeding study that directly compared ultra-processed diets with unprocessed, whole-food diets. In the original experiment, participants spent two weeks eating nothing but ultra-processed foods, then switched to two weeks of only unprocessed, whole foods.

The ultra-processed and unprocessed diets were matched for calories, macronutrients, fiber, sugar, and sodium. Participants were allowed to eat as much or as little as they wanted. Despite those controls, the original study showed that people eating ultra-processed foods consumed significantly more calories per day.

This new analysis wanted to understand the why behind those numbers. So they dug into the daily food logs, tracking exactly what people chose to put on their plates when only whole foods were available.

On the unprocessed diet, participants loaded up on fruits and vegetables in amounts that would make most nutritionists do a double-take. We're talking servings of spinach, broccoli, and green beans that sometimes topped 500 grams in a single meal—that's more than a pound of vegetables on one plate. Meanwhile, foods like pasta sat there, barely touched.

The result was surprising. People eating unprocessed meals consumed about 57% more food by weight, yet still took in roughly 330 fewer calories per day on average.

So why were people piling vegetables on their plates instead of going for the calorie-rich options? The research team believes it comes down to micronutrients, all those vitamins and minerals our bodies actually need to function.

When you're eating whole foods, there's a natural tension that gets created. You need vitamins and minerals, and you need calories for energy. But those two things aren't always packaged together in equal measure. Carrots and spinach are loaded with vitamin A but barely make a dent in your calorie intake. Meanwhile, that plate of pasta has plenty of calories but won't meet your vitamin needs.

The research suggests that if people had only eaten those higher-calorie whole foods and skipped the produce, they eventually would have developed micronutrient deficiencies. Their bodies seemed to know this on some level, even if they weren't consciously thinking about vitamin A or iron or folate. The researchers call this "micronutrient deleveraging." Basically, we prioritize getting the nutrients we need, even when it means passing up some of those energy-dense foods.

Ultra-processed foods aren't actually the "empty calories" we've been led to believe they are. Many of them are fortified with vitamins and minerals. Those French toast sticks and pancakes people ate during the processed food phase? They were delivering vitamin A right alongside all those calories.

And this is where things go off the rails. When calories and micronutrients are bundled together in the same foods, your body loses that natural incentive to seek out lower-calorie, nutrient-rich options. You can meet all your nutritional needs without ever touching a vegetable, so why would your body push you toward one?

The beneficial tension that exists with whole foods just disappears. There's no trade-off to make, no internal calculation that leads you toward produce. Everything you need is right there in those calorie-dense, fortified foods. And that's probably a big part of why we end up consuming so much more energy when we eat this way.

Instead of focusing on eating less, it suggests that eating differently may naturally regulate appetite.

This research adds nuance to the ultra-processed food conversation. Humans may not be wired to chase calories alone; we may be wired to chase nutrients. When food is minimally processed, that instinct appears to guide us toward choices that support fullness, nutrition, and lower energy intake without conscious effort.