A new clinical trial from University College London (UCL) and University College London Hospitals (UCLH) has found that people lose nearly twice as much weight when eating minimally processed foods compared to ultra-processed foods, even when the two diets are matched for nutrition.
The study, published in Nature Medicine, is the first long-term trial to compare these two types of diets in real-world conditions.
The trial involved 55 adults split into two groups. One group first ate an eight-week diet of minimally processed foods (MPF) such as overnight oats or homemade spaghetti Bolognese.
After a four-week break where they returned to their normal eating habits, they switched to an eight-week diet of ultra-processed foods (UPF), such as breakfast oat bars or ready-made lasagna.
The second group completed the diets in the opposite order. In total, 50 participants completed at least one of the diet phases.
Both diets followed the UK’s Eatwell Guide, which recommends a balanced intake of fat, protein, carbohydrates, salt, fiber, fruits, and vegetables. Participants had more than enough food delivered to their homes and were free to eat as much or as little as they wanted. They were not told to limit portions.
After eight weeks on each diet, both groups lost weight, likely because the diets were healthier than what they normally ate. However, the MPF diet led to almost twice the weight loss, with a 2.06% reduction in body weight compared to 1.05% on the UPF diet.
This difference came from an estimated daily calorie reduction of 290 kcal on the MPF diet versus 120 kcal on the UPF diet. The extra weight loss on the MPF diet was due to fat loss and reduced body water, with no loss of muscle mass.
If the results were applied over a year, researchers estimate that men could lose about 13% of their body weight on a minimally processed diet, compared to only 4% on an ultra-processed diet. For women, the numbers would be 9% versus 5%.
The trial also looked at food cravings. Participants on the MPF diet reported much better control over cravings compared to when they were on the UPF diet. They showed double the improvement in overall craving control, four times the improvement in resisting savoury foods, and almost double the improvement in resisting their most craved foods.
Interestingly, despite the negative reputation of UPFs, the trial found no harmful changes in blood pressure, heart rate, or blood tests like cholesterol and blood sugar when participants followed a nutritionally balanced UPF diet.
In fact, there were some neutral or slightly positive changes compared to their regular diets, which often contained even more unhealthy food.
However, the researchers stressed that most people do not follow official dietary guidelines. Less than 1% of the UK population meets all of the Eatwell Guide recommendations. Choosing less processed foods, cooking from scratch, and prioritizing high-fiber options like fruits, vegetables, pulses, and nuts is likely to help with weight control and overall health.
Professor Chris van Tulleken, a co-author of the study, said the findings highlight the need for policies that address the food environment, not just individual choices.
He pointed to measures such as warning labels, marketing restrictions, and taxes to encourage healthier eating. The researchers also noted that more long-term studies are needed to understand how different diets affect health over time.
This study provides strong evidence that food processing plays an important role in weight control, even when nutritional content is matched. Minimally processed foods appear to naturally lead to lower calorie intake and better craving control, making them more effective for sustainable weight loss.
While balanced UPF diets can meet nutritional guidelines, they may still be less effective for weight management. The takeaway is clear: reducing food processing in the diet could offer a practical and lasting way to improve both weight and health.
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The study is published in Nature Medicine.
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