Eating minimally processed foods could significantly enhance weight loss compared to ultra-processed alternatives, even when both diets are nutritionally identical, according to a new clinical

trial led by UCL researchers.

Published in “Nature Medicine”, this is the first long-term trial to compare ultra-processed foods (UPFs) and minimally processed foods (MPFs) in real-life settings. The study involved 55 adults who alternated between eight-week periods of MPF and UPF diets, separated by a four-week break. The MPF diet featured dishes like homemade spaghetti Bolognese, while the UPF diet included ready meals and packaged snacks. Both diets met official UK nutritional guidelines and provided more food than participants needed—allowing them to eat freely without calorie restrictions.

Despite consuming as much as they wanted, participants lost twice as much weight on the MPF diet (2.06% body weight reduction) compared to the UPF diet (1.05%). This difference equated to an estimated daily calorie deficit of 290 kcal on MPF, versus 120 kcal on UPF. Over a year, researchers estimate this could result in a 13% weight loss in men and a 9% loss in women on the MPF diet, compared to just 4–5% on the UPF diet.

Importantly, weight loss on the MPF diet came from reductions in fat mass and total body water, with no loss of muscle mass, indicating a healthier body composition.

Beyond weight, the MPF diet also improved participants’ ability to resist food cravings. Surveys revealed that people on the MPF diet experienced twice the improvement in craving control overall, and four times greater improvement for savoury foods, compared to those on the UPF diet—even though weight loss often intensifies cravings.

Dr Samuel Dicken, first author of the study from the UCL Centre for Obesity Research and UCL Department of Behavioural Science & Health, said: “Previous research has linked ultra-processed foods with poor health outcomes. But not all ultra-processed foods are inherently unhealthy based on their nutritional profile. The main aim of this trial was to fill crucial gaps in our knowledge about the role of food processing in the context of existing dietary guidance, and how it affects health outcomes such as weight, blood pressure and body composition, as well as experiential factors like food cravings.

“The primary outcome of the trial was to assess percentage changes in weight and on both diets we saw a significant reduction, but the effect was nearly double on the minimally processed diet. Though a 2% reduction may not seem very big, that is only over eight weeks and without people trying to actively reduce their intake. If we scaled these results up over the course of a year, we’d expect to see a 13% weight reduction in men and a 9% reduction in women on the minimally processed diet, but only a 4% weight reduction in men and 5% in women after the ultra-processed diet. Over time this would start to become a big difference.”

Interestingly, while weight loss differed between diets, secondary health markers like blood pressure, cholesterol, and blood sugar levels showed no significant adverse effects from the UPF diet during the study period. Researchers emphasize, however, that longer studies are needed to fully assess the long-term health impacts.

Professor Rachel Batterham, senior author of the study from the UCL Centre for Obesity Research, said: “Despite being widely promoted, less than 1% of the UK population follows all of the recommendations in the Eatwell Guide, and most people stick to fewer than half.

“The normal diets of the trial participants tended to be outside national nutritional guidelines and included an above average proportion of UPF, which may help to explain why switching to a trial diet consisting entirely of UPF, but that was nutritionally balanced, resulted in neutral or slightly favourable changes to some secondary health markers.

“The best advice to people would be to stick as closely to nutritional guidelines as they can by moderating overall energy intake, limiting intake of salt, sugar and saturated fat, and prioritising high-fibre foods such as fruits, vegetables, pulses and nuts. Choosing less processed options such as whole foods and cooking from scratch, rather than ultra-processed, packaged foods or ready meals, is likely to offer additional benefits in terms of body weight, body composition and overall health.”

The research was funded by the National Institute for Health and Care Research UCLH Biomedical Research Centre and the Rosetrees Trust. Photo by formulatehealth, Wikimedia commons.