Minimally processed diets yield better fat loss than UPFs, though experts question why

A recent study reveals that people lost twice as much weight eating minimally processed foods (MPFs) compared to ultra-processed foods (UPFs) — even when foods were nutritionally matched. According to the authors, the results suggest that reducing processed food consumption could be a practical strategy for long-term weight management.

The eight-week randomized crossover trial involved 55 adults who were given unlimited access to either MPFs — such as homemade meals — or UPFs consisting of packaged and pre-prepared items.

After a washout period, participants switched diets. Fifty individuals completed at least one of the two diet phases. However, some experts are questioning the validity of the trial and its suggested findings. Moreover, the study notes that daily calorie intake was lower compared to baselines for both groups — an average 503.7 kcal less on MPFs, and 289.6 kcal less when consuming UPFs. 

“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,” says first author of the study, Dr. Samuel Dicken.

“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.”

Assessing impacts

Published in Nature Medicine and conducted by UK researchers at University College London (UCL) and University College London Hospitals (UCLH), the study reveals that the MPF diet led to a healthier body composition, with weight loss driven by reductions in fat mass and total body water, while muscle mass was preserved.

Despite both diets being aligned with the UK’s Eatwell Guide and matched meals for macronutrients and calorie availability, participants lost more weight on the MPF diet. Average weight loss was 2.06% on the MPF diet versus 1.05% on the UPF diet, equating to a daily calorie deficit of 290 kcal and 120 kcal respectively.

“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,” Dicken explains. “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.”

At the same time, Dr. Dimitrios Koutoukidis, an associate professor in diet, obesity, and behavioral sciences at the UK University of Oxford, critiques some aspects of the study, stating that contrary to the researchers’ initial expectation that participants on the UPF diet would gain weight, both groups experienced significant weight loss.

“The researchers found that this assumption was not met, which makes the estimated weight change biased,” Koutoukidis attests. “They have done additional analyses that suggest this concern is addressed.”

“The researchers do not speculate on why people on a UPF diet lost weight, but it might be that having all foods free, ready-prepared, and provided, as in this study, makes people more conscious about the food they eat and reduces the number of decisions they have to make. This can allow them to follow a diet aligned with national UK guidelines.”

Examining corresponding health factors

Beyond weight, participants also reported improved control over food cravings on the MPF diet. Craving control improvements were twice as high overall, four times greater for savory foods, and nearly double for resisting the most craved food.

Secondary health metrics — including blood pressure, cholesterol, glucose, and inflammatory markers — showed no adverse effects from the UPF diet, though researchers noted that longer trials are needed to fully assess these outcomes.

“The global food system at the moment drives diet-related poor health and obesity, particularly because of the wide availability of cheap, unhealthy food,” underscores professor Chris van Tulleken, an author of the study from UCL Division of Infection & Immunity and UCLH. “This study highlights the importance of ultra-processing in driving health outcomes in addition to the role of nutrients like fat, salt, and sugar.”

“It underlines the need to shift the policy focus away from individual responsibility and onto the environmental drivers of obesity, such as the influence of multinational food companies in shaping unhealthy food environments.”

Alternative causes

Koutoukidis from the University Oxford emphasizes different possibilities for the study’s findings. He notes that there were no significant differences between the two groups as heart health markers were mostly similar between groups. He also notes that although triglycerides dropped more on the minimally processed diet, low density lipids fell more with UPFs.

“We know that receiving support to adhere to the diet is one of the strongest predictors of diet adherence and weight loss,” Koutoukidis highlights. “In this study, participants had weekly phone calls with the researchers, but the researchers were not blinded.”

“This might have inadvertently introduced some bias, whereby those on the minimally processed diet got more help and support than when on the ultra-processed diet.”

Additionally, the researchers state that, for health professionals and food industry stakeholders, the study underscores a growing evidence base supporting reformulation strategies and dietary guidelines that emphasize food processing levels and not just nutrient composition.

“Stakeholders across disciplines and organizations must work together and focus on wider policy actions that improve our food environment, such as warning labels, marketing restrictions, progressive taxation and subsidies, to ensure that healthy diets are affordable, available, and desirable for all,” Van Tulleken concludes.