The study, published in Nature Medicine, is the first interventional study comparing ultra-processed food (UPF) and minimally processed food (MPF) diets in 'real world' conditions, as well as being the longest experimental study of a UPF diet to date¹.

The trial split 55 adults into two groups. One group started with an eight-week diet of MPF, such as overnight oats or homemade spaghetti Bolognese. After a four-week 'washout' period during which participants went back to their normal diet, they switched to a diet of UPF, such as breakfast oat bars or a lasagna ready meal. The other group completed the diets in the opposite order. In total, 50 participants completed at least one diet.

The provided diets were nutritionally matched in accordance with the Eatwell Guide, the UK's official government advice on how to eat a healthy, balanced diet. This included levels of fat, saturated fat, protein, carbohydrate, salt and fiber, as well as providing recommended intakes of fruits and vegetables. Participants had plenty of food (i.e. more calories than they needed) delivered to their home and were told to eat as much or as little as they wanted, as they would normally. They were not told to limit their intake.

After eight weeks on each diet, both groups lost weight, likely as a result of the improved nutritional profile of what they were eating compared to their normal diet. However, this effect was higher (2.06% reduction) on the MPF diet compared to the UPF diet (1.05% reduction)².

These changes corresponded to an estimated calorie deficit of 290 kilocalories (kcal) per day on the MPF diet, compared to 120 kcal per day on the UPF diet. To put this in context, the Eatwell Guide recommends a daily energy intake of 2,000 kcal for women and 2,500 kcal for men.

The greater weight loss experienced on the MPF diet came from reductions in fat mass and total body water, with no change in muscle or fat-free mass, indicating a healthier body composition overall.

The findings suggest that, when observing recommended dietary guidelines, choosing minimally processed foods may be more effective for losing weight.

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."

Participants completed several questionnaires to assess their food cravings before starting the diets, and at weeks four and eight during the diets³.

There were significantly greater improvements in the number of cravings and ability to resist them (craving control) on the MPF diet compared to the UPF diet, despite greater weight loss on the MPF diet that might ordinarily be expected to lead to stronger cravings.

On the MPF diet compared to the UPF diet, participants reported a two-fold greater improvement in overall craving control, a four-fold greater improvement in craving control for savoury food, and an almost two-fold greater improvement in resisting whichever food they most craved.

Professor Chris van Tulleken, an author of the study from UCL Division of Infection & Immunity and UCLH, said: "The global food system at the moment drives diet-related poor health and obesity, particularly because of the wide availability of cheap, unhealthy food. 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 on to the environmental drivers of obesity, such as the influence of multinational food companies in shaping unhealthy food environments.

"Stakeholders across disciplines and organisations 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."

The trial also measured secondary health markers, such as blood pressure and heart rate, as well as blood markers such as liver function, glucose, cholesterol and inflammation. Across these markers, there were no significant negative impacts of the UPF diet, with either no change, or a significant improvement from baseline.

Generally, there weren't significant differences in these markers between the diets, and the researchers caution that longer studies would be needed to investigate these measures properly in relation to the changes in weight and fat mass.

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 prioritizing high-fiber 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."

This research was supported by the National Institute for Health and Care Research UCLH Biomedical Research Centre and the Rosetrees Trust.

Notes

1. MPF have undergone very little alteration from their natural state, such as fruits, vegetables, whole grains, meat, fish and dairy products like natural yoghurt. UPF have been significantly altered from their original form through processing, and typically contain ingredients not commonly used in home cooking, such as artificial flavours, preservatives and emulsifiers.
2. Not all participants lost weight, with 10 individuals in each group gaining weight. This is thought to be due to a lack of adherence to the diet, particularly on the second diet that they undertook. When the unadjusted results from the first round of diets (either MPF or UPF) were considered in isolation, the weight loss was greater than when the average across both rounds of diets (4.09% reduction for MPF and 2.12% reduction for UPF).
3. The Control of Eating Questionnaire (CoEQ) assesses overall craving control, craving for sweet foods, craving for savoury foods, positive mood, and the perceived ability to resist eating foods that are craved. The Power of Food Scale (PFS) assesses the appetite for and motivation to consume palatable foods when that food is available (but not physically present), when it is present (but not tasted), and when the food has been tasted (but not yet consumed).