When given nutritionally matched diets, participants lost twice as much weight eating minimally processed foods compared to ultra-processed foods, suggesting that cutting down on processing could help to sustain a healthy weight long term, a new clinical trial led by researchers at UCL shows.
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.
Researchers hope the study will increase understanding of how the type and extent of food processing – including the way the food matrix is altered or degraded – may directly affect health outcomes.
The UCL 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 lasagne ready meal. The other group completed the diets in the opposite order. In total, 50 participants completed at least one diet.
The two 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 fibre, as well as providing recommended intakes of fruits and vegetables. Participants had plenty of food (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).
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. 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 diets.
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.
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. Perhaps surprisingly, the ultra-processed diet resulted in lower levels of low-density lipoprotein, or “bad” cholesterol.
“This study…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”
Shaping unhealthy food environments
Professor Chris van Tulleken, an author of the study from UCL Division of Infection & Immunity, 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.
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.”
This research was supported by the National Institute for Health and Care Research UCLH Biomedical Research Centre and the Rosetrees Trust.
“For decades, the UK Government’s ‘healthy eating’ guidelines have neglected processing, assuming that whole foods and ultra-processed products might deliver the same health outcomes. We now know that to be false”
‘Processing matters’
Responding to today’s study, Soil Association Head of Food Policy Rob Percival said: “This is the most rigorous study of ultra-processed diets ever conducted, and it shows that processing really does matter for health. For decades, the UK Government’s ‘healthy eating’ guidelines have neglected processing, assuming that whole foods and ultra-processed products might deliver the same health outcomes. We now know that to be false. A minimally processed diet is better for your health. You cannot replicate the benefits of whole foods in an industrial manufacturing facility – nature knows better.
“Our government must catch up. Its so-called ‘Eatwell Guide’ should be updated to promote healthy, minimally processed foods, and to warn against ultra-processed diets, or else it risks becoming a ‘Eat Badly Guide’. In tandem, the government must use its Food Strategy to make it easy for everyone to access and enjoy more minimally processed produce. This will require that they support more ethical food businesses and listen to the 20,000 people who have backed our current campaign calling on them to resist the pressure of the ultra-processed food lobby. This new government has an opportunity to do things differently and stop allowing government policy to be shaped by those who selfishly put corporate profits before public health. It’s time for a new approach.”
Unanswered question?
Some researchers suggest that the new study doesn’t significantly advance knowledge about how particular ingredients or processing types may be driving health effects. Professor Ciarán Forde, a specialist in how the sensory properties of foods influence calorie selection, eating behaviour, energy intake and metabolism, told The Guardian that the ultra-processed diet was more calorie-dense than the minimally processed one, which itself could explain the difference in weight loss. He said: “The basic question remains unanswered as to what type of processing or ingredient is driving the observed effects.”