Recent research has found that people who ate minimally processed foods lost twice as much weight as those who ate ultra-processed diets, even though both diets were balanced and participants were allowed to eat freely. This real-world, long-term study showed that the processing of food itself – and not just the nutrients – plays an important role in shaping body weight and health. This is the conclusion of a new clinical study conducted by researchers at UCL and UCLH.
The study, published in Nature Medicine, is the first intervention study to compare ultra-processed foods (UPF) and minimally processed foods (MPF) under ‘real-world’ conditions, and is also the longest experimental study of a UPF diet to date. Minimally processed foods include whole foods such as whole grain products, fresh vegetables and fruit as well as home-cooked meals. Highly processed foods contain additives, preservatives and artificial ingredients.
How the Consumption of Minimally Processed Foods Affects Weight
The study divided 55 adults into two groups. One group began with an eight-week diet of minimally processed foods, such as oatmeal soaked overnight or homemade spaghetti bolognese. After a four-week “washout” period, during which the participants returned to their normal diet, they switched to a diet with UPF, such as oatmeal bars for breakfast or pre-packaged lasagna. The other group followed this diet in reverse order. In total, 50 participants completed at least one diet.
The diets provided were nutritionally balanced according to the Eatwell Guide, the UK government’s official recommendations for a healthy and balanced diet. This included the levels of fat, saturated fat, protein, carbohydrate, salt and fiber, as well as the recommended intake of fruit and vegetables. Participants received plenty of food (i.e. more calories than they needed) delivered to their homes and were instructed to eat as much or as little as they wanted, just as they normally did. They were not asked to limit their food intake. After eight weeks on each diet, both groups had lost weight, probably due to the improved nutritional composition of their meals compared to their normal diet. However, this effect was more pronounced with the MPF diet (2.06% weight loss) than with the UPF diet (1.05% weight loss).
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. By comparison, the Eatwell Guide recommends a daily energy intake of 2,000 kcal for women and 2,500 kcal for men. The greater weight loss on the MPF diet was due to a reduction in fat mass and total body water, with no change in muscle or lean mass, indicating an overall healthier body composition. The results suggest that when following recommended dietary guidelines, choosing minimally processed foods may be more effective for weight loss.
Fewer Food Cravings and Greater Weight Loss
Dr. Samuel Dicken, lead 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 because of their nutrient profile. The main aim of this study was to address important gaps in knowledge about the role of food processing in the context of existing dietary recommendations and its impact on health outcomes such as weight, blood pressure and body composition, as well as experiential factors such as food cravings.”
The primary aim of the study was to assess the percentage weight changes. The researchers found a significant reduction with both diets, although the effect was almost twice as high with the minimally processed diet. A 2% reduction may not seem like much, but it was achieved in just eight weeks without participants actively trying to reduce their food intake. If you extrapolate these results over a year, the researchers say you would expect a 13% weight reduction in men and 9% 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 add up to a big difference.
Participants completed several questionnaires before starting each diet and at weeks four and eight during their diet to assess their cravings for certain foods. There were significantly greater improvements in the number of cravings and the ability to resist them (craving control) on the MPF diet than on the UPF diet, although there was greater weight loss on the MPF diet, which would normally lead to greater cravings. Compared to the UPF diet, participants on the MPF diet reported twice the improvement in overall craving control, four times the improvement in craving control for savory foods, and nearly twice the improvement in resistance to the foods they craved most.
Professor Chris van Tulleken, author of the study from the UCL Division of Infection & Immunity and UCLH, said: “The current global food system promotes diet-related health problems and obesity, particularly due to the wide availability of cheap, unhealthy foods. This study highlights the importance of ultra-processing for health in addition to the role of nutrients such as fat, salt and sugar. It emphasizes the need to shift the policy focus away from individual responsibility to the environmental causes of obesity, such as the influence of multinational food corporations in shaping an unhealthy food environment. ” Stakeholders from different disciplines and organizations need to work together and focus on broader policies that improve our food environment, such as warning labels, marketing restrictions, progressive taxation and subsidies, to ensure that healthy food is affordable, available and attractive to all.
Adhere to Nutritional Guidelines Wherever Possible
The study also looked at secondary health markers such as blood pressure and heart rate, as well as blood markers such as liver function, glucose, cholesterol and inflammation. For these markers, there were no significant negative effects of the UPF diet, either no change or a significant improvement from baseline. In general, there were no significant differences between diets on these markers, and the researchers point out that longer studies would be needed to adequately examine these measures in relation to changes in weight and fat mass.
The study participants’ normal diet did not generally meet national dietary guidelines and was higher than average in UPF, which may explain why switching to a UPF-only but nutritionally balanced diet resulted in neutral or slightly positive changes in some secondary health markers. The best advice for the population would be to adhere to the dietary guidelines as much as possible, with people moderating their overall energy intake, limiting salt, sugar and saturated fat consumption, and favoring high-fiber foods such as fruits, vegetables, legumes and nuts. Choosing less processed foods such as whole grains and home-cooked meals instead of ultra-processed, packaged foods or convenience foods is likely to provide additional benefits in terms of body weight, body composition and overall health.