When it comes to weight loss, it might not just be what you eat but how it’s made.
In one of the longest and most rigorous studies of its kind, researchers from University College London found that participants lost twice as much weight eating a diet made up of minimally processed foods compared to a diet of more processed options, even though the two diets were matched for calories, protein, fat, fiber and other nutrients.
The study, published in Nature Medicine, enrolled 55 adults in an eight-week crossover trial. Each person spent eight weeks on a minimally processed diet, think overnight oats or homemade spaghetti Bolognese, and eight weeks on a more processed diet made up of ready-to-eat or packaged meals, such as oat bars or prepared lasagna. Both plans met the UK’s Eatwell Guide for a balanced diet, and participants could eat as much or as little as they wanted, mimicking everyday habits rather than strict calorie counting.
“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,” said Dr. Samuel Dicken of UCL’s Centre for Obesity Research. “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.”
After eight weeks on each plan, participants lost weight on both, but the minimally processed plan led to about twice the weight loss, an average of 2.06% of body weight compared to 1.05% on the more processed plan. They also reported greater improvements in controlling food cravings, including a fourfold boost in their ability to resist savory temptations.
“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 said. “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.”
That doesn’t mean all processed foods are harmful. In fact, the processed plan in this study still met national nutrition guidelines, proof that processed foods can fit into a balanced diet. But the findings also suggest that how food is made may influence how our bodies respond, beyond what nutrition labels can show.
“The global food system at the moment drives diet-related poor health and obesity, particularly because of the wide availability of cheap, unhealthy food,” said Professor Chris van Tulleken, another study author. “This study highlights the importance of ultra-processing in driving health outcomes in addition to the role of nutrients like fat, salt and sugar.”
So what does this mean for your plate? According to senior author Professor Rachel Batterham, the best approach is to focus on balance and make less-processed choices when you can.
“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,” she said. “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 study was supported by the National Institute for Health and Care Research UCLH Biomedical Research Centre and the Rosetrees Trust.