People lose more weight if they cook minimally processed food from scratch than if they eat ultra-processed and ready-made foods, according to the first study to establish a clear link between ultra-processed foods (UPFs) and weight.

Trial participants were given one of two diets with the same nutritional profile for eight weeks. One diet was made up of UPFs while the other comprised minimally processed foods.

When the first group ate breakfast bars and ready-made lasagne, for example, the second ate oats soaked in milk and natural yoghurt and homemade spaghetti bolognese.

At the end of the trial, participants on the second diet had lost twice as much weight as those on the first.

Prof Chris van Tulleken, an author of the study and of the book Ultra-Processed People, 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.”

The study by University College London and UCL hospitals NHS trust was designed to address the lack of good-quality evidence on the health outcomes of UPFs. The findings suggest that choosing minimally processed foods and cooking from scratch as much as possible rather than buying ultra-processed packaged foods or ready meals may be more effective for losing weight.

Minimally processed means that a food has undergone limited alteration from its natural state, for example fruits, vegetables, whole grains, meat, fish and dairy products such as natural yoghurt. UPFs have been significantly altered and typically contain ingredients not commonly used in home cooking, such as artificial flavours, preservatives and emulsifiers.

People who followed the UPF diet consumed 120 calories fewer than usual and lost 1% of their weight on average during the study. Those who followed the minimally processed diet consumed 290 calories fewer and lost 2% of their weight on average, including much more fat mass. Over a year, this could scale up to a 13% weight reduction for men and 9% for women, the researchers said.

Dr Samuel Dicken, the first author of the study, said that while previous research has linked UPFs with poor health outcomes, not all are “inherently unhealthy”. But, he explained, because UPFs have a softer texture and are more calorie-dense, they do make it easier to overeat.

The study, published in Nature Medicine, split 55 adults with an average BMI of 32, indicating obesity, into two groups, who followed one diet for eight weeks, returned to their normal diet for four weeks, and then swapped to the other diet for another eight weeks.

The diets were nutritionally matched in accordance with government advice on healthy eating from the Eatwell Guide. Participants had food containing more calories than they needed delivered to their homes and were told to eat as much or as little as they wanted, as they would normally.

Participants also completed questionnaires to assess food cravings, which found significant improvements in the number of cravings and ability to resist them on the minimally processed diet compared with the UPF one – despite the fact that weight loss is often associated with stronger cravings.

The team recommended that policymakers use warning labels, marketing restrictions, progressive taxation and subsidies to increase awareness and make healthy diets affordable for all.

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Dr Adrian Brown, another author of the study, said: “The cost-of-living crisis has impacted people’s choices on food. Healthy food is up to two times more expensive than unhealthy food.”

Rob Percival, the Soil Association’s head of food policy, said: “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 Department of Health and Social Care spokesperson said the government had commissioned research to understand more about the health impacts of UPFs and was “committed to tackling obesity and building a healthier Britain”.

Meanwhile, the NHS should do more to help those who have come off a course of obesity drugs or just finished a weight loss programme, to ensure they do not regain the pounds they have lost.

Such people should receive regular check-ups, advice and tailored action plans to help ensure they stay healthy, according to the National Institute for health and Clinical Excellence.

Minimally processedBreakfast: Cinnamon and apple overnight oats Lunch: Mexican chicken with salad and flatbread Dinner: Home-cooked cottage pie with green beans and corn

Ultra processedBreakfast: Oat and fruit breakfast bar Lunch: Meal deal-style packaged chicken tikka wrap Dinner: Ready-made cottage pie with greens and corn