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People who cook minimally processed meals at home lose more weight than if they eat ready-made, ultra-processed meals – even if these foods are healthy, a new UK study has found.

Food experts have long pointed to ultra-processed foods as a key driver of the obesity crisis, which affects about one in eight people worldwide.

These foods are often high in saturated fats, sugar, and salt, for example crisps, frozen dinners, and processed meats. But they also include staples like wholemeal sliced bread and baked beans, meaning ultra-processed foods are not always unhealthy.

That’s prompted much debate over whether it’s the ingredients or the processing itself that really matters when it comes to our health.

The latest study, published in the journal Nature Medicine, indicates that minimally processed diets are better for weight loss – but that diets rich in ultra-processed foods can still be healthy as long as they meet nutritional guidelines.

“Completely cutting [ultra-processed foods] out of our diets isn’t realistic for most of us,” Tracy Parker, nutrition lead at the British Heart Foundation, said in a statement.

“But including more minimally processed foods – like fresh or home cooked meals – alongside a balanced diet could offer added benefits too,” added Parker, who was not involved with the study.

What the study found

The findings are based on a small clinical trial involving 55 adults in England. Before the study, the participants tended to have poor nutrition diets made up largely of ultra-processed foods, and on average, they had a body mass index (BMI) of about 33, which is considered obese.

The trial split adults into two groups. One group started on a diet of minimally processed foods like homemade spaghetti bolognese or overnight oats, while the other ate only ultra-processed foods like ready-made lasagnas or breakfast cereals. After a monthlong break, the two groups swapped, eating meals from the other diet for another eight weeks.

Notably, both of these diets met the United Kingdom’s guidelines for a healthy, balanced diet, which takes into account levels of saturated fat, protein, carbohydrates, salt, and fibre, as well as fruit and vegetable intake. The only difference was how processed these meals were.

By the end of the study, both groups lost weight – but adults lost twice as much weight when they were on the minimally-processed diets. They lost about 2 per cent of their weight on this diet, compared with about 1 per cent on the ultra-processed diet.

“Though a 2 per cent reduction may not seem very big, that is only over eight weeks and without people trying to actively reduce their intake,” said Samuel Dicken, a researcher who helped run the trial at University College London.

“Over time this would start to become a big difference,” he added.

Over the course of a year, men on the minimally processed diet would be expected to lose 13 per cent of their weight, compared with 4 per cent on the ultra-processed diet, the researchers said. For women, the findings translate to a 9 per cent weight reduction on the minimally processed diet and a 4 per cent reduction on the ultra-processed diet.

Dietary differences

People tended to lose more weight on the minimally processed diet because of reductions in their fat mass and body water, which the researchers said suggested they had a healthier body composition overall.

They also reported fewer food cravings on the minimally processed diet.

There were no big differences between the two diets when it came to other health outcomes, such as blood pressure, heart rate, and blood markers that track liver function, blood sugar, cholesterol, and inflammation.

The fact that people still lost weight while on ultra-processed diets indicates that these foods aren’t all bad, independent experts said.

“The most interesting result of the study is that participants on both arms lost weight – which contradicts claims that ultra-processed foods result in weight gain,” Gunter Kuhnle, a professor of nutrition and food science at the University of Reading, said in a statement.

The findings “suggest that a diet meeting current dietary recommendations is not detrimental to weight maintenance, whether it is ultra-processed or not,” added Kuhnle, who was not involved with the study.

The study also has some limitations, namely the fact that it included only 55 people who tried both diets.

Independent researchers warned that it takes time for the body to get used to new eating habits, so longer studies with more people would be needed to understand exactly how ultra-processed foods affect our health.

Even so, researchers said the results are in line with other studies showing that access to nutritious food is critical to our health and wellbeing.

Dr Chris van Tulleken, one of the study’s authors and a researcher studying how corporations affect human health at University College London, pointed to the “wide availability of cheap, unhealthy food” as a key driver of obesity and poor health worldwide.

He called for policy action to make unhealthy options less appealing,  for example through warning labels, marketing restrictions, and taxes.

The study, he said, “underlines the need to shift the policy focus away from individual responsibility and on to the environmental drivers of obesity”.