You can lose weight on a diet of ultra-processed food

It seems that you can lose weight while eating ultra-processed food – but to a lesser extent than if you cut them out.

Food is generally considered to be ultra-processed if it includes ingredients that are never or rarely used in kitchens, such as high-fructose corn syrup, or additives that make the product more palatable or appealing, such as flavourings and thickeners.

Many studies have linked eating ultra-processed food to adverse health outcomes, such as cardiovascular problems, type 2 diabetes and weight gain, but these studies have been observational. Ultra-processed food also tends to be high in sugar, salt or fat – like cookies and microwave meals – sparking a debate over whether it is simply the ingredients that make ultra-processed food unhealthy or if there is something intrinsically detrimental about the processing itself.

To better understand this in the context of weight loss, Samuel Dicken at University College London and his colleagues have carried out a trial in which 55 people who were overweight or had obesity were randomised to eat a diet of either ultra-processed or minimally processed foods.

“Obviously everyone imagines pizza, chips and that kind of stuff when they think of ultra-processed foods,” says Dicken. However, the researchers made sure that both diets aligned with the UK Eatwell Guide, which encourages a healthy, balanced diet that includes at least five portions of fruit and vegetables a day and several sources of protein, such as beans, fish, eggs and meat. The two diets were also matched so they contained roughly the same levels of fat, sugar, salt and carbohydrate.

Food was delivered to the participants, making it the first trial to compare such diets in real-world conditions, rather than in a hospital or lab. With the ultra-processed group, this involved things like breakfast cereals, protein bars, chicken sandwiches and ready-meal lasagnes, but versions that were low in fat and salt. “The kind of foods that if you go into a supermarket, they’re slapped with nutritional health claims,” says Dicken.

The minimally processed diet featured homemade foods like overnight oats, chicken salad, bread made from scratch and spaghetti bolognese. Both groups were given enough food for about 4000 calories a day and were told to eat as much as they wanted. The researchers set it up so that half of the participants were on one diet for eight weeks, half on the other, and then they switched after a four-week break.

The participants were told the study was investigating the health effects of balanced meals made in different ways, rather than looking for weight loss specifically, but both diets still led to people shedding pounds: the minimally processed food diet resulted in a 2 per cent average reduction in weight, and the ultra-processed diet led to a 1 per cent reduction.

“We saw more weight loss on the minimally processed diet, and it’s not just that, we also saw greater fat loss and also a greater reduction in craving,” says Dicken.

The researchers also looked at other measures of health and found that the minimally processed diet reduced the amount of fat in their bodies and its levels in blood. Perhaps surprisingly, the ultra-processed diet resulted in lower levels of low-density lipoprotein, or “bad” cholesterol.

However, Ciarán Forde at Wageningen University in the Netherlands says the ultra-processed diet was more calorie-dense than the minimally processed one, which could have driven the difference in weight loss. “The basic question remains unanswered as to what type of processing or ingredient is driving the observed effects,” he says.

Forde adds that it isn’t surprising the participants lost weight considering they were overweight or had obesity to start with and then went onto a healthy diet. This may mean the weight-loss results won’t apply to the wider population.