Cereal bars might seem healthy - but they contain additives linked to weight gain

UPFs have been the subject of some debate in Britain ever since TV doctor Chris van Tulleken's blockbuster book Ultra-Processed People.

Many suspect the additives packed into everything from cereal bars to pastries, pies, wraps and ready meals - to make them taste nice and last longer - could actually be harming us. But this is very difficult to prove because until now, observational studies have just shown that people who eat more UPFs are more likely to be ill and overweight. This is complicated by the fact that UPFs are also more likely to be high in calories, fat, sugar and salt.

And people who eat a lot of UPFs are also more likely to smoke and do less exercise. So Professor van Tulleken and his colleagues at University College London have run a clinical trial to to tease out the impact of these multiple industrial processes and the addition of ingredients not typically found in home cooking.

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The trial involved 55 people each given eight weeks trying two diets matched exactly for levels of fat, saturated fat, protein, carbohydrates, salt and fibre. The only difference was that one diet was made up of only UPFs, and the second a diet included only minimally processed foods such as porridge oats and home cooked spaghetti bolognese.

Participants lost twice as much weight on the non-UPF diet - proving for the first time there is something going on in the processing.

Professor van Tulleken, of University College London, 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."

Tracy Parker, nutrition lead at the British Heart Foundation, said: “These findings support what we have long suspected – that the way food is made might affect our health, not just the nutrients it contains.”

Chris van Tulleken's book Ultra-Processed People was adapted into a BBC Two documentary called Irresistible: Why We Can't Stop Eating. They explore the addictive nature of UPFs and their impact on our health and society.

UPFs typically contain preservatives, emulsifiers, artificial colours and flavours which are used to enhance taste, texture and prolong shelf life. They are hard to avoid. Most supermarket sliced bread is a UPF as well as shop-bought sandwiches, wraps, pastries, instant noodles, crisps, some breakfast cereals and ice cream.

The new trial ensured both UPF and non-UPF diet options met criteria using the NHS-backed Eatwell Guide, which outlines recommendations on how to eat a healthy, balanced diet.

Despite this, after eight weeks on the non-UPF diet participants lost 2% of their body weight, compared to 1% after eight weeks on the only-UPFs diet. Scaled up over a year it suggests 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.

Those on the trial were also asked to complete questionnaires on food cravings before and after starting the diets. Those eating minimally processed foods had less cravings and were able to resist them better. The UPF diet also did not result in significant fat loss.

Tracy Parker, nutrition lead at the British Heart Foundation, added: “Completely cutting UPFs out of our diets isn’t realistic for most of us. But including more minimally processed foods – like fresh or home cooked meals – alongside a balanced diet could offer added benefits too. Mediterranean-style diets, which include plenty of minimally or unprocessed foods such as fruit, vegetables, fish, nuts and seeds, beans, lentils and whole grains, have consistently been shown to reduce our risk of heart attacks and strokes.”

The Eatwell Guide recommends the average woman should consume around 2,000 calories a day, while an average man should consume 2,500. Participants were generally consuming more than this before they started the trial.

Senior author Professor Rachel Batterham said: "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 prioritising high-fibre foods such as fruits, vegetables, pulses and nuts.

"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."

UPFs have artificial additives to boost flavour and shelf life including emulsifiers, sweeteners, stabilising gums, flavour compounds and colouring agents. These are typically not used in home cooking but can be cheaper in mass produced food than natural ingredients. The findings are published in Nature Medicine.