How worried should you be about ultra-processed foods?

In the past few years, there has been a growing clamour from scientists, doctors and the media about ultra-processed foods (UPFs). Some have warned that the trend towards more and more processed food in our diet is mainly responsible for the huge burden of chronic diseases in the modern world. But what are UPFs, what makes them bad for you, and how worried should you be?

The first question is surprisingly difficult to answer. Humans have processed food for millennia to make it tastier and last longer, for example by milling grains, salting, drying, fermenting, pickling and smoking. The term ultra-processed food was coined in the late 2000s by Carlos Monteiro at the University of Sao Paulo, Brazil, who defined it as food created by breaking down whole foods into their constituents, such as sugars, fats and fibres, and chemically modifying them into products, frequently with additives. Things in this category include breakfast cereals, biscuits, fish fingers, ice cream, cakes, mass-produced bread and carbonated drinks.

Traditionally, healthy eating advice has focused on nutritional composition: we are told to avoid foods that are high in salt, sugar and saturated fat, and choose foods high in fibre and vitamins. The idea of UPFs changed the conversation by suggesting that the degree of processing was more important when deciding which foods are unhealthy. Some countries, such as Brazil, Belgium and New Zealand, have changed their official dietary guidelines to encourage people to avoid these foods.

But is there evidence that UPFs are bad for you? There have now been more than 100 studies that found diets high in UPFs are associated with worse health, including a higher risk of cancer, diabetes, dementia, heart disease, bowel disease and obesity. But most of these show only a correlation. Since this kind of diet tends to be high in things we know are bad for you, these studies don’t prove that processing itself is a problem. It is also hard to separate the impact of diet from other lifestyle and environmental factors, like poverty and pollution. And many of these studies rely on questionnaires, which are notoriously unreliable because people are bad at both remembering or honestly reporting what they eat.

The strongest piece of evidence comes from a randomised trial published in 2019 – but even this was only a small, short-term study. Twenty people were fed a diet high in either UPFs or unprocessed food for two weeks, then they switched to the other diet for another two weeks. The diets were equal in terms of their overall calorie content, energy density, protein, carbohydrates, fat, fibre, sugars and sodium. The volunteers were given three meals a day plus unlimited snacks, and they could eat as much or as little as they wanted.

On the UPF diet, volunteers ate around 500 more calories per day than on the unprocessed diet, and after two weeks, they had gained just under a kilogram on average. On the unprocessed diet, they lost just under a kilogram. This suggests the problem with UPFs is that they make us eat more calories. These foods are engineered to be delicious and moreish, easy to eat and digest, and we just can’t help ourselves.

But some scientists think there are other ways in which they could be harming our health. They could be contaminated with toxins created in food factories. They often contain additives like emulsifiers, and there is some evidence that these chemicals could be harmful. UPFs might also disrupt our microbiome and promote inflammation. Some campaigners argue that UPFs should be subject to the same kind of regulations as smoking: prominent warnings on packaging, restrictions on advertising, bans in schools and heavy taxes.

On the other hand, critics say the evidence isn’t strong enough to support such stringent policies. They say the UPF label is too broad, and it can tarnish foods that are actually pretty healthy, like yoghurt and wholemeal bread. Even nutrition professionals struggle to agree when they are asked to categorise a range of foods according to their level of processing, so how is the public supposed to make sense of which foods to avoid? What’s more, not everyone has the time or the money to cook nutritious meals for themselves every day. Demonising UPFs could take away a valuable source of cheap and convenient nutrition.

So how worried should you be? While there are certainly issues with UPF as a category, it does apply to a lot of foods that are unhealthy and designed to encourage overeating. Most of us would benefit from having fewer UPFs and more whole foods in our diet. But avoiding them completely is impractical and probably not necessary. By all means, cut down, diversify your diet and cook for yourself when you can. But don’t freak out if you fall back on ready-made pizzas from time to time.