Ultra-processed food is back under the spotlight. ‘In the last decade, the evidence has been slowly growing that ultra-processed food is harmful for us in ways we hadn’t thought. We’re talking about a whole variety of cancers, heart disease, strokes, dementia,’ Tim Spector, a professor of epidemiology at King’s College London, told a recent BBC Panorama documentary.

Calls for a crackdown are growing: Guardian columnist Simon Jenkins says a ban is ‘common sense’. The idea sounds appealing: outlawing the sale of foods that some people believe are the main reason for obesity, type 2 diabetes and other disease would clearly have an impact – although not necessarily the one intended. Banning infant formula, classed as ultra-processed food, would clearly endanger many lives. And there will be many other unintended consequences, such as rising food costs.

The focus on ‘ultra-processed food’ is a variation of the ‘clean eating’ idea

As a professor of nutrition and food science, I have seen a lot of diet fads. Most are based on a very narrow and selective interpretation of science and make unrealistic promises. They often require very restrictive diets and, instead of improving health, can lead to eating disorders. The human body is able to thrive on a wide range of different foods. A balanced diet – boring as it might sound – is still the best option.

The focus on the ‘ultra-processed food’ concept is a variation of the ‘clean eating’ idea. It stems from the belief that processing destroys the healthiness and wholesomeness of food. It is derived from the ‘Nova’ classification, a framework to classify foods, according to the extent and purpose of processing instead of the more common nutrient profile. Within Nova, ‘ultra-processed foods’ are seen as foods that should be avoided – but there are several problems with this concept. Even after more than a decade, there is no single agreed upon definition – and those that are available are ambiguous and vague. The most common definition is along the lines of ‘anything that is wrapped in plastic and contains an ingredient that you wouldn’t find in a domestic kitchen’, but this is not a meaningful description.

Fish fingers – a sustainable source of fish often without any additives – are considered ultra-processed because of the way the fish is prepared. Ham and bacon, which usually contain E250 (sodium nitrite) are not considered ultra-processed. Fermented drinks like beer are not considered ultra-processed, even though making beer can include an extensive number of industrial processes. Conversely, distilling, an ancient process, makes any food automatically ultra-processed. In many cases, even experts struggle to make the correct assignment.

This ambiguity has consequences: consumers have become wary of all processed food and start to shun frozen and tinned foods, even though they are an affordable and healthy alternative to fresh produce. The discussion creates anxiety about food choices and is likely to fuel eating disorders. It also makes it very difficult for any regulator to investigate the claims of adverse health effects.

Is the concern about ultra-processed foods justified? The evidence is far from conclusive. Most data are from epidemiological studies – they show that people with high ultra-processed food intake have an approximately 20 per cent higher risk of early death than those with low intake. This is broadly comparable to many other diet-related risk factors like processed meat – and much lower than the risk of smoking, which is often compared to eating ultra-processed foods.

On its own, this figure might justify a warning – like the recommendation to reduce the intake of processed meat. But it is more complicated as the category of ‘ultra-processed food’ is very broad. Studies that have separated different types of these foods give a more nuanced picture: some were even associated with health benefits, such as cereals and whole grains – and for many others there was no meaningful association at all.

There are some reasons why studies show that ultra-processed foods affect health: they are often high in saturated fat, salt and sugar (so called ‘junk foods’) and would already be considered as ‘unhealthy’. Some can also encourage overconsumption, and one of the criticisms is that they have been designed to taste ‘moreish’. However, it is not yet known whether this explains the observed effect on health.

Speculation about how ultra-processed food could cause obesity have also focused on possible changes to the microbiome or the effect of additives – but there is no conclusive evidence for either. Adjustments to the microbiome in response to a new diet are a normal reaction, and there are no data that additives in food cause harm. There are also social factors that are important: people who consume a lot of ultra-processed foods are often poorer – and poverty is one of the main risk factors for health.

There are many ultra-processed foods that have a healthy nutrient profile – wholegrain breads, fish fingers or baked beans – and there is no evidence to call them all unhealthy. Likewise, there is no reason to avoid ultra-processed foods – they all can be part of a healthy and balanced diet.