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CUT down on fatty food. No, sugar. Aim for a Mediterranean diet. And remember to eat more plants…

The variability of healthy eating advice has become a cliché in itself. Yet despite all the contradictions, there is one thing that many agree on: we should avoid junk food. Until recently though, no one could give you a decent reason why. Gastronomic snobbery aside, science lacked an agreed definition of what junk food actually is, and that has made it difficult to know whether we should be avoiding it and, if so, why.

It has long been assumed that processed junk foods are bad because they tend to contain too much fat, salt and sugar. Recent studies, though, suggest that other mechanisms could be at work to make these foods harmful to our health. Getting to grips with what these are could help us not only make healthier choices, but also persuade the food industry to come up with healthier ways of giving us what we like to eat.

One thing’s for sure: we certainly do like it. Factory-made food makes up , and around 60 per cent in the US. But while junk food has a bad name among many food lovers, dietary health research and the public health advice that stems from it have so far concentrated either on individual food groups, like meat and dairy products, or the relative amounts of the three macronutrients – proteins, fats and carbohydrates – that we consume.

In most countries, nutrition guidelines advise people to base their diet on starchy carbohydrates like bread and pasta, while eating plenty of fruit and vegetables, limiting meat and dairy to avoid too much fat and swerving salt and sugar where possible.

Although factory-made foods tend to be high in the frowned-on ingredients of fat, salt and sugar, few national guidelines explicitly advise people to avoid processed foods and instead cook meals from scratch.

That is where the field of nutrition has been going wrong, according to Tim Spector, an epidemiologist at King’s College London who studies food’s effects on health. What policy-makers should focus on, he says, is “getting people to change to eating real food”.

The development of a new way to classify foods by the degree of processing is a first step toward that goal. Called , a nutrition researcher at the University of Sāo Paulo in Brazil, and his colleagues

The ultra-processed group includes not just things usually seen as junk food, like fries or frozen pizzas, but also some breakfast cereals, soups, ostensibly healthy low-fat, low-salt ready meals and even most kinds of mass-produced bread. The defining criteria is that they are made using processes not normally used at home, such as high-pressure fat hydrogenation and production of hydrolysed vegetable proteins using hydrochloric acid. They also contain , sweeteners and flavour enhancers, designed to make the food more palatable.

The NOVA classification system was important because it gave researchers the common language needed to study the links between ill health and factory-made food, says Marion Nestle at New York University, a former US government adviser who writes about the food industry. The concept of ultra-processing is powerful, she says, “because it gets us beyond talking about individual nutrients and talking about food as a whole”.

The first large studies looking at links between consumption of ultra-processed food and health started coming through in the past two decades. A clear association began to emerge between these foods and ill health, with people who ate the most being at a higher risk of obesity and conditions such as heart disease and diabetes.

One study even found that the more ultra-processed food in a person’s diet, the higher their risk of early death. More than four portions of ultra-processed food a day , with each additional portion raising risk by 18 per cent. , published in 2020, also concluded that there were links between junk food intake and worse rates of high blood pressure, and unhealthy cholesterol levels.

Fast-food focus

Importantly, while nutrition research is notorious for its contradictory results – one day a study finds that too much fat is the main problem with our diets, while the next, a paper blames carbohydrates – this doesn’t seem to be the case for ultra-processed foods, says Monteiro. “With red meat, some studies say, ‘Yes, there is an association with ill health’, some, ‘No’. In the case of ultra-processed food, all show an association.”

So far, so bad. But these kinds of studies can only find correlations between eating junk food and bad health. They can’t prove that one causes the other. We know that eating more junk food also correlates with having a , which brings many health disadvantages. The only way to tease apart cause and effect would be to carry out a randomised trial where the only difference between two groups of people is their diet.

For a long time, no such trial existed. Then in 2019, Kevin Hall, a physiologist at the US National Institutes of Health, and his colleagues enrolled 20 participants into just such a . For two weeks, they ate snacks and meals that were mainly ultra-processed food, a careful selection of chicken nuggets, ready-meal ravioli, cookies, cereal and the like. For another two weeks, they ate nothing but wholefoods including fruits, vegetables, meat and yogurt.

“On a processed food diet, people ate 500 calories more per day”

The participants were offered about twice as much food as they should have needed, and they could eat as much or as little of it as they wished. Crucially, the two diets were controlled so that the snacks and meals on offer contained more or less the same amounts of fat, protein, total carbohydrates, sugar, salt and fibre.

The effect on the volunteers was dramatic. On the ultra-processed diet, people put on weight, gaining an average of 0.9 kilograms. On the wholefoods diet, people lost the same amount on average: just under a kilo. As it turned out, when given free rein to eat as much processed food as they liked, people ate about 500 calories more over the course of a day than if they were choosing from wholefoods. “That’s a large difference,” says Hall. “There’s clearly a causal relationship.”

One possible explanation is that people ate more of the processed food because they found it tastier. On questionnaires, the volunteers rated it as only slightly nicer than the wholefood meals, a difference so small it could have arisen by chance. But how much we eat of any particular food isn’t solely down to our conscious opinion of its tastiness.

Ultra-processed foods generally come loaded with just the right amount of fat, salt and sugar for us to find them hard to resist. Think of how common it is to eat more than you intended from a packet of biscuits or crisps, says Nestle. “You can’t just eat one.”

Ultra-processed food could also have led to overeating in another way, by encouraging people to eat faster. Most of this type of food has water taken out of it during manufacturing. Think of crisps compared with boiled potatoes. “You end up with a more concentrated form of the food,” says Hall. “With each bite, you are eating more calories.”

To shed further light on this, Hall’s team is planning a further trial, this time with a third arm that provides ultra-processed meals bulked out with extra vegetables, which would contribute few calories, but would make the meals slower to eat. This would show if the speed at which people eat is actually key to their calorie consumption.

Another explanation, known as the protein leverage hypothesis, is that a crucial regulator of our appetite is the biological need to eat a certain amount of protein every day. Because meat, a key source of protein, is more expensive than other ingredients, processed foods tend to be low in protein but high in fat and carbohydrates. People whose diet is mainly processed food end up hungrier because they aren’t meeting their protein requirements, so they eat more to assuage that hunger – which, ironically, will end up being more of the low-protein processed fare they are used to eating.

The findings from Hall’s study seem to support this hypothesis because when people ate the ultra-processed diet, their extra 500 calories a day came mainly from fats and carbs and their protein intake was slightly lower than when offered wholefoods.

, one of the originators of the protein leverage hypothesis, believes that a lack of protein and fibre in processed foods combine to leave people hungry. “Fibre together with protein are the dietary components that are the most satiating,” he says.

Feed the bugs

Sure enough, fibre also differed between the two diets in Hall’s study. While both had the same total amount of fibre, in the wholefoods diet, it was mostly in the form of insoluble fibre from fruit, vegetables and wholegrains. In the ultra-processed diet, soluble fibre was put in participants’ drinks because that was the easiest and most palatable way of adding fibre. Soluble and insoluble fibre are both thought to be good for us, but they have different effects on the digestive system and our gut bacteria.

People need both types of fibre in their diet to support a healthy gut microbiome, says Robert Lustig, an obesity doctor and researcher at the University of California, San Francisco. In his new book, Metabolical: The lure and the lies of processed food, nutrition, and modern medicine, Lustig argues that a lack of fibre deprives certain gut bacteria of the chance to metabolise fibre into short-chain fatty acids, which have beneficial anti-inflammatory effects on the body.

Spector also believes that one of the biggest dangers of ultra-processed food is that it disturbs our gut bacteria. However, in his opinion, the cause is the artificial additives that it contains, such as sweeteners and the emulsifiers that help fat-soluble substances mix evenly with water. , causing inflammation. “The more artificial foods you have, the more difficulty your gut microbes have in digesting it,” says Spector.

Any or all of these factors could explain Hall’s results, or, indeed, there could be something else going on. Hall himself points out that more research – and larger studies – are needed to tease apart the potential mechanisms. “The science is not yet in on this,” he says.

Nevertheless, Hall’s results have already stimulated action. Soon after the study came out, the UN’s Food and Agriculture Organization published a , and calling for a raft of measures to reduce their consumption, including food labelling, advertising bans and junk food taxes on such products at the point of sale.

So far, only and , including the UK, are planning crackdowns on the sale and advertising of high fat, sugar or salt foods, which encompass many processed options.

Hungry for change

Some campaigners argue that governments should go further still and introduce higher taxes on such foods. This is controversial, though, because processed foods make up such a large part of most people’s diet in high-income countries. Processed foods are often cheaper than home-cooked meals thanks to economies of scale and are quicker to prepare, all of which matters if you are working long hours to support a family on a low income. “The last thing you want to do is make it more difficult for people to feed their families,” says Hall.

Others say that, right now, it is simply unrealistic to ask people to cut processed food out of their diet. “You can’t just tell people to avoid 60 per cent of their current calorie intake overnight, unless you’ve offered them a real solution,” says Ciaran Forde at the Agency for Science, Technology and Research in Singapore, who worked with Hall on the 2019 trial.

Waging all-out war on junk food could also distract from more important dietary advice, like limiting fat or sugar. And it could deter people from potentially healthier processed options, like lower-calorie ready meals, says Forde.

Food processing might even turn out to be part of the answer. “Maybe certain elements of the modern food environment have got us into the situation where it’s easy to overconsume. But reformulating foods to slow the rate of consumption is possibly also part of the solution,” says Forde.

There are already food products in development to combat the lack of fibre in modern diets, and these would definitely be classed as ultra-processed. For instance, firms are developing soup, yogurt and bread with extra fibre or the beneficial short-chain fatty acids produced when gut bacteria digest it. A US firm called BioLumen is also .

Such novel foods are likely to be more expensive, however, and so out of reach for poorer families who rely on ultra-processed products. And with so much of the modern diet coming from factory-made foods, it is hard to imagine a complete substitution with the engineered options.

Another thing is that no one can yet agree on exactly what it is that makes processed food unhealthy, so we don’t know which processed food components to limit and which from wholefoods we need to boost. “We have 26,000 chemicals in our food. We don’t really understand what’s in real food, we’re still getting to the bottom of it,” says Spector.

All things considered, while we may not yet fully understand the mechanisms, the evidence is accumulating that it is probably best to avoid eating too much ultra-processed food. While we wait for the science to catch up, the take-home message is simple: eat wholefoods as much as budget and circumstances allow. Your body may thank you for it.