they are black beast That's what many nutritionists are thinking - mass-produced but tastier foods like chicken nuggets, packaged snacks, sodas, ice cream and even sliced ​​brown bread.

So-called ultra-processed foods (UPF) Accounts for 56% of calories consumed in the UKthe numbers are even higher for children and people living in poor areas.

UPF is defined by the number of industrial processes it undergoes and the number of ingredients (often difficult to pronounce) on the packaging. Most are high in fat, sugar or salt; many you would call fast food.

What they all have in common is their synthetic look and taste, which makes them a target for some clean-living advocates.

There is growing evidence that these foods are bad for us. But experts can't agree on exactly how or why they affect us, and it's unclear whether science will give us answers anytime soon.

While recent research indicates many widespread health problems, including cancer, heart disease, obesity, and depression, linked For UPF, there is currently no evidence that they are caused by them.

For example, an observational study of more than 500,000 people in the United States was recently presented at the American Society of Nutrition meeting in Chicago. The study found that those who ate the most UPF were about 10 percent more likely to die early, even taking into account their body mass index and overall diet quality.

In recent years, many other observational studies have shown a similar link - but this is not the same as proving it how Food processing processes can cause health problems, or determine which aspect of these processes may be the culprit.

So how can we learn the truth about ultra-processed foods?

Dr. Nerys Astbury, a senior researcher in diet and obesity at the University of Oxford, said the research needed to definitively prove that UPF causes health problems would be extremely complex.

It involves comparing a large number of people on two diets - one higher in UPF and the other lower in UPF but with exactly matched calories and macronutrient content. This is actually very difficult to do.

Participants need to be locked away so their food intake is strictly managed. The study will also need to recruit people with similar eating habits as a starting point. This will be logistically very challenging.

To deal with the possibility that people who eat less UPF may simply be adopting healthier lifestyles such as getting more exercise or getting more sleep, participants in these groups need to have very similar habits.

"It would be an expensive study, but you could see changes in diet relatively quickly," Dr. Astbury said.

Funding for such research may also be difficult to obtain. There may be accusations of conflicts of interest because researchers motivated to conduct such trials may know what conclusions they want to draw before starting.

In any case, these trials don't last long—too many participants are likely to drop out. Telling hundreds of people to stick to a strict diet for more than a few weeks is unrealistic.

What exactly would these hypothetical experiments prove, anyway?

Duane Mellor, head of nutrition and evidence-based medicine at Aston University, said nutritional scientists cannot prove whether specific foods are good or bad, or what effect they have on individuals. They can only show potential benefits or risks.

"The data doesn't show more or less," he said. Claims to the contrary are "bad science," he said.

Another option is to study the effects of common food additives in UPF on laboratory models of the human gut—something scientists are busy doing.

However, there's a broader issue - there's a lot of confusion over the actual definition of UPF.

Typically, they contain more than five ingredients, several of which you'd be hard-pressed to find in a typical kitchen cupboard.

Instead, they are often made from cheap ingredients such as modified starches, sugars, oils, fats, and protein isolates. Then, to make them more appealing to the taste buds and eyes, flavor enhancers, colors, emulsifiers, sweeteners and glazing agents are added.

They range from the obvious (sugary breakfast cereals, carbonated drinks, American cheese slices) to the perhaps more unexpected (supermarket humus, low-fat yogurt, some cereals).

This raises the question: How helpful is it to label chocolate bars on the same level as tofu? Do some UPF affect us differently than others?

To find out more, BBC News spoke to the Brazilian professor who coined the term "ultra-processed foods" in 2010.

Professor Carlos Monteiro also developed the Nova classification system, which ranges from "natural foods" at one end (such as beans and vegetables), to "processed cooking ingredients" (such as butter), to "processed foods" (like canned tuna and salted nuts) all the way up to UPF.

The system was developed after obesity rates continued to rise in Brazil as sugar consumption fell, and Professor Monteiro wanted to know why. He believed that our health is affected not only by the nutritional content of the food we eat, but also by the industrial processes used to make and preserve it.

He said he didn't expect the attention UPF would receive so far, but claimed "it is driving a paradigm shift in nutritional science."

However, many nutritionists say fears about UPF are overblown.

Gunter Kuhnler, professor of nutrition and food science at the University of Reading, said the concept was "vague" and the message it sent was "negative", leaving people confused and fearful about food.

Indeed, there is currently no concrete evidence that the way food is processed harms our health.

Processing is something we do every day—chopping, boiling, and freezing are all processes that are harmless.

When manufacturers process food on a large scale, it helps ensure food safety, lasts longer and reduces waste.

Take frozen fish sticks, for example. They use up the leftover fish, provide the kids with some healthy food, and save the parents time - but they still count as UPF.

What about meat alternatives like Quorn? Granted, they don't look like the original ingredients from which they are made (and thus fall under UPF's Nova definition), but they are considered healthy and nutritious.

"If you make a cake or brownies at home and compare it to a cake or brownie that comes in a bag with flavor enhancers, do I think there's any difference between the two foods? No, I don't know ," Dr. Astbury told me.

The Food Standards Agency, the UK's agency responsible for food safety, acknowledged reports that people who eat large amounts of UPF are at greater risk of heart disease and cancer, but said it would not take any action against UPF until there was evidence that it caused specific harm.

Last year, the government's Scientific Advisory Committee on Nutrition (SACN) looked at the same report and concluded there was "uncertainty about the quality of the available evidence". It also has some concerns about the practical application of the Nova system in the UK.

For his part, Professor Monteiro is most concerned about processes involving high temperatures, such as the production of breakfast cereal flakes and puffs, which he claims "can reduce the quality of the natural food matrix".

He points to a small study that suggests this leads to a loss of nutrients that makes us feel less full, meaning we're more likely to make up for the shortfall with extra calories.

It's also hard to ignore the creeping sense of self-righteousness and snobbery surrounding UPF, which can make people feel guilty about consuming it.

Dr Adrian Brown, a professional nutritionist and senior researcher at University College London, said demonizing a food is unhelpful, especially when what and how we eat is such a complex issue. "We have to be careful about the moralization of food," he said.

Living life without UPF can be expensive - and cooking meals from scratch takes time, effort and planning.

one Food Foundation's recent report Healthier food costs twice as much per calorie as less healthy food, and the poorest 20% of the UK population need to spend half their disposable income on food to meet government requirements, study finds Healthy eating advice. The richest people only pay 11%.

I asked Professor Monteiro if it was possible to live without UPF.

"The question here should be: Is it feasible to prevent the rising consumption of UPF?" he said. "My answer is: It's not easy, but it's possible."

Many experts say the current traffic light system on food labels - which marks high, medium and low levels of sugar, fat and salt - is simple enough to help you when you're shopping.

There are now several smartphone apps available for unsure shoppers, such as the Yuka app, which allows you to scan barcodes and learn more about how healthy a product is.

Of course, there's also the advice you already know - eat more fruits, vegetables, whole grains and legumes while cutting back on fat and sugary snacks. Whether or not scientists prove that UPF is harmful, it's still a good idea to stick to it.

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