Could ultra-processed foods be harmful for us?

"It's a bit scary isn't it after only two weeks to see those results."

Aimee, 24, has spent two weeks on an ultra-processed diet as part of a test carried out by scientists from King's College London for BBC Panorama.

Her identical twin, Nancy, was also on a diet containing exactly the same amount of calories, nutrients, fat, sugar and fibre - but she was consuming raw or low-processed foods.

Aimee gained nearly a kilo in weight - Nancy lost weight. Aimee's blood sugar levels also worsened and her blood fat levels - lipids - went up.

This was a short-term study on just one set of twins, but the results highlight growing fears among some scientists about the possible impact of so-called ultra-processed foods on our health, which BBC Panorama has been investigating.

Prof Tim Spector, is a professor of epidemiology at King's College London, who studies trends in disease and oversaw the test.

He told BBC Panorama: "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."

The term ultra-processed foods - or UPF - was only coined 15 years ago but it makes up about half the things we now eat in the UK.

From sliced brown bread to ready meals and ice cream, it is a group of foods made with varying - but often substantial - levels of industrial processing. Ingredients used, such as preservatives, artificial sweeteners and emulsifiers, do not typically feature in home cooking.

"Ultra-processed foods are among the most profitable foods companies can make," says Prof Marion Nestle, a food politics expert and professor of nutrition at New York University.

As our consumption of ultra-processed foods go up - the UK is one of the biggest consumers per head in Europe - so have rates of diabetes and cancer.

Some academics think the link is not coincidental.

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In January, one of the most comprehensive studies on ultra-processed food - by Imperial College's School of Public Health - was published in The Lancet medical journal.

The study of 200,000 UK adults found that higher consumption of ultra-processed foods may be linked to an increased risk of developing cancer overall, and specifically ovarian and brain cancers.

And, as of last month, the World Health Organization (WHO) is now cautioning against the long-term use of artificial sweeteners - citing potential health risks.

It follows dozens of studies linking increasing consumption of UPF to increased risk of developing serious illnesses.

But proving that specific ingredients cause human harm can be challenging - there are a range of other factors in our lifestyles that can cause these diseases. For example: lack of exercise, smoking or sugary diets.

Common ultra-processed foods:

The first investigations into mortality and consumption of ultra-processed food started in France at the University Sorbonne Paris Nord, as part of the ongoing study into the eating habits of 174,000 people.

"We have 24-hour dietary records during which they tell us all the foods, the beverages and so on, that they are eating," explains Dr Mathilde Touvier who heads up the study.

The ongoing research has already published results showing UPF may drive an increased burden of cancer.

More recently, they have been looking into the impact of one specific ingredient - emulsifiers - which act as a glue in ultra-processed foods to hold everything together.

Emulsifiers are the Holy Grail for the food industry - they improve the appearance and texture of food, and help to extend the shelf life far beyond that of less-processed food.

They're everywhere, in mayonnaise, chocolate, peanut butter, meat products. If you eat, you're likely to be consuming emulsifiers as part of your diet.

BBC's Panorama was given exclusive access to Dr Touvier's early results.

They are yet to be peer reviewed - a crucial verification step for scientific studies - but she said they are still concerning.

"We observed significant associations between emulsifier intake and increased risk of cancer overall - and breast cancer notably - but also with cardiovascular diseases," she says.

This means that a pattern has been observed between consuming ultra-processed food and disease risk, but further research is needed.

Despite the growing body of evidence, the UK's Food Standards Agency (FSA) - which regulates the food industry in England, Wales and Northern Ireland - has yet to issue any regulation restricting emulsifiers.

When Panorama asked the FSA about the growing body of evidence that these additives could cause harm it said: "We have not been presented with any evidence - by this programme or otherwise - of any specific emulsifiers which are believed to pose a risk to health."

But the FSA said it planned to hold a public consultation.

Could the food industry itself be playing a role in pushing back on regulation?

The BBC Panorama team spent the past eight months investigating.

"Food companies are not public health agencies... their job is to sell products," food politics expert Prof Nestle told the BBC.

She said the food industry has been known to fund research, sponsor experts and disparage existing studies to prevent regulation.

The International Life Sciences Institute (ILSI) is a body that receives funding from some of the world's largest food companies.

It says its mission is to "provide science that improves human health" - but it has previously published studies globally undermining regulation and public guidance on healthy diets. In 2012, the European Food Safety Agency was so worried about potential conflicts of interest, it insisted anyone associated with ILSI either had to resign from the institute or leave the agency.

Prof Alan Boobis, emeritus professor at Imperial College London, is an unpaid director of ILSI Europe and a former vice president of its board of directors. But he also heads up a group of UK scientists, known as the Committee on Toxicity which provides advice on the risk of chemicals in food to the FSA.

More than half the members of the committee have recent links to the food or chemicals industries. And over the past 10 years, the committee hasn't supported a single restriction on the use of any chemical additive in our food.

Prof Boobis told Panorama his advice wasn't slanted to favour industry, and he had always been "totally committed to conducting and identifying the very best scientific research... whoever is funding it."

The Food Standards Agency said it had a "clear code of conduct… for declarations of interest" and that it had "no evidence" bias has affected its decisions.

ILSI said: "[We] operate within a framework of the highest principles of scientific integrity."

One of the most controversial additives in UPF is the sweetener aspartame.

Two-hundred times sweeter than sugar, it has been heralded as a great low-calorie alternative - turning once unhealthy sugary drinks, ice cream and mousses into products marketed as "healthy".

There have been questions about its potential harm over the past two decades.

Then, last month, the World Health Organization said, although the evidence is not conclusive, it was concerned that long-term use of sweeteners like aspartame may increase the risk of "type 2 diabetes, heart diseases, and mortality".

In 2013, the European Food Safety Authority (EFSA) decided - after looking at all the available evidence - that aspartame was safe. The UK's Food Standards Agency accepted this position.

The Committee on Toxicity looked at a study into aspartame in 2013 and concluded that the results "did not indicate any need for action to protect the health of the public".

Six years later, Prof Erik Millstone, emeritus professor of science policy at the University of Sussex, decided to review the same evidence considered by EFSA - to see who had funded the different studies.

He discovered that 90% of the studies defending the sweetener were funded by large chemical corporations that manufacture and sell aspartame.

And that all the studies suggesting that aspartame may cause harm were funded by non-commercial, independent sources.

A spokesperson for the Food and Drink Federation, a membership body for manufacturers, told the BBC that companies took "the health of consumers, and safety of the food they produce, seriously - and adhere to the strict regulations".

A spokesperson for the International Sweeteners Association said: "Low/no calorie sweeteners are safe to use, are amongst the most thoroughly researched ingredients in the world and have been approved by all major food safety bodies, including the European Food Safety Authority (EFSA) and the U.S. Food and Drug Administration."

The FSA says it will look into WHO's ongoing assessment of aspartame. And the government says it is aware of the growing concerns around UPF and has ordered a review into the evidence on ultra-processed foods.

Watch Panorama - Ultra-Processed Food: A Recipe For Ill-Health? - on BBC iPlayer and listen to 5Live listeners have their say with Nicky Campbell, Is ultra processed food okay?