Amy and Nancy are 24-year-old twins.

So, Amy and Nancy, what we want to do is to look at how food processing impacts your health.

They're taking part in a test with Doctor Sarah Berry and Professor Tim Specter from King's College London.

What you're going to do is for two weeks, Nancy, you're going to be doing the unprocessed diet and Amy, you're going to be eating the ultra-processed diet.

Then we're gonna get you back here and then we're going to do the same test again and look at how these different foods have impacted your health.

The twins diet will be matched exactly for calories, nutrients, fat, sugar, and fiber.

Ultra-processed food accounts for more than 50% of our energy intake and it's an even bigger problem in children.

So it accounts for about 65% of our energy intake for children.

This is actually increasing at an alarming rate.

We believe ultra-processed foods are having an unfavorable effect on our health is because the actor processing foods breaks down the structure of the foods.

Often ultra processed foods have very little fiber in them, or the fiber is removed.

Many of these ingredients most people won't have heard of.

But what we don't know is how all of these different chemicals might interact together in terms of our health.

I do eat some processed foods just out of convenience, but I do love lots of fresh food and have quite a healthy diet. I think.

So I think it is gonna be a bit of a shift.

For breakfast this morning I had a quinoa porridge with berries on top and it's done a really good job at keeping me energized and full.

So it's the end of the day now and I've got a bit of a headache, to be honest.

So yeah, ending the day, still hungry, feeling a bit sort of rubbish and yeah, a bit of a headache.

Lots of the things that you've just described are about your headache.

All point to the fact that you're probably having what we call a glucose dip, which is a dip in blood sugar that happens about 2 to 4 hours after people consume really heavily processed refined carbohydrates.

This is a mousse made with fruit puree and only 73 calories.

Yeah, you'd think that would be a healthy option.

Oh my goodness.

Pork gelatin, glucose fructose syrup, acidity regulator, citric acid, sodium hydroxide, modified maize starch.

There's a term we use called the health halo where, you know, you have packaging says "High in fiber or low in sugar, low in salt, plant-based,"

sort of these kind of magic words that make people think, "Wow, these are really healthy."

If anything ever says "low, reduced or no," I'd always be a bit suspicious because how else have they therefore made that food taste great.

In the last decade, the evidence has been slowly growing that ultra processed food is harmful for us in ways we hadn't thought.

The twins find there is one ingredient they keep seeing again and again on food packets.

And then emulsifier again, we've seen that a few times, haven't we?

Emulsifiers are essentially a glue.

They stick components together so that they have a good mouth feel and they don't fall apart in your mouth or on the plate.

So here we have carboxymethyl cellulose CMC, which is one of the commonest emulsifiers used in the food industry.

And I'm just gonna demonstrate what happens when you just add water to it and it very rapidly becomes like a glue.

The food industry uses around 60 different emulsifiers to enhance the appearance, texture and shelf life of ultra-processed foods.

Some are naturally occurring, others are chemically produced.

It's found in an incredible number of our foods and they are potentially harmful.

Doctor Mathilde Touvier is leading one of the world's biggest studies into food additives, publishing regular reports based on the health and eating habits of 174,000 people.

She's been looking emulsifiers' long-term impact on health.

The BBC has had exclusive access to the early results.

They are yet to be verified by peer review, but she says they're concerning.

We observed a significant association between emulsifiers' fire intake and increased risk of cancer overall and breast cancer, not only but also with cardiovascular diseases

In England, Wales and Northern Ireland, food safety is regulated by the Food Standards Agency, the FSA.

It says it's about to launch a public consultation on some emulsifiers.

A spokesperson for the Food and Drink Federation said,

"Food manufacturers take the health of consumers and safety of the food they produce seriously and adhere to the strict regulations.

In the UK, all additives are independently and rigorously assessed to safety by the Food Standards Agency before they can be used in food."

The results of the twins test are now in.

Amy, you were on the ultra-processed food diet.

Your results were really quite different to your sisters.

They were far worse.

Your blood fat levels actually went up; your lipids which are markers of heart disease, they were increased; your blood sugar was noticeably worse.

You actually gained weight nearly a kilogram and you actually lost weight.

It's a bit scary, isn't it? After only two weeks to see those sorts of results.

So just imagine what that would be over 20 years.

After just two weeks of the test, the impact of Amy's ultra-processed diet will be fully reversible.

But the results are in line with a growing body of evidence, linking chemical additives and ultra processing to serious health consequences.

We already have the most obese children in Europe.

That means more type two diabetes, more cancers, more heart disease, more misery, more mental illness.

This really is a future time bomb.

There are now dozens of scientific studies warning of the dangers of diets high in ultra-processed food.

The twin's test showed the potential impact these foods can have on you in just two weeks.

Esme Stallard, BBC News.