Caffeine nation - Am I an addict?

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Media captionThe BBC's Ken Macdonald is given an MRI scan to find out how his brain reacts with and without caffeine.

It's a lovely morning in Cardiff, but I can't see too much.

For a start, I'm in the basement of Cardiff University's Brain Research Imaging Centre (Cubric). Second, my top half is inside a magnetic resonance imaging (MRI) machine.

This is proving to be something of a tight fit for the chunkier gentleman. Just as well I don't suffer from claustrophobia.

I'm assured there will be no harmful side-effects, despite a magnetic field 100,000 times stronger than that of the planet Earth.

Keys, coins and all other metal objects have been taken away to prevent them becoming unintended missiles.

I have earplugs to deaden the worst of the rattles and thumps which will occur over the next half hour or so. My head is held firmly in a cradle to stop it moving around while they take pictures of my brain.

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Media captionThere are calls for tighter controls to prevent children consuming high caffeine energy drinks

And all this without a cup of coffee.

In fact, for the past 24 hours I've been avoiding caffeine in all its forms: no morning java, no Diet Coke, not even chocolate. (This may be the longest period I've ever avoided the latter.) As caffeine has a half-life in our bodies of about six hours, that should be enough time to create the new, decaffeinated me.

And oh how I miss the stuff. Like 80% of the world's population I'm a regular caffeine user. A nice big Americano (with just a splash of milk, thanks) is my dividing line between night and day.

Now, after 24 hours of cold turkey, I feel a bit bleary. And that headache - that's just a touch of sinusitis, isn't it?

I am extracted from the MRI scanner without the need for a cave rescue team. Time to have a look inside my head.

The good news? To my great relief, science has confirmed that I have a brain.

But that's only half the process. I'll be going in again in about half an hour. But first - at last - a mug of delicious coffee. This Americano has a triple shot in it; that's about 150 milligrams of caffeine. All in the interests of science, you understand.

Ken's brain scan image
Image caption Following the brain scan, experts told Ken he was dependent on caffeine, and did not function normally without it

What we've done so far is establish a baseline. We now have scans showing what my brain looks like without caffeine.

And as the drug - because that's what it is - sweeps through my bloodstream towards my noggin, it seems a good time to ask Professor Peter Rogers what's going on. He's professor of biological psychology at Bristol University, and studies the psychopharmacology of caffeine.

He says it's linked to a chemical called adenosine.

"It's a substance that's produced naturally in the body," he says.

"It's what is called a neuromodulator. It subtly affects various physiological functions.

"What it tends to do is inhibit neural activity, and what caffeine does is it block that inhibition. It takes the brake off adenosine's natural inhibiting function.

"In other words caffeine acts as something of a stimulant."

Caffeine is the drug 80% of the world consumes on a regular basis. And it certainly seems to be working on me. Having felt sluggish until now, I now find myself delivering a detailed disquisition on Belgian constitutional politics. I know I'm running off at the mouth a bit, but I don't care because suddenly I feel great.

But is that psychology or biochemistry? It's time to put me back in the scanner again. Will there be a visible change in my brain?

Back into the scanner. More squeezing, rattles and thumps and we have the answer.

Professor Richard Wise of Cubric shows me two images of my brain - before and after caffeine. My caffeinated brain is noticeably darker.

"The blood flow here shown by darker colours is reduced with respect to before your coffee," he says.

The image suggests caffeine has cut the blood flow to my brain by between 20% and 25%.

But here's the catch: a brain with reduced blood flow may actually be my normal state.

Roasted coffee beans
Image caption More than three quarters of the world's population regularly consume caffeine

"Your brain will be used to a certain amount of blood flow and it's got used to having caffeine in your system," Professor Wise explains.

"This represents a more normal state for your brain. So there is some suggestion that that might explain caffeine withdrawal headaches."

So if my boost in mood is really down to getting rid of my withdrawal symptoms, does that make me a caffeine addict?

According to Professor Rogers, not quite.

"You clearly like caffeine and you're dependent," he says.

"Actually, caffeine is a good example of the difference between dependence and addiction. You're dependent in the sense that you don't function normally - but I don't think you're an addict.

"Addiction is something else beyond dependence, addiction is when you have a real compulsion."

I'm relieved, I think. And the fact is most of us seem to get used to using caffeine.

However there remain questions about how we can tell how much caffeine we're consuming - which has implications during pregnancy - and issues surrounding high-caffeine energy drinks, children, and alcohol. All of which we examine in the documentary.

But, first, I could do with another Americano.

BBC Scotland Investigates: Caffeine Nation is on BBC1 Scotland on 10 July at 19:30, and is available across the UK on the iPlayer for a week afterwards.

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