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The brains behind the obesity problem

Woman eating chocolate

Why do we over-eat? As concerns grow about the nation's obesity problem, BBC science presenter Michael Mosley explains the science behind pleasure-seeking urges such as eating and having sex.

We tend to think of the pursuit of pleasure as something a bit frivolous, frothy, all about the moment. But in fact, the drive to seek pleasure is a vital part of our survival mechanism.

If we didn't get pleasure from doing things we wouldn't make friends, we wouldn't eat, and we certainly wouldn't do anything as messy and dangerous as sex. In which case, goodbye homo sapiens.

Pleasure means different things to different people, but for scientists pleasure is simply a short-term reward for doing what your body wants. It is largely about fulfilling basic human needs and one of the most basic of our human needs is food.

Inside your brain there are a number of different pleasure circuits that control food intake, but one of the most important is the dopamine reward system.

This system is particularly important when it comes to triggering the desire to eat, so much so that if you block the activity of dopamine in animals they stop eating altogether and soon starve to death.

When it comes to eating, as well as knowing when to start, it is equally important to know when it is time to stop. A recent study, which involved eating chocolate, revealed just what goes on in the brain when the pleasure circuits say "no more".

There is something primal about chocolate, it appeals to the cavemen in all of us. It's not just the fat and the sugar, but the way it melts just below blood temperature.

With the right chocolate, when you put it on your tongue and allow it to melt, you get wonderful flavours coming through into your nose. Mmmm.

Anyway, in this particular study volunteers were asked to eat as much chocolate as they comfortably could while lying in a brain scanner.

The researchers were particularly interested in the orbito-frontal cortex (OFC), the part of the brain behind the eye sockets that tells us how much we like something.

So the participants ate and ate, and at the very moment that they signalled to the experimenters that they had had enough, activity in the OFC changed. This was the moment that enjoyment turned to revulsion. It's this reaction that gives pleasure its defining feature - transience.

Over half the population in the UK is overweight and part of the problem is that people go on eating long after they have ceased getting pleasure from it. The switching off of pleasure is an important signal and we ignore it at our peril.

The neurotransmitter, dopamine, is heavily involved in prompting us to eat, but it does far more than that. It links up neural circuits deep inside our brains that are associated with not just pleasure and reward, but also novelty seeking.

We do many things in life, including eating, having sex, gambling and risk taking, because doing them results in our brains being flooded by feel-good dopamine.

Take the longest bungee jump in Europe, a terrifying plunge off the Verzasca dam in Switzerland.

It's 220m high from the top to the bottom and it's where James Bond (well, a stuntman) made his death defying leap in the opening sequence of GoldenEye.

Thrill seekers go there from all over the world to experience the thrill of free falling. Most of them are young and most of them are male. But is this sort of thrill seeking behaviour simply a form of bravado, or is there also some sort of genetic link?

There is certainly a cultural element to thrill seeking, but there is also some evidence that the urge to seek out novel experiences is a personality trait that you are born with.

In other words, it is unlikely that someone who is normally cautious will suddenly, overnight, become a dare devil.

In addition, a recent study found that these urges to do new things are associated with higher levels of dopamine in the brain, which may explain why people who constantly crave stimulation are at greater risk of addictive behaviour, such as drug abuse and gambling, than the rest of the population.

I did the bungee jump at the Verzasca dam. I stood on the edge, peered down, felt terrified, but I still jumped.

The sheer embarrassment of having come that far and not jumping was too great, I couldn't just quietly withdraw.

As I dived head first towards the rocks I felt a mix of terror and exhilaration. I can't say that I really enjoyed it, yet I'm glad that I did it. Some pleasures, I think, are best never repeated.