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Monday, 9 December, 2002, 14:13 GMT
Anxious coffee-drinking explained
Coffee beans
The effects of coffee may be far-reaching in some people
Some people have an attack of nerves when they have a cup of coffee - and now geneticists believe they have found out why.

So-called "caffeine-induced anxiety" affects only a small proportion of the millions who drink it regularly.

But for those who are afflicted, a cup of coffee is more likely to make them bite their nails or feel unusually nervous.

Now research suggests that these people have two linked genetic variations which separate them from the rest of the population.

Tests were carried out by teams at the University of Chicago, alongside scientists at the universities of Munster and Wurzburg in Germany.

Twin genes

They recruited 94 healthy, occasional caffeine drinkers, and their responses - both physical,and in terms of mood - were analysed, after they had been given either caffeine or a placebo containing no caffeine.

They also took blood samples from the volunteers.

Scientists already know that two brain proteins are known to interact with caffeine, so they looked at the genes which produce the proteins.

They found that, among the group, there were four variations on this "adenosine receptor gene".

However, research subjects who had two of these variants were much more likely to show anxiety after a dose of caffeine, even though physically their reactions were exactly the same.

Dr Harriet de Wit, from Chicago, one of the lead authors of the work, presented at a US conference on Sunday, said: "This is the first time that anyone has identified why people have different behavioural reactions to the same drug."

Junk jewels

Other research presented at the same conference suggests new genes which may play a role in behavioural problems

Scientists previously thought that some sections of the human genome - the complete genetic map of a person - were "junk", and contained no influential genes.

However, increasingly, they are being forced to reconsider this.

Researchers from Mt Sinai School of Medicine have found a candidate gene in one of these areas which appears to have a link to another gene which makes a behaviour modifying hormone.

Corticotrophin releasing hormone plays a key role in the response of humans and other mammals to external threats.

Alterations in this hormone's activity appear to contribute to a number of mental illnesses including depression, anxiety disorders and anorexia.

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