SCIENCE / My vegetable love will grow: Even carrots, surely the epitome of goodness, can be addictive, suggests recent research. What, then, causes addiction? Rob Stepney investigates

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ALCOHOLICS washed up on a park bench in a sea of cans and bottles. Junkies shooting up in toilets. Images of compulsive behaviour are central to our concept of addiction. Extend the definition just a little and it takes in less extreme drug habits such as heavy nicotine and caffeine use, and the 'medicinal' taking of Valium. Broaden it further and it seems to fit obsessive gambling and binge eating. Though not based on the ingestion of mind-altering chemicals, these last two behaviours can be as difficult to quit as drugs, and may cause as much social and personal damage. But can the term 'addiction' also be applied to the obsessive eating of carrots?

Writing recently in the British Journal of Addiction, psychiatrists Ludek and Karel Cerny describe three cases of profound dependence on carrots encountered in their practice in Prague. 'At first I thought it was a spoof,' says the journal's editor, Professor Griffith Edwards of the Maudsley Hospital, London. 'I considered with great care whether to publish the article, but there was the roughness of truth about it.'

All three patients had to consume an ever greater number of carrots to satisfy their craving. They were profoundly irritable and anxious when their supply was interrupted, and attempts to kick the habit were frustrated by repeated relapse. The patients' social lives were disrupted by their bizarre behaviour, and all suffered at least short-term damage to their health.

One patient first turned to carrots during pregnancy, when she reckons to have consumed two-and-a-half sacks of them. Her craving then subsided, only to return with undiminished intensity 15 years later. So strong was her need that she preserved carrot peelings as a reserve supply, ate carrots unwashed if no water was available, and on one occasion took carrots from a stable where they had been delivered as horse fodder. Faced with social disapproval, she began eating carrots in secret.

Several features are characteristic of more conventional addictions. The first is tolerance: increasingly large doses of a substance are required to produce the same subjective effect. Second, there is development of a withdrawal syndrome, leading to craving when the addictive substance is removed. Both elements are clearly present in compulsive carrot eaters.

Usually, there is a third component. Potentially addictive behaviours generally confer pleasure, at least at the beginning. Why carrots should be so powerfully rewarding is difficult to understand. Carotene is not known to have any psychological effects, although carrots contain sugar, and sweetness can

become addictive. ''But there are more convenient sources of sugar, even in Prague,' Griffith Edwards comments. 'So we have a puzzle. But any puzzle in science may be a key to opening doors.'

For decades, scientists have sought an explanation for the many different types of addiction. The most popular at the moment is the idea that many compulsive behaviours are caused by a deficiency in the brain of a substance called serotonin which is involved in transmitting messages beween nerves. This substance certainly plays a part in the regulation of mood and may be involved in the control of impulsive behaviour.

A new class of drugs which increases serotonin levels in the brain has been said to improve recovery prospects for people suffering from compulsive eating disorders such as anorexia nervosa and bulimia. 'There is also evidence in alcohol dependency that use of these agents decreases the frequency and severity of relapse,' says consultant psychiatrist Tim Kidger of St Andrew's Hospital, Northampton. But the case is not sufficiently strong for him to use serotonin-enhancing drugs in routine clinical practice. Nor is it clear why such drugs should have a beneficial effect. The evidence that serotonin levels are abnormal in people with eating disorders, for example, is patchy at best.

Dr David Schmidt and colleagues from Vanderbilt University in the US recently tested the hypothesis that people crave sweets in an attempt to restore depleted levels of serotonin in their brains. Eating high-carbohydrate, low-protein foods maximises the passage into the brain of tryptophan, a precursor of the mood enhancer serotonin. The researchers therefore argued that binge eating of sweets should occur during periods of low mood. But their study of obese women prone to this problem, published this summer in the journal Addictive Behaviors, found no relationship between binge eating and episodes of depression linked to low serotonin levels.

In the 1980s, the discovery that the mammalian brain produces its own opiate-like chemicals, termed endorphins, seemed to offer a breakthrough in our understanding of addictions. The idea was that taking drugs like heroin directly switched on our opiate system, while other behaviours, such as compulsive exercise, might well achieve the same effects indirectly.

Hannah Steinberg, now visiting professor at Middlesex University's school of psychology, recalls: ''The link between endorphins and exercise was initially by analogy: there were reports of the 'exercise high' and of increased pain tolerance - and then we were also able to show a real withdrawal

syndrome.' With Maria Morris and colleagues at University College London, Professor Steinberg investigated runners recruited from participants in the St Albans marathon. Subjects were divided into two groups: one carried on running their usual 40-plus miles per week, while the other was asked to stop for a fortnight. Compared both with the controls who continued to exercise and with their own previous mental state, the runners in enforced abstinence showed a marked increase in symptoms of anxiety, depression and insomnia.

As well as this observation of withdrawal, there is now more direct evidence of a link between endorphins and compulsive exercise. 'It has been shown that the use of large doses of an agent that blocks opiate receptors decreases the pleasure derived from exercise,' Professor Steinberg says. She is sufficiently impressed with the link to suggest a controlled trial of exercise in people withdrawing from narcotics, arguing that stimulation of opiate production by our own bodies may wean addicts off external supplies.

Others are less clear that endorphins represent the final common pathway linking the various forms of addictive behaviour. 'There is indeed an indication that exercise might generate endorphins. And there is some evidence that acupuncture, which can be useful in heroin withdrawal, stimulates opiate production by our own bodies,' Dr Kidger says. 'But what we have are no more than tantalising hints. The same can be said for serotonin. These substances are almost certainly important, but in ways we don't yet understand. Their promise has still be be fulfilled.'

There have also been attempts to explain addictions in terms of physiological arousal, perhaps mediated by some natural substance such as the hormone adrenaline. In The Devils of Loudun, Aldous Huxley talks about people who obtain a 'kick' from their 'psychically stimulated endocrines'.

Of all the compulsions, pathological gambling is the one which most closely fits this model of addiction, and research published earlier this year adds weight to the idea that fruit- machine gamblers are addicted to the excitement involved. Dr Mark Griffiths, of Plymouth University's psychology department, took 30 adolescents, wired them up to portable heart-rate monitors, gave them some money and sent them to an amusement arcade.

Both the subjects who were regular players of fruit machines and those who were relative novices showed a similar rise in heart rate of about 10 beats per minute during games. This posed the question: does the firm evidence of arousal show that gambling could increase in frequency?

More important is the fact that the study demonstrated the classical addictive phenomenon of tolerance. In youths who did not play regularly, heart rate remained high for a long period after each game. But in regular players it returned to normal within a minute. This supports the idea that compulsive playing is caused by the desire to maintain a continued high level of excitement.

A broad concept such as physiological arousal, and the idea of dependence on some external or internal chemical agent, help to explain many addictive behaviours. But psychologists are increasingly arguing that these theories still do not do justice to the full variety of compulsions.

'People are moving away from the view that we can reduce such a complex array of behaviours to just one or two factors,' argues Dr Martin Plant of the Royal Edinburgh Hospital. Another scientist expressing this view is Jim Orford, professor of clinical psychology at Birmingham University. Professor Orford, who will expand on these ideas this year in a second edition of his book Excessive Appetites, says: 'It is mistaken to think that dependence or addiction necessarily requires some psychoactive drug or clearly defined physiological action on the central nervous system.'

Rather than looking for some common biochemical or physiological mechanism, Professor Orford finds it more constructive to define addictions in terms of behaviour itself. Those behaviours that are potentially addictive do something powerful in terms of changing mood and consciousness, and perhaps in distracting from pain.

For a behaviour to be addictive, he continues, its effects must be short-lived, so that there is frequent repetition. 'It also has to be something that, sooner or later, causes harm - either because the time or money required makes it socially disruptive, or because the behaviour is ultimately toxic.

'Any strong habit or attachment to an activity or substance which starts to get the person into trouble can be considered an addiction. The element of conflict is the essence, because it is this conflict that induces anxiety and gives the behaviour its compulsive quality.'

On these criteria, carrots could be considered as addictive as cocaine. If addictions are simply habits that have got out of control, whether or not they involve consumption of drugs may be less crucial than previously thought.-

(Photograph omitted)