WOULDN'T IT be better for all of us if we didn't like sweet food? After all, we know sugar is packed with empty calories and that it encourages dental caries. In fact, there are people who vomit at the taste of anything sweet. But their aversion, far from being a healthy blessing, can make childhood so difficult that they end up in psychiatric care. And the condition they are suffering from can kill them.

An unusual genetic disorder, called Hereditary Fructose Intolerance (HFI), means that their bodies cannot cope with most forms of sugar. Fructose is not only found in fruit and honey, but is also a constituent of sucrose, the normal cane and beet sugar.

While it is acceptable for sufferers to eat the lactose in milk, and pure glucose, anything containing fructose - sugar in tea, cake, even an apple - can cause severe stomach pains, even convulsions and unconsciousness.

Jane Brown, now in her late twenties, was diagnosed as having HFI a few years ago. 'As a child I hated going out because I never knew when I was going to be given something that would make me throw up and feel really ill,' she recalls. 'My mother used to say I was a really difficult baby, always crying and vomiting and refusing to eat.' Because food became associated with distress and vomiting, she now finds it hard to cook for her own family.

Like many people with the disorder, Ms Brown says she was a lonely child who always felt an outsider, despite her mother's sympathy.

Because sugar is contained in so many foods, parents of children with HFI often do not spot the disorder. For the child, the effect can be psychologically damaging, if not life-threatening.

'Some of these people were referred to psychiatrists for what is called 'anomalous eating behaviour',' says Professor Timothy Cox, from the Department of Medicine at Addenbrooke's Hospital in Cambridge, who is Britain's leading authority on the condition.

In one case, he says, a young girl used to eat only the skin of an apple, which contains far less sugar than the actual fruit.

'For her it was very sensible, but to others it looked perverse. Even more extreme are the patients who are referred to the psychiatrist for rejecting their mothers. This happens when the mother thinks the child is just being faddy, and forces the child to eat food with sugars in.

'The mother then becomes associated with nausea and pain, and not surprisingly her relationship with the child becomes strained. However, if no one identifies the connection with sugar, no form of therapy will help the family.'

Because of a genetic defect inherited from both parents, sufferers of HFI lack an enzyme, called Aldolase B, which is needed by the body to break fructose down into glucose.

If they eat food containing fructose, the build-up of unprocessed fructose in the liver inhibits the liver's normal production of glucose, a sugar which is a crucial source of the body's energy. Professor Cox says that doctors are only just beginning to understand why this happens.

'Normally the liver is constantly releasing glucose into the bloodstream. But in these patients the build-up of undegraded fructose in the liver creates an environment in which the production of new glucose cannot occur.'

The consequences can be very serious. Low blood sugar leads to hypoglycaemia. The result is pallor, sweating, trembling, fear, convulsions and ultimately loss of consciousness because the brain cannot tolerate being deprived of glucose. At the same time, repeated exposure to fructose causes epilepsy, while a build-up of unprocessed sugar can cause liver and kidney damage.

Some HFI victims have had unnecessary operations because the constant vomiting caused by fructose exposure has led doctors to suppose that they were suffering from a narrowing of the stomach outlet. Others have needed transplants to deal with liver cirrhosis. The one bright spot in this sea of unpleasant symptoms is that nearly all sufferers have a complete absence of tooth decay.

No one knows the true incidence of HFI, but Professor Cox believes that it affects about one person in 20,000. So far, since the disorder was first identified in 1956, fewer than 100 people have been diagnosed as having HFI in Britain. According to Professor Cox, that leaves about 3,000 who have managed to survive by painfully constructing their own very restricted diets, but who still do not understand why food is such a source of misery.

Professor Cox argues that if the deficiency could be spotted at birth, the mental and physical traumas caused by HFI would be avoided. He and his colleagues have now developed a simple genetic test to spot HFI and he is currently investigating how this could be applied to all newborn babies.

'What distinguishes HFI from most of the several thousand genetic diseases we can now detect is that it is completely treatable with diet,' he says. 'If you don't eat any sugar - and that includes alcoholic drinks - you are completely fine.'

He points out that all babies in Britain are tested at birth for another genetic disorder, phenylketonuria, because it can be treated with diet. 'I think we should do the same for HFI,' he adds.

Professor Cox estimates the cost of adding an HFI test to those already done on the 700,000 babies born every year would be around pounds 1 per head. The total cost would probably be less than the medical time and resources inevitably consumed by people whose HFI has not been diagnosed.

HFI has become more of a problem because of the way our diet has changed. 'Three hundred years ago we ate a few grams of sugar per day,' observes Professor Cox. 'By 1800 it was 30 grams and by 1968 it reached a high point of 168 grams; it has dropped over the last 25 years to about 120 grams. But these are still lethal amounts for HFI sufferers.'

As long as Jane Brown keeps her daily sugar intake down to less than 40mg per kilogram of body weight - about three grams - she is perfectly healthy. As a measure of how difficult that can be, one apple can contain five grams of fructose.

Once their disorder is detected, HFI sufferers are often put on something like the fashionable 'Stone Age' diet: meat, fish, potatoes, green leaf vegetables and nuts. By the standards of chocolate cake fanciers it may not be very yummy, but it means that in future having HFI could herald a long and healthy life.

For more information contact: Department of Medicine, Level 5, Addenbrooke's Hospital, Hills Road, Cambridge CB2 2QQ. Mark envelope HFI.