Nine-year-old Elliot thinks about food all the time. Unsupervised, he'd fast become obese. And it's all because of a rare genetic disorder, reports Jeremy Laurance

An obsession with eating does not sound like the worst medical disorder in a society that celebrates gastronomy. Yet there is something uniquely cruel in a condition that makes people eat without ever feeling satisfied.

An obsession with eating does not sound like the worst medical disorder in a society that celebrates gastronomy. Yet there is something uniquely cruel in a condition that makes people eat without ever feeling satisfied.

That is Elliot Humberstone's lot. Half an hour after breakfast he will ask what's for lunch. Leave a plate of food on the table and it will be demolished in minutes. When he comes home from school he is not allowed to raid the fridge or the biscuit tin like other nine-year-olds. He must wait till his mother, Sue, gives him a snack. "His next meal is never far from his mind," she says.

Elliot has the appetite of someone twice his age but the calorific need of someone half it. He has Prader Willi syndrome, a rare genetic disorder whose most distinctive feature is a constant desire to eat.

Named after the two Swiss doctors who first described it, Prader Willi Syndrome affects an estimated 2,000 people in Britain. Sufferers have been known to consume 4,000 calories in two hours (a day-and-a-half's meals for an average person). They will steal food from the kitchen and squirrel it away for snacks later. They will scavenge in rubbish bins and have been known to eat shampoo (the fruity kind), soap, wallpaper and carpet. One died after eating large quantities of jam and salt.

They face a lifelong battle to control their weight. Some have lost it and ballooned to a grotesque size. A 13-year-old Californian girl who died in 1997 weighing 49 stone, was thought to be an undiagnosed Prader Willi case. She had spent her last months lying on the sofa in front of the television unable to leave the room. She could find no clothes to fit and was so heavy she could not get up to wash. Her body was covered in bed sores and fungal growths and her mother was charged with child abuse.

That was a tragic case in which the mother was wrongly blamed. There is no cure for Prader Willi syndrome and, left untreated, sufferers will eat themselves to death. The key is early diagnosis so that the war against weight gain is not lost before it starts.

For parents the condition is a nightmare. They must monitor their child's eating day and night by imposing strict controls, including locks on kitchen cupboards, the fridge and freezer and carefully rationed portions at meal times.

For the child, it is worse. It is not just that they have a larger than normal appetite. They also have a lower than normal need for food. Prader Willi children have an abnormal body composition with more fat and less muscle which requires less energy. Normal children grow on 11-14 kilocalories per centimetre of height, but Prader Willi children need only 10-11 kcals per cm. To lose weight that has to be cut to 8-10 kcals per cm.

Sue Humberstone, a single parent from Matlock, Derbyshire, has brought up Elliot to accept that he will always have less than everyone else. She works for Derby City Council and has an older daughter, Grace, aged 13.

"If we have crisps, Elliot gets half a packet. Before we go to a restaurant we agree how much I am going to take off his plate and then he knows he can eat everything left on it. There were some tantrums at first, but not now. He knows that if he eats too much he will get fat. He has accepted that."

Elliot is 4ft 5ins tall and weighs six stone. He is taking growth hormone to improve his muscle tone and stamina, not to boost his height. He is only mildly affected by Prader Willi and his weight is under control, but Sue worries what will happen when he gets to secondary school and has more freedom.

"Food is always on his mind. If we are going out he will ask where are we going to have lunch. If we are going for a picnic he will want to know what kind of sandwiches we are having and whether we will be back in time for him to have a milkshake at tea."

One of the ironies of the condition is that people with the syndrome start life with the opposite problem - an inability to feed. They tend to be floppy babies who lack the capacity to suck and appear sleepy, listless and disinterested at feeding time. They grow slowly and gain weight with difficulty.

Sometime after their first birthday, however, this failure to thrive is replaced by a seemingly insatiable appetite. Not only insatiable, but indiscriminate. Frozen food, raw meat, scraps - anything that fills the belly. As they grow older, food becomes an all-consuming passion. If given the chance, they will eat without stopping. They lack the crucial appetite control that tells us when we are full. They rarely vomit and they eat until, literally, there is no more room.

For most people the sense of fullness comes long before this, through a complex mechanism of chemical messengers travelling between gut and brain. It had been hoped that elucidation of this mechanism in Prader Willi patients might one day yield clues to the western world's greatest obsession: the control of eating.

American researchers thought they had the answer when they published evidence showing that the hormone ghrelin was present in higher levels in Prader Willi patients. But a trial earlier this year in which the hormone was reduced in patients with the condition showed no effect on appetite.

"There was great excitement about this theory and then hopes were dashed," said Jackie Waters deputy chief executive of the Prader Willi Association (PWSA). "There is nothing else on the horizon."

Without a prospective drug treatment, the only hope rests with the families of sufferers in their never-ending struggle to control the appetite and weight of their children. Even if they succeed while the children are young, the real challenge comes when they get older. Jackie says: "The minute the child gets any independence, the problems begin. As soon as there is a loophole in the daily routine that allows access to food they are in trouble. They have the feeling of being hungry all the time. But there is also an obsessional aspect. They think about food to the exclusion of other things."

That was Rebecca's undoing, one of the sufferers who tell their stories on the PWSA website. While at school, Rebecca would slip out of classes to go on binges. Once, she stole a £10 note from her mother's purse and spent the lot in the local supermarket, gorging herself on cakes and sweets. Yet somehow it was never enough.

She writes: "I am 19 years old. I lived my life not even knowing what was wrong with me. At school I kept going down to my local newsagent and I brought loads of cakes and biscuits and I ate them all. I was depressed because people were calling me names and stuff. I felt so upset by this I suddenly decided it might be better if I wasn't here any more.... I suppose people will say you greedy pig, that's right I was greedy. At the age of 14 I gained lots of weight and at 17 I ended up 25 stone."

Only then, after being referred to a consultant at her local hospital, did she learn that she had Prader Willi. The syndrome was first described in the UK in The Lancet 40 years ago by the paediatrician Bernard Laurance (my father, as it happens) and is caused by a defect involving chromosome 15. It is now detectable in almost all cases by a blood test.

The defect is believed to occur at or near conception, but most of those affected do not develop sexually, so cannot pass it on. About 70 per cent of cases are inherited from the father and 30 per cent from the mother, but the syndrome rarely affects other siblings in the family.

One of the key objectives of the association is to reassure parents that their children can be helped to lead full lives and avoid the disability that comes with gross increases in weight.

Jackie said: "We all thought that is how our children would end up. Now we know it isn't so and it is important not to be too depressing for parents whose children are newly diagnosed. There is reason to assume they may not stay delicately thin but there is no reason why they should become grossly bloated and obese, with proper management."

Prader Willi Syndrome Association (01332 365676;