Today's fatter children risk developing diabetes, heart disease and cancer. And it's easy to become obese, as one 13-stone boy tells Julia Stuart, but harder to know what to do about it

Jack Glassett knows he might die before his parents. Sprawled on the family settee, dressed in baggy jeans and a black T-shirt, the 13-year-old admits: "It's bad." The reason for Jack's potentially early demise is that he eats too much. So much so, in fact, that on occasions he has vomited. His soft, rounded frame of over 13 stones has earned him the nickname "Fat Controller".

Jack is not the only obese child at his school. There are about five in his year of a similar size and two who are even bigger. This is nothing extraordinary: one in five schoolchildren in the UK is now overweight and one in 20 is obese. The figures have virtually doubled within the last 10 years. Health experts are understandably horrified. "It's an epidemic. Because it creeps up on us slowly we don't recognise it, but it's just as serious, if not a lot more so, than things like Aids or mad cow disease," says Dr Tim Lobstein, childhood programme co-ordinator for the International Obesity TaskForce.

Jack, who lives in Milton Keynes, doesn't know why he eats so much. "I like food," he suggests. His brother and sister, aged 17 and 19, are both slim. So was Jack until the age of five, when he started school. "I just started eating a lot and started getting bigger. I was eating crisps, biscuits, chocolate bars. I don't know why," he says.

Over the last few years, Jack would skip breakfast so he could stay in bed longer, and then eat the sandwiches he had brought in for lunch at break time, along with a packet or two of crisps. At lunchtime he would buy more sandwiches from the canteen and a sugary drink. On the way home, he would buy a chocolate bar and another drink. When he got in he would make himself another sandwich. After supper with the family he would slip back into the kitchen for crisps and biscuits when his parents weren't looking. And this is just what he will admit to.

"We've had locks on the cupboards because he would sneak in and just eat and eat and eat," says his mother, Karen, 36, a receptionist. "He'd come down in the night and nick packets of crisps and I would find all these wrappers everywhere. The year before last we had a letter home saying that we owed over £50 in school dinner money. He was spending his dinner money on sweets and that, and then would tell them he had forgotten his dinner money and would get a lunch voucher. Then he got himself a little job on the tuck trolley and was eating more than he was selling. So I had to take him off it. When my husband's mum was alive he would have dinner here then go down there and she would feed him. On Sundays he'd be up before us and eat whatever he wanted. At the age of nine or 10 he could eat a dinner bigger than his father could.

"It dumbfounds us how he got so big. He's not one of those kids who comes in and sits and watches the telly all night. He's always been quite an active kid, but obviously not active enough for the amount of food that he was taking in. He's had a terrible time growing up with that weight. He used to cry all the time about it."

Karen is rightly concerned about his health. "Last year when he was asleep I was panicking because his breathing was strange. He sounded like an old man. It worries me sick that he could get ill because of his weight and die before me. He knows all about diabetes and high blood pressure - don't you Jack?" she says, at her son. He nods from the sofa. His parents have taken him to see a dietician, their GP and a child psychologist. "We've been down so many roads you would not believe. We tried to get him into a gym, but not many will take children because of insurance policies. We've tried everything," says Jack's father, Jim, 42, a landscape gardener who also trains greyhounds.

The doorbell goes. It's a friend. Jack goes out to talk to him in the front garden. Karen and Jim won't let Jack venture any further, as he's grounded. If Jack gets into trouble at school, he's punished at home too. "His school work has been suffering quite a lot," admits Jim. "I think a lot of that is down to his appearance. When he gets teased it affects him mentally, so his schoolwork suffers. What he tries to do is to make friends and he does things that he shouldn't be doing in class. He tries to make them laugh, but they're not laughing with him, they're laughing at him. So his schoolwork is affected and he finds himself in trouble and gets suspended or in detention. He's been suspended twice for misbehaving in class. He's had an extremely hard time. We support him in every way we can. I tell him every day that I love him, how beautiful he looks, how gorgeous he is, how handsome he is."

It is surprisingly easy to become obese. If you consume about 150 calories more than you expend each day you will start to put on weight. There are about 150 calories in a soft drink or half a Mars bar. If you continue in that way for a couple of years you will go from a normal weight to frankly obese, says Dr Lobstein.

As obese children progress into early adulthood they start to show early signs of chronic disease associated with obesity, such as diabetes, heart disease and cancer. Typically, 20 to 25 per cent of kidney and gall bladder cancers, 35 per cent of womb cancers, 10 per cent of colon cancers and five to 10 per cent of breast cancers are attributable to excess weight in adults. Obese children who don't slim down are very likely to be obese adults - and with shorter lives.

Obesity in adulthood reduces life expectancy by about five to 10 years. "We don't know yet how much their life is going to shorten if obesity carries through from childhood. The fears are that they could be curtailed by 20 or 30 years," says Dr Lobstein. "If you as an adult live to 75 but you're child only lives to 50 and you had that child before you were 25, they would die before you." One thing is certain. Their care is going to cost the Health Service billions.

So why are children getting fatter? Certainly, they are being encouraged to eat and drink like never before. Three quarters of all children's television advertising is for food, and 95 per cent of those adverts are for products which are high in fat, sugar or salt. The medical journal The Lancet recently called for legislation banning the use of celebrities such as David Beckham and Paula Radcliffe in endorsing fatty, sugary food. It also singled out the BBC for criticism for allowing its television characters the Tweenies to be linked to McDonald's fast-food. Cadbury's recent Get Active scheme encouraged children to eat chocolate in exchange for sports equipment.

Nor is there any escape at school. Typical canteen meals are more like those from high street fast food outlets. And dispensers selling chocolate and fizzy drinks can earn a school up to £15,000 a year. Last year a survey revealed that 81 per cent of parents would like them removed.

But parents can't escape blame. The Food Standards Agency found that nine out of 10 children's lunch boxes contained double the recommended daily intake of sugar and nearly half the safe daily allowance for salt and saturated fats.

At the same time children are moving less. Parents are more wary of letting them play outdoors, and many are driven to school, rather than walking there. And school PE lessons have been cut back, break times are shorter and playing fields have been sold off.

So what should be done? "It requires a pretty strong Government rethink," says Dr Lobstein. "You can't just tell children to ignore the environment - the promotion of sweet cereals in the supermarkets, free gifts in the chocolates at the check outs. There should not be battles between parent and child, there should be battles between public health authorities and the food industry to prevent this promotion of inappropriate diets. We have a duty of care to children not to expose them to environments which are prejudicial to their health.

"As parents we should make a fuss at supermarkets checkouts - say that you don't want to have these battles with the child; make a fuss to the supermarket about the way foods are promoted; write to your MP saying you are disgusted at the school's lack of attention to physical activity, play areas and cycle sheds; write to your local councillor about the need for getting a child to school safely. It's political action. It's get angry time."

In November, Coca-Cola announced it would stop advertising any of its drink brands during children's programmes - though the heads of McDonald's, Cadbury Schweppes and Pepsi told the House of Commons Health Select Committee that their products were not to blame for Britain's obesity epidemic. Andrew Cosslett, managing director of Cadbury Schweppes, said: "There is no correlation between confectionery consumption and obesity."

The committee is considering whether to recommend a ban on television advertising of high-fat and sugary foods during children's viewing times, and the introduction of cigarette-style health warnings on junk food, and is due to publish its recommendations in the spring. And this month the Government announced an £80m investment to tackle obesity among the young. All children aged between four and six will get fruit every day and the money will also fund cookery classes, healthy eating advice, and trials of vending machines selling water, milk, and fruit.

And there is some hope for Jack. This summer his family and relatives had a whip-round to find the £2,225 needed to send him to the Carnegie International Weight Loss Camp for children, in West Yorkshire. In six weeks he lost 20 pounds and enjoyed himself so much in the process that he cried when he got home.

But, as is often the case with dieters, Jack has already put some of the weight back on. He came back from a week at his grandmother's five pounds heavier. They are, however, slowly coming back off again. Since camp, Jack eats cereal and toast for breakfast, a sandwich or jacket potato for lunch and whatever Karen cooks for supper - now low fat meals, rather than ready-made or takeaways. She no longer serves him as much as her husband and there are fewer crisps and biscuits in the house. Jim has also tracked down a gym which accepts children, which Jack is attending.

The boy's next goal is to get under 13 stone. How long it will take is anyone's guess - Jack himself admits that he doesn't always stick to his new regime. "You can't be with him 24 hours a day and keep an eye on what he's doing," says Karen. "Even now, if I shut my eyes for 10 minutes he'll be straight into that kitchen for food. At school there are great big vending machines with every sort of drink and sweet that you can think of." And there's nothing like temptation to crack a dieter's resolve.