A nine-year-old I know has recently “taken up” cycling. A few pounds over the mark, though by no means obese or even significantly overweight, he believes that exercise in regular bouts is the solution to his podginess. So on Saturday mornings he heads out with one of his parents on a predetermined bike route with the sole aim of burning fat. The farther he cycles the greater his sense of accomplishment, not in terms of sporting endeavour but because he can congratulate himself that day on expending more calories than he has consumed.

This boy equates exercise with slimness, and slimness with conforming to society's ideal of how a person of his age should look. He can tell you which foods are calorie-laden and the number of daily steps that need to be taken to stay healthy. On the face of things, perhaps, such a level of bodily awareness should be applauded among a generation so prone to obesity. Yet this boy is deeply self-conscious about his appearance and weight - warning signs, say experts, that he is one of a growing breed of youngsters with an extreme fear of obesity known medically as baryphobia.

Evidence that his mentality is becoming more common among his peers came last week in a survey of 150,000 children by the education watchdog Ofsted. Findings indicated that almost one quarter of ten-year-old boys and one third of girls the same age are worried about their body image and shape.

There is no disputing the fact that more children than ever are too fat. Figures for 2006-07, published earlier this year by the NHS Information Centre from the National Child Measurement Programme, revealed that 22.9 per cent of children aged 4 to 5 in primary school reception classes were overweight or obese. Among those aged 10-11, the proportion rose to 31.6 per cent. But experts now say that such widespread condemnation of the obesity “epidemic” is triggering a new set of problems. With an overwhelming pressure to stay slim comes a fear of fatness that is now engrained in the psyche of many children long before they start secondary school. By the time they are teenagers, many are disturbingly obsessed about their shape and neurotic about totting up calories. For some, the fixation with food and weight becomes so extreme that they develop the eating disorders anorexia or bulimia nervosa.

At a recent conference of the British Dietetic Association, Claire Mellors, a dietitian in childhood and adolescent eating disorders for the Nottinghamshire Healthcare Trust, described how “a real fear of obesity is an emerging and worrying trend” that is causing a dramatic rise in the number of children with eating disorders. “From my clinical experience I would say that as healthy living messages have become more prevalent, there has been a corresponding rise in referrals for children with disordered eating,” Mellors says. “We have no hard facts to back this up yet, but it is clearly happening.”

It is difficult to escape the bombardment of public campaigns that encourage children to lead more active lifestyles. From schools to supermarkets and from cereal packets to chocolate wrappers, the overwhelming message is that being fat is bad - very bad - while being thin is all things admirable and good. At the checkout in Sainsbury's you are handed vouchers for its Active Kids campaign, designed to combat obesity, and the chain also has teams of food advisers to deliver dietary advice in schools.

On the back of cereal packets you can cut out coupons for free football, gym- nastics or swimming sessions. Through its Schools on the Move programme, the Department of Health gave pedometers to 9,000 pupils at 54 schools across England with the instruction for them “to count the number of steps they make every day”.

But, for all their good intentions, these and other campaigns have become so pervasive in society that they have prompted a worrying shift in perspective about weight and exercise. Whereas it was once an integral part of daily life for children, physical activity is now billed as a means to an end, that end being slimness and a low BMI. To anyone who grew up in the 1970s or 1980s, compartmentalising exercise into periodic bouts of sweaty effort is something alien. Back then, being fit, even being fat, was not a concept that readily entered the conscience of a child of school age. Professor Andrew Hill, a medical psychologist at the University of Leeds, has been studying issues related to obesity for the past 15 years.

While it is important to raise awareness of the risks of being overweight, he says, the warnings should perhaps not be targeted at children. “We have spent a huge amount of effort getting obesity on to the public health map in recent years, but we have not been very selective as to which sections of the population the messages have been broadcast to. The result is we now have two problems - children with obesity and those who develop disordered eating habits because they are terrified of it.”

It is vital, Hill says, that children develop an understanding of why exercise is important. “There are risks of inactivity that they do need to know about. But why does it always have to be hung on a peg of obesity? They should grow up enjoying activity, not doing it because it makes them thin.”

Mellors says that some young people she sees are taking eating and exercise messages to extremes, then justifying it to themselves and their parents by claiming that “they are only being healthy”. It is something that has been noted, too, by Susan Ringwood, chief executive of the eating disorders charity Beat. “Like parents, the Government needs to understand that it can inadvertently pass on the wrong messages with its health campaigns,” she says. “A nightmare scenario would be for a young boy or girl to stop eating and then present a leaflet to their GP with government advice that they need to exercise more and eat less.”

Last year, research at the University of Sydney demonstrated that a fear of fatness is causing more teenagers to starve themselves, abuse laxatives and smoke in an effort to avoid putting on weight. In the study on nearly 9,000 adolescents, funded by the Australian Research Council, there was almost a doubling of those aged 12-18 with “eating- disordered behaviour”, claimed Jenny O'Dea, the professor of nutrition who led the investigation.

According to O'Dea, the heavy focus on childhood obesity and the media attention paid to “super-skinny celebrities” were to blame. “I think there has been undoubtedly a media panic and a moral panic about childhood obesity in the past six years,” she says. “I would certainly suggest that some of that comment has got into the minds of teenagers who think that losing weight will make them a better and happier person. But that's a big myth.”

Still, obesity is continually purported to be sinful. Most months a new study emerges linking childhood fatness to problems such as poor exam results, relationship difficulties and health issues.

Such is the pressure to avoid fatness that children find themselves threatened at an age when weight should not cross their minds. If they were left to skip, skateboard and play football they would learn far more than campaigns can teach.

“They need direction, but children with the right information can make their own association with weight and whether they are too heavy,” Hill says. “It is coun- ter-productive to provide them with endless reasons not to be fat.” Childhood memories, he says, should be of play and having fun, not of totting up your 10,000 steps a day.

The eating disorders charity b-eat (b-eat.co.uk) has a Youthline to help children and teenagers with eating disorders. Tel 0845 6347650; e-mail fyp@b-eat.co.uk.

'He was doing 500 sit-ups': Plight of troubled 11-year-old

Sue first became concerned about her son, Patrick, when he began wearing layers of jumpers in the summer. He also started to exercise excessively. “He was doing 500 sit-ups at a time and cycling a lot,” says Sue, 50, of South London. “He was eating little, but then bingeing late at night. He brought a toaster into his room and would get through a loaf of bread in no time.”

When he lost a stone (6.4kg) in a month, Patrick was admitted to hospital. Sue was told that her 11-year-old son had developed a non-specified eating disorder. Now 17, Patrick has been readmitted to the eating disorder clinic several times. His battle has stunted his growth; he is 5ft 1in (1.55m), the same height he was at 13.

Sue says: “He gets back to a safe weight but then it only takes a trigger, like his dad not being able to take him to a football match, and he goes back to dieting and losing weight.” As a mother, Sue felt powerless to help, with little information available about her son's condition. “People should be aware of it. The age of children with eating disorders is getting lower, and it is also happening to boys,” she says.

Names have been changed