Harry worries constantly about getting fat. Before bed, he regularly looks down at his stomach, grabs at his flesh and wonders aloud if he is putting on weight. Harry is six years old.

“He thinks that if he suddenly developed a belly, it would be a reason to tell him off,” his mother said. “He’s looking for confirmation that he hasn’t been naughty. And I think: how dare anyone put that into his head?”

Harry is tall, skinny, active, and a casualty of the obsession over obesity. Organisations specialising in eating disorders worry that the focus on young people’s weight - and the way some adults demonise certain foods - is creating a generation obsessed with growing fat and increasingly guilty about eating.

Since Jamie Oliver fought his first battle against turkey twizzlers, the Government and the media have been on the warpath against childhood obesity. For schools, it has meant altering what they serve in kitchens and changing the contents of their vending machines.

The focus on healthy eating is also driven by one of the five aims of the Every Child Matters agenda, introduced five years ago, which requires that children are helped to lead physically healthy lives.

But nutritionists argue that schools’ efforts to promote balanced diets are contributing to an unhealthy fetishisation of foods. Susan Ringwood, chief executive of Beat, an eating disorder charity, said: “It’s dangerous to put adult attitudes on to children’s food. Too much of anything has unhealthy aspects. What’s unhealthy are people’s emotional attitudes.”

But that is not to say children aren’t getting fatter. The Government says the percentage of overweight children in England rose from 5.4 to 9 per cent among boys, and from 9.3 to 13.5 per cent among girls between 1984 and 1994. Figures released last week indicate that 22.9 per cent of reception pupils and 31.6 per cent of Year 6 pupils were overweight or obese in 2006-07. This is one of the highest rates in Europe.

So it is not surprising that schools are keen to join the battle against the bulge. Chocolate desserts have been replaced with fruit salad, and vending machines selling sweets and sugary drinks restocked with healthier alternatives.

But such decisions could be misguided. Body mass index, the measurement used to determine obesity, is designed for adults who have reached their full height. It is not a definitive measurement of childhood obesity.

In adults, high body-mass index is linked to increased mortality rates. But Carolyn Summerbell, professor of human nutrition at Teesside University, said there is no such link in children.

“A big two-year-old could be very fat or very thin as an adult,” she said. “And in puberty, things are changing all over the place. When a child gets to 16, we’re more confident about the risks they’re going to have when they’re older. We’re less clear about the risks associated with overweight younger children. There are some immediate risks: bullying, being isolated, lacking in self-confidence. But not mortality.”

The Department of Health defends body-mass index as the best measure of obesity across the population. But it points out that it would never be used to diagnose obesity in individual children, as results can be skewed by childhood growth spurts. Children can grow three or four inches within months, so a child who starts the summer holidays short and overweight might return to school tall and slim.

“Children need a higher percentage of fat in food than adults,” said Ms Ringwood. “Fat plays an important role in development of brain function and growth development. So it’s actually dangerous for them to be fed low-fat or diet foods.”

Jane Freeman, of the British Dietetic Association, has seen children who have been dangerously deprived of sugar, fat or carbohydrates in the fight against obesity.

“I cringe when mothers say to me, ‘My child doesn’t have sugar’ or ‘My child doesn’t have fat’,” she said. “Increasing numbers are coming in with extreme diets. When a child is growing ... the body needs glucose and carbohydrates for energy.”

The negative side-effects of the obesity battle are not purely physical. Last November, Girlguiding UK published a report based on interviews with 80 Brownies aged seven to 10. Each was shown cartoon images of different-sized girls. Almost all the Brownies made a connection between being slim and being happy, popular and academically successful. Overweight girls would be miserable, unsuccessful and likely to be bullied.

Cecilia, from Kent, is 11, 5ft 2in and weighs 9 stone. She eats a balanced diet and has dancing lessons three hours a week. She also worries constantly about her weight.

“She definitely feels she’s obese,” her mother said. “And she feels people are making moral judgements about her: that she’s the way she is because she’s lazy. It’s wrong that we should openly discuss children’s weight in this way. It’s state-sponsored bullying.”

All pupils are now weighed in the final year of primary school. But Cecilia’s mother refused to sign the consent form. “It’s public humiliation,” she said. “It’s an appalling world kids now live in.”

Ms Freeman believes that schools should not divide foods into good and bad: chocolate desserts can be an entirely valid part of school dinners. She suggests serving a dessert that includes both chocolate and fruit. “It would give children energy through the afternoon,” she said.

Ms Ringwood believes chocolate, biscuits and sweets should not be given purely as treats. “If it’s a treat, children feel they’re only entitled to them if they’ve been good. Then if you help yourself to a biscuit you feel guilty. It can lead to secret eating,” she said. She recommends that schools teach children to grow and prepare food, giving practical lessons in how to balance a diet.

Ms Freeman runs lessons in preparing balanced meals at her local primary. “There’s a lot of confusion around food,” she said. “People need guidelines to understand a balance is needed. If you boost the variety of what children eat, that can be vital to a healthy attitude to food.”

Beat: www.b-eat.co.uk

British Dietetic Association: www.bda.uk.com

AIMS FOR EVERY CHILD

Schools seem expected to fix all of society’s problems. Nowhere is this clearer than in the five “outcomes” they are supposed to ensure for children.

The aims were first introduced nearly five years ago as part of the Government’s Every Child Matters strategy. But their relevance is growing: schools are now judged against them by inspectors, and the outcomes underpin the Children’s Plan launched in December, which covers the next 10 years.

In this special six-part series, The TES is examining what schools are doing, what they are missing and whether they can realistically make a difference.