Just over a year ago, around the time of his ninth birthday, Zach Hewitt, a Surrey schoolboy, began doing a few exercises. The odd stomach crunch or bicep curl - nothing too extreme. Then one day he looked in the mirror and decided that he had to do much, much more: he had to get serious about fitness.

“I saw that I was extremely fat compared with what I'd like to be and I ran out of puff a lot more than friends. I looked at myself and thought ‘I didn't used to look like this; I need to get into shape'. I try my hardest not to be fat because I'm scared of it.”

Getting in shape for Zach meant a gruelling daily schedule, which a year later he still maintains. According to his mother Nicci, a part-time graphic designer: “He gets up, has breakfast and goes to school. He comes home and goes straight out to the trampoline where he works out for 45 minutes. Even if it's raining, even in the depths of winter, he's out there. After a healthy supper, where he carefully watches what he eats, he will do additional exercises before bed: press-ups, sit-ups and stomach crunches.”

So how fat is Zach? The shocking truth is not at all, not by any stretch of any fattist imagination could he be described as fat. With his shirt off you see Zach's size for what it really is: spot on for a boy his age, a boy who's just turned 10 and still fits into his Year 8 or 9-sized clothes. Nicci says that he's has always been the same: “Zach's never been fat, he was cuddly as a toddler, but he's never been a fat child.”

So where has this misplaced belief that he is large come from and why is a normal-looking ten-year-old boy so “scared” of being fat? Andrew Hill, professor of medical psychology at Leeds University and an expert in eating disorders, says that 20 per cent of nine and ten-year-old girls claim that they are dieting to lose weight and twice that number say that they've tried it in the past. For boys, who have evidently “always been part of the picture”, the figure is around 5 to 8 per cent and increasing. Hill points out that children are all too aware of how important appearance is and how people are judged on it. “There's a great emphasis in society on appearance. You see it in magazines, in newspapers, on billboards. You don't have to be a certain age to understand its importance. It's reinforced through your peer group, with your parents, with other significant adults.”

Nicci blames magazines and TV: “Most of the role models he sees are muscular men, either models, or guys in boy bands, or wrestlers. Zach wants to be like them and his aim is to get a six-pack. You don't see many normal people with their tops off in magazines so it's hard for him to get a view of what's normal.”

Understandably, Nicci is worried that Zach may “become too obsessed” with his healthy eating and fitness regime, but hopes that she can stop this happening by keeping an eye on him and talking regularly about it to him.

Teleisha Coulson, from Northampton, is a ten-year-old girl who is acutely conscious of her body and weight. “I don't like my stomach area because I think I'm fat. I worry about my stomach.” Teleisha monitors the fat and calorie content of what she eats; she swims three times a week, regularly works out on a cross trainer and runs and cycles constantly. “I think it's important to look good because it sets a good impression. I want to look good so that other people think nice things about me. If not, I think maybe people would talk behind my back and say things such as ‘Oh she's big'.”

Like Zach, Teleisha is not fat, she is very slim. But put her in front of a mirror and she will poke her tummy wistfully and say how much she would like a flatter stomach.

Again, why is such a young child so scared of getting fat? Her mother Louise - like Nicci Hewitt - blames the media. She believes that they promote an impossible ideal while also using “scare tactics” to promote healthy lifestyles. “There is so much on TV about children being fat,” she says. However, Louise admits that it might be her influence as well. A chronic yo-yo dieter, who once had an eating disorder, she has been many different clothes sizes - from a 6 to an 18, the photographic evidence of which is pinned on her fridge. “Teleisha may be afraid of getting fat because she's seen me go up and down the scales so maybe she doesn't want to go that way.”

The sad part, Hill says, is that children are as brutal as the rest of us over their interpretation of what a desirable appearance means: being thin. “Their reasoning is very black and white; it's either good or it's bad. So a fat child or a fat adult is seen as being lazy, or not eating healthily, having few friends, not being liked by their parents, not doing so well at school.”

A study carried out in the 1960s looked at ten and 11-year-olds' perceptions of being overweight in relation to other disabilities. It found that the obese child was selected less often as a friend compared with the amputee, a child in a wheelchair, or a child with a port-wine stain. Forty years later, the same experiment was repeated: this time the rejection of the fat child was even more extreme. “As obesity has increased it has not become more acceptable - it has become less tolerated,” Hill says.

With these perceptions deeply embedded in formative young minds, it's no wonder that children such as Zach and Teleisha are terrified of becoming fat - a fear that, for girls especially, is exacerbated by the onslaught of adolescence.

“Boys grow into their desired body shape to a degree more than girls,” Hill explains. During puberty both boys and girls experience height growth and weight gain - however, girls put on twice as much subcutaneous fat as boys. For the female this is biologically normal and necessary for fertility, but try explaining that to a young teenager. “Girls putting on fat at the age of 12, 13 or 14 is the last thing that they want,” Hill says.

Adolescence also heralds the peak age for the onset of eating disorders - anorexia nervosa at 14, and bulimia at 16 to 18 years. Although it's by no means a given that because your child diets or is very focused on weight that he or she will end up with an eating disorder, “it's certainly a first step”, Hill says. “When you're 14 or 15 and you're feeling crap about your life and your appearance, what will you turn to? You will turn to something that you've done in the past.”

Bryan Lask, professor of child and adolescent psychiatry at St George's University of London and a consultant to Huntercombe Hospital Group UK, has been studying eating disorders for 20 years. His research shows that if your eight-year-old is excessively preoccupied with weight now, it is the strongest indicator that he or she may be one of the minority of adolescents who go on to develop an eating disorder. “If your eight-year-old child keeps saying ‘I'm too fat and I don't like my body', or ‘could I stop eating such and such a food', then listen to the warning sign.”

As well as a preoccupation with weight and a change in eating habits, other signs of eating disorders include self-induced vomiting and excessive vomiting, and changes in personality. A child may “disengage” from what's going on around him or her - from family, friends and school life. She may become secretive, isolated and develop perfectionist tendencies. “What often starts as a need to restrict certain foods spills over to other areas and a need to restrict and control their worlds around them,” says Susan Ringwood, of Beat (beating eating disorders) - the main UK charity for families affected by eating disorders. But, as she points out, what often makes identifying an eating disorder so difficult, especially in the early stages, is because “in a very cruel way it mimics adolescent behaviour” - that is, the secrecy, the wanting to be on your own and the disengaging from the family.

What the increasing research and evidence does reveal is that, in a cruel way, children such as Teleisha and Zach mimic adult behaviour. They are becoming frighteningly aware of their bodies from an early age - an age when you hope that they would be enjoying their time free of future angst over weight, size and shape.

Too Young to Diet? is on Sky Real Lives, Sky Channel 253, at 10pm on Friday. (www.sky.com/reallives)

What parents can do
Talk to your GP or a health professional. Contact Beat (www.b-eat.co.uk), a charity for families with eating disorders.
Try talking to your child. Choose your moment - the dinner table with a plate of food in front of you is not the best place to confront such things.
Help your child to see that there are more important aspects to his or her world than weight and shape. Try to find a different emphasis to life.
Set an example. If you eat plenty of fruit and vegetables, have regular meals, exercise within reason and feel good about your appearance, your children are far more likely to do so too.

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