When you see your skinny four-year-old standing in front of a mirror, saying "Does my bum look big in this?", you don't know whether to laugh or cry. Whatever you do, take it seriously, advises Susie Orbach, the author of Fat Is A Feminist Issue, Hunger Strike and Towards Emotional Literacy. Eating disorders are at epidemic levels and our children are on the front line, she believes.

"This is a public-health emergency much more serious than the heroin problem," says Orbach, who will be in Dublin on Friday to give a public lecture, "Eating, Body Image and Emotional Literacy", which has been organised by the Irish Analytical Psychology Association and the Irish Forum for Psychoanalytic Psychotherapy.

Irish research also indicates that there is cause for concern. Dr Mary Flynn has found that teenage girls are suffering more than anyone else from "fat phobia". Three out of four teenage girls are normal weight or underweight, yet half of normal-weight girls and four-fifths of underweight girls want to be even lighter. Thirty-five per cent of girls, some as young as nine years old, are on diets or think they should be.

The National Health and Lifestyle Surveys, published in March 1999, gathered the views of children aged nine to 18 and found that, on average, 12 per cent of girls and 4 per cent of boys were on weight-reducing diets. An additional 23 per cent of children - an amazing one in four - believed they should be on diets. Prof Cecily Kelleher, the author of the study, said at the time that she found it "disturbing" that young people felt they should be dieting, when in most cases there was no medical reason to do so.

Orbach's daughter, who is 12 years old, has, since the age of seven, known of students at her school with bulimia, anorexia and preoccupations with dieting. Considering that one in three Irish girls is on a diet or thinks she should be, there is no doubt that food is also a serious issue in classrooms in the Republic. How has this happened?

"Their mothers are anxious about food," says Orbach. "Mothers are looking in the mirror and hating themselves. They're cooking for the family and not sitting down to eat anything themselves, or they are cooking something different than the rest of the family."

Mothers are giving their daughters the message that food is something that must be carefully controlled. "If you look at the feeding relationship in infancy, mothers are worrying about how much milk they're giving their babies. When women are breastfeeding, it's an anxious breast they bring to the feeding of their babies," she says.

Whether mothers feed by breast or bottle, they are already worrying about making their babies fat, especially if those babies are girls. "Mothers get uptight if their baby daughters are robust: they see them as greedy," says Orbach. "By the age of seven, eight or nine, the girls are worrying about their own eating habits, and by the age of 12 it becomes a control issue."

It's when girls - and an increasing number of boys - start trying to control their lives by controlling their weight that life-threatening eating disorders get a hold. What should parents, especially mothers, do?

"Don't have fights at the dinner table, and keep your own food craziness under control," says Orbach. The worst thing that parents - and schools - can do is to "talk about good foods and bad foods". She describes as outrageous the trend of schools preventing children from bringing sweet foods in their lunch boxes. Children need sweet, high-calorie foods, she says. Even if children eat the treats first, followed by the sandwich and fruit, that's OK. "Give them the sugar hit and then let them eat the rest," she advises.

"Schools are teaching kids that there are good and bad foods. What parents and schools should be doing is asking children: 'Do you feel full, and are you satisfied?' " Limiting children's access to teeth-wrecking sweets would be fine if we did not have a culture that was so messed up about food, Orbach believes. "What we really need to do is to help kids with knowing what they are really hungry for, and make sure they enjoy it."

In Hunger Strike (Penguin, £7.99 in UK), Orbach writes that there are 150,000 women dying in the United States from the effects of anorexia. As many as one in 10 girls and women suffers from bulimia. This is also a problem in the UK, where Orbach lives and practises as a counselling psychologist. She has been appalled by the refusal of the British government to take the fact on board. Last year, she helped to organise "On Eating", a body-image summit that cited eating disorders as the principal health problem for women. Anorexia and bulimia are "silent destroyers of lives", she says.

Orbach's contention that there is an epidemic is a serious one. Marie Campion of Marino Therapy Centre, who is researching the incidence of eating disorders in the Republic, says that when her statistics are published, the problem "will probably be bigger than we realised". In her work in primary schools, she has found that 40 per cent of boys and girls are concerned about their body image.

There are few statistics for the Republic, but we usually keep pace with the United States in the incidence of mental illness. In the US, estimates of the incidence of bulimia range from 3 to 10 per cent, depending on which study you read. Anorexia affects as many as 2 per cent of women. Among secondary schoolgirls, up to 3 per cent are affected, and among university-age women, up to 4 per cent are affected, according to worst estimates. A study in Connecticut found that 6.8 per cent of girls and 2.8 per cent of boys suffered from anorexia. The health consequences may be severe. Bulimia can lead to tooth erosion and low potassium levels, resulting in weakness and semi-paralysis. It is often related to self-destructive behaviour and alcohol abuse.

Anorexia is the most commonly diagnosed mental illness in teenage girls, and it is also the mental illness most likely to be fatal. US research has shown that between 4 and 20 per cent of anorexia cases are fatal. Health problems include heart disease, electrolyte imbalances, neurological problems, reproductive abnormalities, blood problems, gastrointestinal disorders and hypoglycaemia.

The link between eating disorders and suicide has not yet been appreciated, says Campion. There have been 6,000 callers to her helpline in the past year. A survey of 200 clients found that 80 per cent had suicidal thoughts and that 50 per cent had attempted suicide.

"There is a shameful stigma around eating disorders," she says. Campion believes fathers' attitudes are as important as mothers'. Insecure men tend to find fault with their own bodies and those of their families, she says. "We need to learn to like children's bodies in every shape and size, and to like our own bodies as well."

Orbach's next book, to be published in the new year, will be focused on the mass market. Penguin will run a publicity campaign declaring that there is a public-health emergency. Anyone who doubts this is the case may be in denial or simply not realise how food dominates their lives, she says.

"You know there's something wrong, and you don't even realise that it is wrong. Our obsession with food is like gravity. It's become a fact of life. We don't question it and children think it's normal."

If you're a parent, keep the lines of communication open with your children, she advises. Be willing to talk about food, but in a healthy way.

Susie Orbach will give her lecture, introduced by Kathryn Holmquist, at Milltown Park, Sandford Road, Dublin 6, at 8 p.m. on Friday (members £10, non-members £15, concessions £7)