Eating disorders can affect children as young as six years old, the first ever national figures show.
Girls are more likely to suffer from eating disorders than boys
Over 13 months, 206 children under 12 years were treated for an eating disorder in Britain and Ireland - including one six-year-old girl.
Half of the children had to be admitted to hospital and 45% had exercised excessively to keep their weight down.
The figures will be presented at the Royal College of Paediatrics and Child Health annual meeting.
Eating disorders commonly affect adolescents and young adults but until now it has been unclear how many young children are diagnosed and treated.
Researchers at the British Paediatric Surveillance Unit asked 2,600 psychiatrists and paediatricians how many eating disorders they had diagnosed in children aged five to 12 years.
From the results they estimated that 3.5 children in every 100,000 in the UK are treated for an eating disorder, including anorexia, bulimia and binge eating.
Around 18% of cases identified were seen in boys, which is a higher proportion than in older age groups.
At the extreme end of the spectrum, one six-year-old girl presented to a paediatrician with food avoidance, excessive exercising and fear of weight gain but had not been diagnosed with anorexia because she was not severely underweight.
The youngest child to be diagnosed with anorexia was eight years old.
Initial findings showed that around 60% of the children had now improved but the researchers will be doing a more accurate follow-up study in a year's time.
In teenagers, eating disorders are often linked to changes in body shape associated with puberty.
And skinny celebrities and models are often blamed for promoting an unhealthy and unrealistic ideal.
But in children the issue seems to be more complicated.
Study leader Dr Dasha Nicholls, who is child and adolescent psychiatrist at the Eating Disorders Unit in Great Ormond Street Hospital said each child was different.
"Children are conscious of their bodies so those factors do come into it but the strongest influence in any mental health disorder is genetic and then there are psychological and social factors that come into play."
She added that doctors need to be aware that eating disorders can occur in young children because they are unlikely to go away without intervention.
"Children get sicker quicker, often they stop eating and drinking at the same time and because they are small they show signs of starvation very quickly."
She said children with eating disorders commonly start by cutting out favourite foods such as sweets and crisps but other signs include becoming withdrawn.
Dr David Wood, consultant psychiatrist at Ellern Mede Centre in North London, a specialist service for young people with eating disorders said: "GPs sometimes don't think of anorexia as a cause of weight loss in young children so it's important to recognise these disorders do occur in young children."
He added: "The resources available are very few and far between in the under 12s."
Susan Ringwood, chief executive of eating disorder charity, Beat, said: "We know from speaking to parents that young children are being affected."
She added that puberty is well recognised as a factor in the development of eating disorders.
"Puberty is happening younger and the hormonal changes occur at least two years before the physical changes so that could be a reason."