One hundred children are admitted to the Child and Adolescent Eating Disorders Unit at Westmead Hospital each year and 25 per cent are under 12.

"At least two-thirds to three-quarters of the children present typically with classic anorexia nervosa, weight loss because of body image," said unit co-director Dr Sloane Madden.

"That's to do with weight, shape and a drive to be thinner."

The remaining third are mostly children who present with weight loss due to stress and anxiety not driven by body image concerns - children who stop eating due to stress and other reasons.

Dr Madden, a child and teen psychiatrist, said concerns about body image were apparent "certainly in very young kids".

"A third of kids as young as six or seven report being unhappy with their weight and shape and a reasonable proportion of those are dieting," he said.

"Part of that is the current focus on risks of overweight and obesity, so we have to balance that message. The other thing is that by the time they get to 12 or 13, body image dissatisfaction is very common. It's unusual for people to be happy about weight and shape at that age.

"Where people are focusing on fat shaming and numbers rather than health, then that message can be quite detrimental to young people."

Dr Madden said health experts working across the two fields had a common goal to focus on health, rather than numbers on scales.

"If you look at children, even at a very young age, they are targeted by billboards, magazines and TV shows.

"TV hosts are young, slim and attractive, the same goes for adult game shows, newsreaders, children's TV promoters," he said.

There's a very common body type fronting those shows.

"A lot of messages, without directly saying it, portray that idea that with slim comes being attractive, successful [and] hard working, and being overweight goes with being lazy and undisciplined."

Dr Madden said it was important people understood eating disorders were a serious illness and not a lifestyle choice.

"While a lot of people aren't happy with weight and shape, fortunately only a small proportion get a devastating illness," he said.

"We know there's a strong genetic basis to anorexia and that while the environment plays a role, it's that genetic risk that really predisposes people to getting an eating disorder.

"It has a very high rate of both medical complications and starvation, people also die from disorders due to suicide."

But if it's recognised and treated early the outcome is good.

"Certainly while it's a worry that younger people are developing the illness ... we are more effective at treating people early and at a younger age," Dr Sloane said.

'We now know there's growing evidence to show that with the right intervention you can prevent many of the risk factors for eating disorders."