Doctors have started treating a new type of eating disorder and warn aggressive anti-obesity campaigns may be harmful to some healthy children.

The phenomenon has been observed by the state of Victoria's, in Australia, three leading pediatric services, with doctors hospitalizing children who have lost up to a third of their body weight over a few months in an irrational desire to stay thin.

Royal Children's Hospital chair of adolescent health Susan Sawyer said this eating disorder, affecting children at the upper end of the healthy weight range, was only starting to be documented.

"When you're older and overweight it's a very simple message that weight loss is good for you," Sawyer said.

"The difficulty with young people is that even if they are moderately overweight, they are still growing height-wise and are at risk of over-interpreting public health messages of 'low fat is good' to suggest that 'no fat is better.'

"For all intents and purposes, these adolescents have anorexia nervosa in terms of how unwell they are, the distorted body image and the amount of weight loss, but they are at a normal weight.

"This is very new."

Austin Hospital's medical director of mental health, Richard Newton, said he believed some of the nine and 10-year-olds being treated were becoming ill from "the panic" created by anti-obesity campaigns.

"We need to be giving healthy weight messages that don't vilify fatness, but actually encourage health," Newton, an associate professor, explained.

"Some of the health messages we give create panic. We have to reassure young people that if they do have a weight problem, it doesn't mean that makes them a bad person.

"We need to encourage people to not just consider physical health, but emotional wellbeing as well."

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