Professor Bryan Lask told BBC Newsnight that he is seeing ever younger patients, with increasing numbers becoming ill under age 10.
His pioneering research suggests a strong genetic predisposition to anorexia, which experts say needs to be tackled through early intervention.
New understanding of the disease is coming from research led by Prof Lask, who set up the eating disorders unit at Great Ormond Street Hospital.
His team have been researching a tiny part of the brain, the insula, which photo imaging technology shows to be underactive in people with anorexia.
"We are suspecting that there is an abnormality in the insula, it is not quite working properly and... there's a knock on effect around the body," he told Newsnight.
Prof Lask believes his work shows there is a genetic pre-disposition to getting anorexia which should transform attitudes to the illness.
"For so long people thought that this is essentially middle class girls getting it. But it's not like that at all. It's an illness we do not choose to get," he says.
"This is a much more serious disease than was previously understood."
With thanks to www.bbc.co.uk
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