Teenagers are now being treated for cirrhosis of the liver caused by overeating.
The disease, where the liver becomes scarred, lumpy and shrunken, is widely associated with alcoholism.
But leading doctors say they are starting to see it in teenagers who have been obese for most of their lives.
They believe the cases are part of a shocking "hidden epidemic" of liver problems caused by obesity.
Dr Jude Oben, a consultant hepatologist at Guy's and St Thomas' NHS Foundation Trust in London, is treating about a dozen teenagers with fatty liver disease, including a 13-year-old who is 21 stone.
A number of those in their late teens have already developed full-blown cirrhosis, he told The Mail on Sunday.
He said, "To have patients so young is shocking beyond belief. It is a reflection on parents, who have to realise that bad diets and overeating can do enormous damage, even at an early age."
Cirrhosis occurs when the liver is forced to process large quantities of alcohol or food over a prolonged period and can result in the organ failing, necessitating a transplant.
Meanwhile, a major new study which involved MRI scanning of 5,000 adults has revealed that a "shocking" one in five middle-aged Britons is harbouring fatty liver disease - and most do not know it.