The child weighed more than 40 kilos, almost three times as much as a normal three-year-old, and could not stop eating and gaining weight.
Researchers found that the “satiety hormone” that tells the body to stop eating was inactive, meaning the child was always hungry.
But in an article in the New England Journal of Medicine, they described how they were able to bring their patient's eating and weight under control within days by giving the child an artificial form of the hormone leptin.
Leptin is produced in fatty tissues and acts on the brain to stop it feeling hungry.
“When the body's energy reserves are full, lots of leptin is produced and the appetite is restrained”, the clinic said in a statement.
If the hormone isn't released when someone has eaten enough, their brain doesn't get the signal to stop and they may continue eating.
In the child's case, it was producing enough leptin – but the doctors found that due to a genetic mutation, its leptin receptors couldn't be activated by the natural hormones produced by its body.
“The body was acting as if it was lacking this hormone, but it couldn't be measured with usual methods as the blood concentrations were normal”, head researcher Martin Wabitsch said.
“We expect that this isn't a one-off case. We've already identified other patients with this diagnosis”, he added.
No silver bullet
But other experts were quick to warn that the the treatment would likely not work for most obese people.
“A mutation like this is of course extremely rare,” German Society for Internal Medicine board member Petra-Maria Schumm-Draeger said, meaning that despite the “extremely interesting” results the discovery might have few applications.
She added that doctors have understood how leptin – whose name comes from the Greek word “leptos”, meaning “thin” – works on appetite for 20 years.
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