The hormone leptin governs whether we've had enough to eat - when we are sated. It sends a signal to the brain regarding how full our fat storage is. Leptin regulates the feeling of hunger in the human body, thus indirectly influencing our weight.

If enough leptin is present in the bloodstream, it reaches the brain, where it docks to receptors and says: "I'm full!"

But with the overweight toddler, this didn't work.

He ate without end, never felt satisfied, and always cried because he was hungry. And he kept gaining weight.

"That quickly becomes a problem, also for the family," said pediatrician Pamela Fischer-Posovszky, adding that parents are then criticized for overfeeding their child.

The two parents consulted doctors, who could provide neither an explanation nor further guidance. But at the university clinic in the German city of Ulm, a working group took up the problem - with Fischer-Posovszky at its helm.

A new condition

It's been known for more than 20 years that leptin deficiency prevents satiety - and that it's also linked to obesity. Such a deficiency can be detected through a simple blood test, which was also done with the obese child.

But in his case, leptin levels were completely normal. Nor did the toddler's leptin receptors - where the hormone binds - vary from those of a normal person. So the working group had to dig deeper.

"The next step was to sequence the leptin gene," Fischer-Posovszky told DW. "That's how we discovered the mutation."

A hormone mutation was discerned to be the cause of the boy's insatiability. Due to the mutation, the information typically transported by the chemical messenger didn't arrive where it needed to. Although the hormone was present in the body, it was inactive, thus failing to signal satiety.

A rare case

The university clinic in Ulm claimed it was on to a "new illness." The team's results are special in that they show that the hormone can be produced while still not functioning.

"Our discovery will now change diagnostic procedures for this type of illness," Fischer-Posovszky said.

The young patient is being treated with a synthetic hormone, which the parents and an ethics committee had to first approve.

The treatment worked, causing the formerly obese child to lose weight. He's now at a normal weight, Fischer-Posovszky reported.

It's not known how many people have this genetic mutation - although it is known that other cases exist.

The team's research, discovery and subsequent development of treatment have allowed the boy to live a normal life - and has taken pressure off his parents.

"Research is often a lengthy process, and can involve long dry periods," Fischer-Posovszky said.

"But then there's that moment that makes up for everything."