WEDNESDAY, Dec. 31, 2014 (HealthDay News) -- A small number of extremely obese people may be fat because their bodies produce a malfunctioning form of the appetite-controlling hormone leptin.
That's the conclusion of a case study that appears in the Jan. 1 issue of the New England Journal of Medicine.
The researchers base this new theory on a 2-year-old boy of Turkish descent with an insatiable appetite. He was chronically obese for much of his short life.
At first doctors were puzzled by the case, since the boy appeared to have high blood levels of leptin, the hormone the body releases when a person has eaten enough, said Dr. Martin Wabitsch, a researcher in the Division of Pediatric Endocrinology and Diabetes at the University of Ulm in Germany.
A genetic sequencing test revealed that the boy produces a mutated form of leptin that can't effectively signal to the central nervous system that no more food is needed, Wabitsch said.
"The leptin is produced and secreted in a normal way, but it does not bind and does not activate the satiety receptor in the central nervous system, due to the mutation," he said.
When doctors treated the boy with injections of synthetic leptin, he started eating less and experienced substantial weight loss, the study authors reported.
Wabitsch said this condition likely is very rare, but added that he already has identified a second and third case that he will write about in another upcoming article.
"I am sure there are many other patients who have this disease with biologically inactive leptin, and they are not detected because doctors test the leptin in their blood and find there is no deficiency," he said.
Doctors have known about the role that leptin plays in appetite since the 1990s, Wabitsch said, but studies aimed at treating obesity through leptin injections have tended to fail.
That's because some people who have leptin-associated obesity have receptors that don't receive the hormone's signal properly, he said. It doesn't matter how much healthy leptin is circulating in their system -- their nervous system can't receive the message that the person is full.
Dr. Mitchell Roslin, chief of obesity surgery at Lenox Hill Hospital in New York City, agreed with Wabitsch, and added that the wider clinical implications of this case study are "minimal."
"Leptin therapy itself has had little efficacy for obesity, but still may be important as part of a cocktail," Roslin said. "For most, obesity is related to environmental factors that are altering our genetic expression. Pharmaceutical [drug] solutions are coming, but are not yet ready to make a significant impact."
Wabitsch said future research should focus on the "feedback circle" of appetite that's driven by hormones like leptin and ghrelin, which increases appetite when the body needs fuel.
"In the future, people will focus on stimulating the leptin receptor with other drugs," he said. "Our body weight and our appetite is regulated by hormones, and this feedback circle is very important. An individual has only limited possibilities in controlling appetite and body weight by their own willpower. Anything you can interfere with in this circle will really have an effect on your appetite and your body weight."
In the meantime, doctors faced with an obese child who can't stop eating should consider the possibility that the child might have this leptin mutation, Wabitsch added.
For more on childhood obesity, visit the U.S. Centers for Disease Control and Prevention.