Most people who are obese are eating too much and exercising too little, but for a small number, there might be a defective hormone to blame.
A toddler who is extremely obese and had an insatiable appetite was found to be producing a defective version of the hormone leptin. Leptin is the hormone that the body releases when it has eaten enough food. It signals the brain to stop being hungry. Extreme obesity has been seen in people whose bodies do not produce leptin. However, this child had high blood levels of leptin, but still ate insatiably.
When researchers examined the leptin produced in the boy's body they found that it was defective, and probably produced by a mutated gene. This mutated form of leptin did not signal the boy's central nervous system that he had eaten enough, the researchers said. Apparently, his version of leptin does not bind to the receptor for leptin in the hypothalamus of the brain. If this receptor is not activated after a meal, a person would still be hungry and could continue eating.
When the boy was treated with injections of synthetic leptin, he started eating less and lost a substantial amount of weight, the researchers reported.
Having a mutated gene that produces a defective leptin is very rare. However, the researchers who reported on this boy say they have found two more cases of the same mutated gene.
The role of leptin in the modulation of hunger and appetite has been known for about 20 years. However, attempts to treat obesity by injecting leptin have failed. People who have obesity that is associated with problems with leptin apparently have receptors that are not receiving the leptin signal correctly.
A report on this case was published in the New England Journal of Medicine.