Scientists say they have discovered a hormone that suppresses the appetite.
They hope that the hormone, obestatin, which belongs to the same family ghrelin, which stimulates the appetite, may be a key to the fight against obesity.
Previously scientists, had believed all the key hormones involved in appetite control had been identified, but the discovery of obestatin could explain why treatments based on existing hormones have failed.
In a study by researchers at Stanford University School of Medicine rats injected with a synthetic version of obestatin ate half as much as rats given no injection.
The treatment also slowed the movement of digested food from the stomach to the intestine.
The search for treatments for the global epidemic of obesity has engaged thousands of scientists around the world for decades, but progress in the search for treatments for obesity has been slow, and despite the discovery of the hormones leptin and ghrelin in the nineties, neither fulfilled the promise of holding the key to obesity.
The discovery of obestatin offers an explanation.
Deleting the gene for ghrelin also takes out obestatin.
So the rats lost their appetite-stimulating and appetite-suppressing hormones at the same time.
According to Aaron Hsueh, an endocrinologist and professor of obstetrics and gynaecology at Stanford, who led the study, it is rare for more than one hormone to come from a single gene sequence, and even more unusual for two hormones from the same gene to have such diverse effects.
He says Obestatin behaves almost as an "anti-ghrelin".
Professor Hsueh says that a better understanding of the roles of ghrelin and obestatin may be essential for the successful treatment of obesity.
He believes that need is urgent as it is predicted that obesity could overtake smoking as the UK's biggest killer.
There are at present 24 million adults who are classed as overweight.
Experts attribute the cause to an imbalance between the calories people consume and the energy they expend.
Basically, though we are in general eating fewer calories we are far less active.
The study is published in the current edition of Science.