Scientists have demonstrated for the first time how a hormone may hold the key to explaining why people carry on eating, even if they have already eaten enough to fill them up.

The discovery of the role of the hormone PYY could lead to new treatments to reduce obesity, which affects almost one in four adults in Britain.

PYY is produced naturally in the gut in response to food, and tells the brain when the appetite has been satisfied. For the first time, researchers from University College and King's College, London, have succeeded in infusing the hormone into eight human volunteers and measuring its effect in two different areas of the brain, using MRI scanners.

The tests found that even after a 14-hour fast, the volunteers with high PYY levels wanted to eat less than they normally would. The researchers will now start to examine how PYY works in anorexics, who are known to have high levels of the hormone.

Rachel Batterham, obesity specialist at University College, London, who led the study published in Nature, described the results as a "complete surprise".