Researchers led by Rachel Batterham at University College London found that a powerful appetite-suppressing hormone called PYY is behind why people find it difficult to stop eating.
As a part of the study published in Nature magazine, the team used functional magnetic resonance imaging to scan the brains of eight people while they received an intravenous drip either of saline or PYY, reports the New Scientist.
Half-hour after the scan, the subjects were given an all-you-can-eat buffet of their favourite meals.
The researchers noted that when the subjects were given PYY, which effectively mimics having just eaten a big meal, activity increased in their hypothalamus, the part of the brain that controls basal metabolism.
However, activity also increased in higher processing areas of the brain involved in reward and pleasure, notably the orbital frontal cortex.
They also found that the extent of these changes in brain activity correlated accurately with how many calories each subject would go on to eat at the lunch buffet. For volunteers who were given saline, the researchers noted that it was levels of activity in their hypothalamus that predicted how many calories they would consume, but for those who received PYY, activity levels in their pleasure centres determined how much they ate.