Find it hard to say no to dessert? Blame it on your brain, for after you've eaten your fill, it's the pleasure centers that tell you when to put down the fork.
The discovery comes from an experiment that measured the brain activity of volunteers offered an all-you-can-eat lunch buffet. Rachel Batterham at University College London and her colleagues used functional magnetic resonance imaging to scan the brains of eight people while they received an intravenous drip either of saline or PYY, a powerful appetite-suppressing hormone that is naturally secreted by the gut after eating. Half-an-hour after being scanned, Batterham dished out an all-you-can-eat buffet of each subject's favourite meals.
The fMRI scans revealed that when the volunteers 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.
What's more, 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. If a volunteer had fasted on saline, 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 centers determined how much they ate.