Scientists have identified the brain circuits that 'decide' how much we eat. Using live brain scanning techniques and an innovative study design, researchers have discovered how the brain controls food intake in humans. Their findings are published in Nature.
The group of scientists from UCL (University College London) and King's College London used peptide YY (PYY), a naturally occurring hormone that regulates appetite, to investigate which areas of the brain are involved in controlling food intake. Studies in animals had suggested that PYY acted on the homeostatic regions of the brain (hypothalamus and brainstem), which govern primitive functions. However, there was no information on how this hormone regulates eating in humans.
This latest research, led by Medical Research Council clinician scientist Dr Rachel Batterham, used functional Magnetic Resonance Imaging (fMRI). It showed that PYY not only targets the primitive homeostatic parts of our brain that control feeding but also acts in the cortico-limbic brain regions that determine the rewarding and pleasurable aspects of eating.
Dr Batterham, based at UCL, said: "We were surprised to discover that the greatest change in brain activity in response to PYY was within the orbito frontal cortex (OFC), a region thought to act as an integrative hub and implicated in reward processing. More surprisingly we found that the change in OFC activity predicted how much food the volunteers subsequently ate. The greater the change in activity in that area, the less the subjects ate."
Eight normal weight men took part in a double blind placebo-controlled study. After 14 hours without food subjects were given an intravenous drip of either PYY or placebo for 100 minutes while their brain was scanned continuously. Thirty minutes later they were offered an unlimited meal. Each subject was tested twice one week apart, once with PYY and once with the placebo in a random order. PYY infusion reduced food consumption in all 8 subjects and on average caused a 25% reduction in the calories eaten.
Dr Batterham explains: "In the food-deprived state brain activity within the hypothalamus predicted how much food the subjects ate. However, in the presence of increased PYY levels, mimicking a meal, there was a switch in the circuits controlling eating so that brain activity within the orbitofrontal cortex now predicted feeding behaviour."
It is hoped studies using fMRI-based physiological approaches may help determine whether pharmacological or other interventions target appetite regulating regions.
Dr Batterham added: "The obesity crisis continues unabated with 23 per cent of the UK adult population and 33 per cent of the US adult population now classified as obese. Whilst we know that body weight is determined by a balance between food intake and energy expenditure a clearer understanding of the brain circuits that regulate these processes could allow us to tackle one of the biggest health burdens western societies face today."
Dr Batterham concludes that: "Our study provides insights into how biological signals affect not only how full we feel but also the enjoyment of eating and may determine whether we reach for a second helping or are satisfied with the first. Understanding which brain regions control eating in different environmental conditions may help us to develop more targeted treatments for people with weight problems. Further research is now needed to investigate whether underweight and overweight people have abnormalities in these circuits."