Researchers in the U.S. have found that the same brain circuits are involved when obese people fill their stomachs as when drug addicts think about drugs and the discovery they say suggests an association between overeating and addiction.
Dr. Gene-Jack Wang of the U.S. Department of Energy's Brookhaven National Center for Translational Neuroimaging in New York, says they wanted to know why, when people are already full they still continue to eat a lot.
Dr. Wang says they simulated the process that takes place when the stomach is full, and saw for the first time the pathway from the stomach to the brain that turns 'off' the brain's desire to continue eating.
Wang and his team tested seven obese volunteers who had had gastric stimulators implanted for one to two years, a device which tricks the body into thinking the stomach is full.
Like a pacemaker, the simulator provides low levels of electrical stimulation to the vagus nerve, causing the stomach to expand and produce peptides that send messages of "fullness" to the brain.
The seven patients received two separate positron emission tomography (PET) brain scans two weeks apart: one with the gastric stimulator on, the other with the stimulator off.
Participants were not told whether their stimulator was on or off and prior to the scans, subjects were injected with a radioactively labeled form of glucose, which the scanner could track to monitor brain metabolism.
They then used a PET scan to see which parts of the brain activated when the stimulator was activated.
They also carefully questioned their volunteers, all of whom were very obese, about why and when they overate.
The volunteers were all genuinely hungry when the PET scans were conducted as they had been fasting for 16 or 17 hours and the stimulator succeeded in making them feel less hungry.
The researchers had assumed that the activated area of the brain must be in the satiety center, supposedly in the hypothalamus, but in fact they did not see the activity there.
The activity was seen in all areas of the brain, especially in the hippocampus, the region linked to learning, memory, sensory and motor impulse and emotional behavior.
The researchers say the hippocampus was 18 percent more active when the gastric stimulator was on.
The stimulators also sent messages of satiety to brain circuits in the orbitofrontal cortex and striatum, which have been linked to craving and desire in cocaine addicts.
Dr. Wang says this provides further evidence of the connection between the hippocampus, the emotions and the desire to eat, and gives new insight into the mechanisms by which obese people use food to soothe their emotions.
The researchers believe the finding may help in the development of more effective treatments for obesity which is an ever increasing problem in many parts of the developed world.
The study is published in the Proceedings of the National Academy of Sciences.