They found that only fat appeared to turn on the endocannabinoid system by a signal that traveled to the brain and then to the intestines via a certain nerve bundle called the vagus, and that happened early in the process of digestion. The endocannabinoids, in turn, trigger a craving for more fat.
"The fat hits the tongue, the cannabinoids kick in and more hunger follows," Piomelli said.
The system appears to be a product of evolution's interest in making sure that animals eat lots of fat when it's available, he said. The problem comes in modern life, when the animals known as humans often have plenty to eat.
"In modern life, fat is everywhere," Piomelli said. "There are McDonalds and Burger Kings. But before the invention of the refrigerator, fats were hard to find."
What to do with this new information? Piomelli said it provides more support for finding ways to manipulate hunger -- particularly your desire to eat more than you need -- by disrupting how the endocannabinoid system works.
The problem, he said, is that drugs designed to do just that have made people irritable, depressed and anxious. "That's why they're no longer being developed," said Piomelli.
The new study was supported by the U.S. National Institutes of Health and other federal agencies.
Tim C. Kirkham, a professor of experimental psychology at the University of Liverpool in England, said the challenge is finding a drug that affects the endocannabinoid system but doesn't enter the brain and cause the psychological side effects.
There's still hope, study co-author Piomelli said: "Imagine being able to block this mechanism so that when you reach for your pint of ice cream, you have one or two spoonfuls and that would be fine."
The study appears in this week's issue of the Proceedings of the National Academy of Science
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