There's more to feeling full than squirming when your waistband gets tight.
Investigators have now mapped out the signals that travel between your gut and your brain to generate the feeling of satiety after eating a protein-rich meal.
Understanding this back and forth loop between the brain and gut may pave the way for future approaches in the treatment and/or prevention of obesity.
Food intake can be modulated through mu-opioid receptors (MORs, which also bind morphine) on nerves found in the walls of the portal vein, the major blood vessel that drains blood from the gut. Specifically, stimulating the receptors enhances food intake, while blocking them suppresses intake.
Investigators have now found that peptides, the products of digested dietary proteins, block MORs, curbing appetite.
The peptides send signals to the brain that is then transmitted back to the gut to stimulate the intestine to release glucose, suppressing the desire to eat.
Mice that were genetically engineered to lack MORs did not carry out this release of glucose, nor did they show signs of 'feeling full', after eating high-protein foods. Giving them MOR stimulators or inhibitors did not affect their food intake, unlike normal mice.
Because MORs are also present in the neurons lining the walls of the portal vein in humans, the mechanisms uncovered here may also take place in people.
"These findings explain the satiety effect of dietary protein, which is a long-known but unexplained phenomenon," senior author Dr. Gilles Mithieux of the Universite de Lyon, in France, said.
"They provide a novel understanding of the control of food intake and of hunger sensations, which may offer novel approaches to treat obesity in the future," he added.
Investigators have reported the findings online in the Cell Press journal Cell.