A new study says that the brain may answer why we overeat even when we are full.
The new study conducted over mice showed that the so-called hunger hormone ghrelin works in the brain to make some people keep eating "pleasurable" foods when they're already full.
"What we show is that there may be situations where we are driven to seek out and eat very rewarding foods, even if we're full, for no other reason than our brain tells us to," said Dr. Jeffrey Zigman, assistant professor of internal medicine and psychiatry at UT Southwestern and co-senior author of the study.
Scientists have previously linked increased levels of ghrelin to intensifying the rewarding or pleasurable feelings one gets from cocaine or alcohol.
Zigman said his team speculated that ghrelin might also increase specific rewarding aspects of eating.
According to him, rewards, generally can be defined as things that make us feel better.
"They give us sensory pleasure, and they motivate us to work to obtain them. They also help us reorganize our memory so that we remember how to get them," he said.
For this study, the researchers conducted two standard behavioural tests. In the first, they evaluated whether mice that were fully sated preferred a room where they had previously found high-fat food over one that had only offered regular bland chow.
They found that when mice in this situation were administered ghrelin, they strongly preferred the room that had been paired with the high-fat diet. Mice without ghrelin showed no preference.
"We think the ghrelin prompted the mice to pursue the high-fat chow because they remembered how much they enjoyed it. It didn't matter that the room was now empty; they still associated it with something pleasurable," said Dr. Mario Perello, postdoctoral researcher in internal medicine and lead author of the current study. The researchers also found that blocking the action of ghrelin, which is normally secreted into the bloodstream upon fasting or caloric restriction, prevented the mice from spending as much time in the room they associated with the high-fat food.
For the second test, the team observed how long mice would continue to poke their noses into a hole in order to receive a pellet of high-fat food. "The animals that didn't receive ghrelin gave up much sooner than the ones that did receive ghrelin," Dr. Zigman said.
The study appears online in Biological Psychiatry.