Knowing when to eat, and when to stop, is important enough that evolution really should have got it right. Looking over the Christmas debris of Christmas lunch, we may ruefully ponder why the sensation of fullness seems to have so little power in the face of additional servings of pudding and chocolate. The question is even harder to answer now research has shown that the brain provides two reward signals upon eating, one when the food is first consumed and a second as it fills the stomach.
Like so many other pleasurable things, our food response relies on the dopamine system. When we consume food, the brain is triggered to release dopamine, which improves our sense of well-being. Dr Marc Tittgemeyer of the Max Plank Institute for Metabolism Research, like many scientists before him, wanted to understand this mechanism in more detail, but unlike his predecessors, he had the technology to do it.
"With the help of a new positron emission tomography (PET) technique we developed, we were not only able to find the two peaks of dopamine release, but we could also identify the specific brain regions that were associated with these releases," Tittgemeyer said in a statement.
Tittgemeyer and colleagues gave 12 men either a tasty milkshake or a nutritionally similar tasteless drink and used the PET imaging to observe their brain response. To maximize the impact, all those involved had fasted overnight before the test.
They report in Cell Metabolism that the dopamine response kicked off as soon as the milkshake was drunk, but fairly quickly dropped away. However, a second peak of similar strength occurred 15-20 minutes after drinking in response to the stomach registering the presence of food. Many different parts of the brain are seen lighting up in response to these stimuli, with some variation between those that responded to taste and those to filling the stomach. As expected, the tasteless drink only produced the second response.
The findings are consistent with animal studies, which also show two pathways for the brain's food response.
The team also looked at the areas of the brain that showed a hunger response prior to being given the drinks, and unexpectedly found a strong negative correlation between these and the places that responded to receiving the food. The paper speculates a high desire to eat inhibits the release of dopamine in the putamen, located in the forebrain region. This could indicate that the more you want food, the less effective the message is that you've had enough, encouraging over-consumption, particularly of the most desired foods.
It is hoped the more we know about how the brain responds to food, the better we will be able to control our appetites, both to stop over-eating and tackle conditions like anorexia.