The mystery of why our eyes are often bigger than our stomachs could have been solved, scientists said.

Every time we eat we get rewarded by the release of the feel-good hormone dopamine twice: once when the food touches our lips and once when it reaches our bellies.

And this double dose of dopamine may actually increase craving at the expense of brain signals that tell us we are full, scientists said.

The study in humans found dopamine release happens in two stages and affects different segregated areas of the brain as we eat.

It found evidence for an immediate dopamine release in the area of the brain that drives what we want to eat and then a 15- to 20-minute delay that makes us feel full.

And an unproven theory is that “wanting” brain signals suppress satiety signals which fuels our desire to overeat.

Senior author Dr. Marc Tittgemeyer of the Max Planck Institute for Metabolism Research in Germany explained: “Recent evidence from animal models indicates that both the pleasant taste and the nutritional value of food act as reinforcers in food selection behavior.

“Highly desired food items, in turn, can enhance food intake and may lead to overeating and obesity.

“In the light of the recent obesity epidemic, a growing number of studies have investigated brain signaling mechanisms underlying food intake and their modulation by the desire to eat.

“However, the physiological mechanisms still remain poorly understood.”

So the study gave 12 healthy volunteers with an average age of 56 and a BMI of 25.57 either a tasty milkshake or a tasteless solution.

Scans then measured their brain signaling as they drank.

Tittgemeyer, head of the Institute’s Translational Neurocircuitry Group added: “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.

“While the first release occurred in brain regions associated with reward and sensory perception, the post-ingestive release involved additional regions related to higher cognitive functions.”

Interestingly, the volunteer’s craving or desire for the milkshake was proportionally linked to the amount of dopamine released in particular brain areas at the first tasting.

But the higher the craving, the less delayed post-ingestive dopamine was released.

Co-author Dr. Heiko Backes, group leader for Multimodal Imaging of Brain Metabolism, said: “On one hand, dopamine release mirrors our subjective desire to consume a food item.

“On the other hand, our desire seems to suppress gut-induced dopamine release.”

Suppression of gut-induced release could potentially cause overeating of highly desired food items.

Backes explained: “We continue to eat until sufficient dopamine is released.

“A potential interpretation for this mechanism is that wanting suppresses satiety-related signaling, which would then lead to overconsumption of highly desired food.

“This hypothesis, however, requires further investigation.”

Tittgemeyer said: “The current findings also have high relevance for understanding the obesity epidemic.

“Prolonged high-fat diet and compulsive eating, as well as weakened impulse control, are associated with D2-receptor downregulation in rodents and reduced striatal activation in response to food consumption in humans.

“Nutrient sensing in dopamine circuits is critical for learned food preferences and directly involved in the initiation of feeding motor programs.

“Hence, post-ingestive dopamine deficiency in obesity has behavioral consequences.

“In rodents, it was associated with reduced motivated food-seeking behavior and increased preference for high caloric food.

“The behavioral consequences of wanting-dependent dopamine release reported in our study have to be investigated in future studies.

“Our results highlight the role of brain and periphery in interacting to reinforce food intake in humans.”

Earlier experiments demonstrated gut-induced dopamine release in mice, but this is the first time it has been shown in humans.

The study was published in the journal Cell Metabolism.