Eating, feeling satisfied, stopping eating and not overeating, all involve quite a dance between the stomach and the brain. And it’s the messages from the stomach to the brain that makes us stop eating.

But this stomach/brain messaging service breaks down in people with obesity. It seems the brain doesn’t heed messages from the stomach.

This blunted brain response may explain why it’s difficult for some people to lose weight and keep it off, say researchers at Yale University, US.

“We need to find where that point is when the brain starts to lose its capacity to regulate food intake and what determines that switch” says Yale professor of medicine, Mireille Serlie.

Researchers fed glucose or fat directly into the stomach of 28 people identified as lean (BMI 25 or less) and 30 people as obese (BMI 30 or higher). Then they ­examined brain activity with functional magnetic resonance imaging (fMRI).

Among lean participants, brain activity reduced across various regions of the brain following the infusion of both glucose and fat. Conversely, no changes in activity were seen in ­participants with obesity.

“This was surprising,” said Prof Serlie. “We thought there would be different responses between lean people and people with obesity, but we didn’t expect this lack of brain activity change in people with obesity.”

Prof Serlie and her colleagues then took a closer look at a brain region called the striatum, which plays a key role in regulating eating behaviour through its ‘reward’ neurotransmitter, dopamine. Using fMRI to evaluate dopamine release following nutrient infusion, researchers found glucose induced a release in both groups of participants, while fat only caused dopamine release in lean participants, meaning there’s reduced “nutrient sensing” in obese people.

Participants with obesity then underwent a 12-week weight-loss programme, and those who lost at least 10% of their body weight were re-imaged. Weight loss did nothing to change the brain’s response to nutrient infusion. “None of the diminished responses were recovered,” said Prof Serlie which goes a long way to explaining why people who lose weight then regain it.

It also explains why people with obesity can eat a full meal and still feel hungry – defective nutrient sensing.

“We need to find where that point is… when the brain starts to lose its capacity to regulate food intake and what determines that switch. If you know when and how, you may be able to prevent it,” says Prof Serlie.

“People still think obesity is caused by a lack of willpower,” she adds.

“But we’ve shown there is a real difference in the brain when it comes to nutrient sensing.”