Obesity can mess with the way your brain reacts to food to stop you feeling full — and the damage may be permanent

• A study found the brains of people with obesity did not react normally to them eating food.
• This didn't change as they lost weight, which could explain why weight loss can be so difficult.

Obesity can mess with the way your brain reacts to food to stop you from feeling full, and those changes could be permanent, according to a study.

The research, published in the journal Nature Metabolism on Monday, may help explain why some people struggle with keeping weight off.

"People still think obesity is caused by a lack of willpower," Mireille Serlie, a study author and professor of endocrinology at Yale School of Medicine, said in a press release. "But we've shown that there is a real difference in the brain."

The brain is not getting the message

The study looked at the signals the stomach sent to the brain after a meal in people who were leaner and in people who have obesity, as defined by having a body mass index (BMI) over 30.

Scientists gave the participants fats and sugar suspended in water through a tube in their nose — to bypass the influence of taste and desire for the food — and looked at their brain's response to the meal using brain scans.

They focused on a part of the brain called the striatum, which has been linked to the "motivation to actually go and look for food and eat it," Serlie told CNN.

Our gut and body send signals to our brains to show we've had enough calories. Usually, the brain reads these and stops stimulating the areas that tell us to eat.

In the brains of leaner people that's exactly what happened. Though both leaner people and people with obesity's brains lit up with hunger before a meal, that brain activity subsided after the meal in the brains of leaner people.

The scans also showed a rush of dopamine to the striatum, a chemical that gives a satisfying feeling of reward for the meal. But that relief didn't happen in people with obesity.

"This was surprising," said Serlie, in a press release. "We didn't expect this lack of changes in brain activity in people with obesity."

Francesco Rubino, chair of metabolic surgery at King's College London, who was not involved in the study, told Insider the research shows how obesity is more than a question of willpower.

"Somebody with a normal weight would normally make the brain feel there's been enough nutrients intake," but for people with obesity, "those mechanisms don't work well enough," he said.

"The brain is 'hungrier' in people with obesity, if you wanna put it that way, but it's not because of a conscious decision," he said.

The brain stayed unreactive after weight loss

Scientists then asked whether weight loss could trigger the brain to get back to normal.

Participants were asked to follow a diet to lose 10% of their body mass in three months. Losing this amount of weight has been shown to reduce health complications associated with being overweight, but studies have shown that people with obesity struggle to keep this weight off in the long run.

Scientists hoped that the brain would catch up with the body and would start giving people that release associated with feeling full. But that didn't happen.

"Nothing changed — the brain still did not recognize fullness or feel satisfied," Serlie told CNN.

This finding might "explain why people lose weight successfully and then regain all the weight a few years later — the impact on the brain may not be as reversible as we would like it to be," she said.

Rubino and Naveed Sattar, professor of Metabolic Medicine at the University of Glasgow who was not involved in the study, both told Insider more research is needed to say that these changes are definitely irreversible.

It may be that the effect could be reversed if we waited longer after the weight loss intervention, said Sattar. Or the brain might react differently if the weight was lost through different methods like bariatric surgery or pharmacological intervention like semaglutide, said Rubino.

"You cannot make definitive conclusions based on a 12-week study," said Sattar.

Is the problem the brain or the gut... or something else?

Obesity is a complex disease, and scientists are still struggling to understand exactly what is going on to cause overweight and its associated health issues.

How and when these changes start to occur is hard to say from this study alone.

"We don't know when these profound changes in the brain happen during the course of weight gain. When does the brain start to slip and lose the sensing capacity?" said Serlie to CNN.

It's also not clear if the problem starts with the brain or the gut, Rubino and Sattar told Insider.

"The question is whether the difference between people who have obesity and people have normal weight is due to change in the brain physiology or changes in the gut physiology," said Rubino.