Diet and Brain Health

Obesity changes how the brain views food—even after weight loss

Obesity is a major epidemic in the United States with roughly one out of three adults (36%) fitting the criteria. And because people with obesity are more likely to develop one or more health problems, including but not limited to heart disease, stroke, diabetes, sleep apnea, osteoarthritis and certain cancers, researchers are eager to better understand the disease in order to address it.

A new study looking at the effects of obesity on the brain has revealed two concerning discoveries:

• Obesity appears to damage the brain’s ability to recognize the sensation of fullness and be satisfied after the person has consumed fats and sugars.

• The changes to the brain may not be reversible—even after significant weight loss.

The Study

A team of researchers from Yale School of Medicine conducted a controlled clinical trial with 30 people of “normal” weight and 30 people who were medically obese. All 60 people were fed one of the following directly into their stomachs via a feeding tube on multiple days:

• sugar carbohydrates (glucose)

• fats (lipids)

• water

The feeding tube was used to focus on the gut-brain connection without the person seeing, tasting, or smelling food.

The scientists used single-photo emission computed tomography (SPECT) and functional magnetic resonance imaging (fMRI) to see how the brain responded over 30 minutes. Specifically, the researchers wanted to know how glucose and fats would each trigger the striatum, the part of the brain involved in the motivation to actually look for food and eat it. 

“The MRI shows where neurons in the brain are using oxygen in reaction to the nutrient—that part of the brain lights up,” explains University of Cambridge Professor of Metabolism and Medicine Dr. I. Sadaf Farooqi, who was not involved in the study. “The other scan measures dopamine, a hormone that is part of the reward system, which is a signal for finding something pleasurable, rewarding, and motivating and then wanting that thing.”

The Findings

The results were different for both groups of participants.

In those who were not obese, the signals in the striatum slowed when the participants were fed sugars or fats, indicated that the brain acknowledged that the body had been fed. Simultaneously, dopamine levels rose, demonstrating that the brain’s reward centers had been activated.

But when the participants who were obese were fed the same nutrients, the researchers found that the brain activity didn’t slow and dopamine levels didn’t rise—especially when the nutrient was lipids or fats. The results were somewhat surprising, considering that foods with higher fat content are typically more rewarding to the brain.

For the next phase of the study, the participants with obesity were asked to lose 10% of their body weight—an amount that’s been shown to not only improve blood sugars, but also reset metabolism—within 3 months.

After the weight loss was achieved, the tests were repeated. Unfortunately, even losing the weight wasn’t enough to reset the participants’ brains.

“Nothing changed—the brain still did not recognize fullness or feel satisfied,” explained Yale School of Medicine Professor of Endocrinology Dr. Mireille Serlie. “Now, you might say three months is not long enough, or they didn’t lose enough weight. But this finding might also explain why people lose weight successfully and then regain all the weight a few year later—the impact on the brain may not be as reversible as we would like it to be.”

The study results seem to point to the truth: that just eating less, exercising more, and using willpower isn’t necessarily enough to prevent weight gain.

“I think it’s important for people who are struggling with obesity to know that malfunctioning brain may be the reason they wrestle with food intake,” says Serlie. “And hopefully this information will increase will increase empathy for that struggle.”

The results of the study were published in the June 13, 2023 edition of the journal Nature Metabolism.


Wendy Burt-Thomas writes about the brain, mental health and parenting.

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