Obesity may permanently impair the brain’s ability to process signals that are supposed to make people feel satisfied and full after eating foods rich in sugars and fats, a new study suggests.
When a person eats food, the gut normally transmits signals to the brain announcing the presence of nutrients — a process that scientists believe may be crucial for helping to regulate eating behavior. But in the new study, published in the journal Nature Metabolism, researchers found that in people with obesity, the brain has a greatly diminished ability to respond to these signals.
For the study, researchers wanted to directly test how the brain responds to two kinds of nutrients — sugars and fats — in people with a body mass index (BMI) of 25 or less, which is considered a healthy weight, and in people with a BMI of 30 or higher, which is considered obese. Scientists used feeding tubes to send sugars or fats directly into participants’ stomachs, then used functional magnetic resonance imaging (fMRI) to examine chemical responses to these foods in the brain.
Among the people with lower BMIs, infusions of sugars and fats produced reduced activity in several regions of the brain involved in hunger regulation, the study found. But scientists didn’t detect any brain response to these nutrients in obese people.
“This was surprising,” senior study author Mireille Serlie, MD, PhD, of the Yale School of Medicine in New Haven, Connecticut, said in a statement.
“We thought there would be different responses between lean people and people with obesity, but we didn’t expect this lack of changes in brain activity in people with obesity,” Dr. Serlie said.
Even After Weight Loss, Brain Signals for Fullness and Satiety Didn’t Return
To see if it might be possible to reverse this lack of brain response in people with obesity, researchers then asked participants with obesity to follow a weight loss program for 12 weeks. But even in the subset of those participants who lost at least 10 percent of their body weight, the brain’s lack of response to sugars and fats remained constant.
“None of the diminished responses were recovered,” Serlie said.
One limitation of the study is that scientists performed the brain imaging within about 30 minutes of feeding people sugars and fats. With this timing, it’s possible that the study failed to detect brain responses to these nutrients that were simply delayed and not entirely eliminated in people with obesity.
Another drawback of the study is that it only included people 40 years and older, and it’s possible that the results might be different for younger individuals.
Lack of Brain Signaling Could Contribute to High Calorie Intake
However, one strength of the study is that scientists infused nutrients directly into the stomach, avoiding the potential for the brain responses to be influenced by how foods taste, says Samuel Klein, MD, a professor and director of the Center for Human Nutrition at the Washington University School of Medicine in St. Louis, who wasn’t involved in the study.
“These results suggest alterations in how the brain responds to ingested nutrients in people with obesity could contribute to high calorie intake that causes obesity and to weight regain after diet-induced weight loss,” Dr. Klein says.