New research helps explain why delicious food is sometimes so hard to resist.
Why is it that, despite our best intentions, we impulsively devour that tub of ice cream or bag of popcorn?
Back in 2008, the CDC estimate, the annual medical cost of obesity to the United States was $147 billion.
In 2015–2016, the organization reports, 39.8% of adults in the U.S. had obesity. This condition increases the risk of several health problems, including type 2 diabetes, heart disease, stroke, and certain cancers.
But what underlying mechanism is behind overeating, and could identifying it eventually help people who are experiencing this health issue?
Now, a study appearing in Nature Communications has identified a specific circuit in the brain that may affect our ability to resist temptation.
Impulsivity, or doing something without considering the possible consequences, not only affects the ability to turn down food when sated — it is also a common thread linking issues such as excessive gambling and drug addiction.
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While nothing is wrong with impulsivity per se, the new study's authors note, it can lead to undesired consequences.
So, the team set out to understand what happens in the brain to prompt impulsive behavior, in the hope that their findings might lead to novel therapies for people who battle related disorders.
Key brain cells increase impulsivity
Researchers trained rats to receive a "delicious, high-fat, high-sugar" pellet by pressing a lever.
The rats had to wait 20 seconds before pressing the lever again. If they were quicker than this, they had to wait an additional 20 seconds.
Then researchers then introduced an injection of melanin-concentrating hormone (MCH). This is a transmitter produced in the hypothalamus at the base of the brain, and previous research has shown it to play a role in impulsive behavior.
Using an advanced technique, the team activated an MCH neural pathway from the hypothalamus to the hippocampus, which is part of the brain linked to learning and memory.
"There's underlying physiology in your brain that is regulating your capacity to say no to impulsive eating," says Emily Noble, Ph.D., an assistant professor in the Department of Foods and Nutrition at the University of Georgia, in Athens.
"In experimental models, you can activate that circuitry and get a specific behavioral response."
The researchers found that after the activation of the neural pathway, the rats pressed the lever more frequently, even though this would delay the delivery of the sugary pellet by 20 seconds — a less efficient means of getting a reward.
While previous research has shown that MCH levels in the brain affect food intake, this is the first study to demonstrate the role of the hormone in impulsive behavior, the authors report.
"We found that when we activate the cells in the brain that produce MCH, animals become more impulsive in their behavior around food," says Noble.
The results suggest that the MCH did not affect the rats' enjoyment of the food or how hard they were prepared to work for it, but it did impact their ability to resist trying to get a pellet, even though they had learned that pressing the lever more frequently would cause further delays.
"Activating this specific pathway of MCH neurons increased impulsive behavior without affecting normal eating for caloric need or motivation to consume delicious food," Noble explains.
"Understanding that this circuit, which selectively affects food impulsivity, exists opens the door to the possibility that, one day, we might be able to develop therapeutics for overeating that help people stick to a diet without reducing normal appetite or making delicious foods less delicious."
Emily Noble, Ph.D.