Stress was found to turn off the brain's natural response to satiation, which is to stop eating, and thus, encourage rewarding continued eating, in a new study that could explain why we crave high-calorie 'comfort food' when chronically stressed.
The study found that this happened in the brain's lateral habenula, which when activated usually dampens reward signals and, thus, stops a person from eating when satiated or full.
"Our findings reveal stress can override a natural brain response that diminishes the pleasure gained from eating - meaning the brain is continuously rewarded to eat," said Herbert Herzog, senior author of the study and Visiting Scientist at the Garvan Institute of Medical Research, Sydney, Australia.
The researchers also said that their study showed how chronic stress could promote weight gain and obesity and, thus, highlighted the need for a healthy diet during stressful times. Their work is published in the journal Neuron.
While some people are seen eating less during times of stress, others eat more than usual and choose calorie-rich options high in sugar and fat.
Wanting to understand the varied eating behaviours of people in response to stress, the researchers studied mouse models for how different brain areas' reacted to chronic stress under diverse diets.
"We discovered that the lateral habenula was active in mice on a short-term, high-fat diet to protect the animal from overeating.
"However, when mice were chronically stressed, this part of the brain remained silent - allowing the reward signals to stay active and encourage feeding for pleasure, no longer responding to satiety regulatory signals," explained first author Kenny Chi Kin Ip from the Garvan Institute.
"We found that stressed mice on a high-fat diet gained twice as much weight as mice on the same diet that were not stressed," said Ip.
At the heart of the weight gain was the molecule NPY, produced naturally by the brain in response to stress, the researchers found when they blocked NPY from activating brain cells in the lateral habenula of stressed mice on a high-fat diet.
The mice were, thus, found to consume less comfort food, resulting in less weight gain.
The researchers further found, again in mice, that the stressed ones on a high-fat diet consumed three times more sucralose (artificially sweetened water here) than those on a high-fat diet alone, suggesting that stress drove a craving for sweet, palatable food.
"Crucially, we did not see this preference for sweetened water in stressed mice that were on a regular diet," said Herzog.
"In stressful situations, it's easy to use a lot of energy and the feeling of reward can calm you down - this is when a boost of energy through food is useful.
"But when experienced over long periods of time, stress appears to change the equation, driving eating that is bad for the body long term.
"This research emphasises just how much stress can compromise a healthy energy metabolism.
"It's a reminder to avoid a stressful lifestyle, and crucially, if you are dealing with long-term stress, try to eat a healthy diet and lock away the junk food," said Herzog.
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