It is no secret that over-consumption of high-calorie foods can be harmful to health, but it turns out that in stress, watching what you eat can be all the more important. A team led by Professor Herbert Herzog, head of the Nutrition Disorder Lab at Garvan Institute of Medical Research, found in an animal model that a high-calorie diet combined with stress results in more weight gain than in the same diet-free environment. Researchers have discovered a molecular pathway in the brain controlled by insulin that stimulates further weight gain.
Publish their discoveries in the magazine Cell Metabolism on 25 April 2019 (EST).
"This study shows that we need to be much more conscious of what we eat when we are under stress to avoid faster development of obesity," said Professor Herzog.
The Center for Food Comfort of the Brain
Some people eat less when they are under stress but most of them will increase food intake – and most importantly, eating high-sugar and high-fat foods is high in calorie intake.
To understand what controls this "stress diet", Garvan researchers explore different brain areas in mice. While food intake is mainly controlled by a part of the brain called hypothalamus, another part of the brain – the amygdala – processes emotional responses, including anxiety.
"Our study has shown that when loaded over a long period of time with high-calorie food, mice become faster than those who consume the same high fat food in a stress-free environment," says Dr. Kenny Chi Kin Yep Learning.
At the center of this weight increase, scientists have discovered is a molecule called NPY that the brain produces naturally in response to stress to stimulate nutrition in both humans and mice.
"We found that when we excluded NPY production in amygdala weight gain, it was reduced." Without NPY, gaining weight on a high-fat diet with stress is the same as weight gain in a stress-free environment, Ip. "This shows a clear link between stress, obesity and NPY."
Spiral down to obesity
To understand what the NPY stimulus can control under stress, scientists analyzed nerve cells that produced NPY in the amygdala, and found they had receptors or "docking stations" for insulin – one of the hormones that controlled food intake.
Under normal conditions, the body produces insulin immediately after a meal, which helps the cells absorb glucose from the blood and sends a signal to stop feeding to the hypothalamic feeding center of the brain.
In the study, researchers found that chronic stress only increases insulin levels in the blood, but in combination with a high calorie diet, insulin levels are 10 times higher than mice that are stress-free and have a normal diet.
The study showed that these prolonged, high levels of insulin in amygdala cause nerve cells to become desensitized to insulin, which stops them from fully detecting insulin. In turn, these desensitized nerve cells increased their levels of NPY, which both promoted nutrition and reduced the body's normal energy-burning response, the study said.
"Our findings have revealed a vicious cycle where chronic, high levels of insulin, caused by stress and high-calorie diet, encourage more and more nutrition," explains Professor Herzog. "It really strengthened the idea that while it's bad to eat unhealthy food, eating high-calorie foods under stress is a double stroke that causes obesity."
While insulin imbalance is at the center of a number of diseases, the study shows that insulin has more widespread effects in the brain than previously thought.
"We were surprised that insulin had such a significant impact on amygdala," says Professor Herzog. "It is becoming increasingly clear that insulin does not only affect the peripheral areas of the body but also regulates the functions in the brain, and we hope to investigate these effects in the future."