London, June 16 : People often try to beat their stress or blues by making a beeline for the fridge. Now researchers have explained why gloominess is linked to binging on food.
Researchers at UT Southwestern Medical Center said that the so-called "hunger hormone" ghrelin, which has long been known to increase when a person doesn't eat, might also help defend against symptoms of stress-induced depression and anxiety.
"Our findings in mice suggest that chronic stress causes ghrelin levels to go up and that behaviors associated with depression and anxiety decrease when ghrelin levels rise. An unfortunate side effect, however, is increased food intake and body weight," Nature quoted Dr. Jeffrey Zigman, assistant professor of internal medicine and psychiatry at UT Southwestern and senior author of the study, as saying.
"Our findings support the idea that these hunger hormones don't do just one thing; rather, they coordinate an entire behavioral response to stress and probably affect mood, stress and energy levels," said Dr. Michael Lutter, instructor of psychiatry at UT Southwestern and lead author of the study.
Fasting is known to cause ghrelin production in the gastrointestinal tract, and then aiding in sending hunger signals to the brain. Researchers have suggested that one way to help control weight by decreasing food intake and increasing energy expenditure might be to block the body's response to ghrelin signals.
"However, this new research suggests that if you block ghrelin signaling, you might actually increase anxiety and depression, which would be bad," said Zigman.
In their quest to determine how ghrelin affects mood, the researchers restricted the food intake of laboratory mice for 10 days, causing their ghrelin levels to increase four times.
When matched with the control mice, which were allowed free access to food, the calorie-restricted mice displayed decreased levels of anxiety and depression when subjected to mazes and other standard behavior tests for depression and anxiety.
Besides this, mice genetically engineered to be unable to respond to ghrelin were also fed a restricted-calorie diet. But, these mice did not experience the antidepressant-like or anti-anxiety-like effects unlike their calorie-restricted wild-type counterparts.
In order to test if ghrelin could control depressive symptoms due to chronic stress, mice were subjected to daily doses of social stress, using a standard laboratory technique that induces stress by exposing normal mice to very aggressive "bully" mice.
The researchers stressed both wild-type mice and altered mice that were unable to respond to ghrelin and found that after experiencing stress, both types of mice had significantly elevated levels of ghrelin that persisted at least four weeks after their last defeat encounter. The altered mice, however, displayed significantly greater social avoidance than their wild-type counterparts, indicating an aggravation of depression-like symptoms. They also ate less than the wild-type mice.
Zigman pointed out that the findings seem to be important when considered from an evolutionary standpoint. On the other hand, Lutter said the findings might be relevant in understanding conditions such as anorexia nervosa.
The study appears online and in a future print edition of Nature Neuroscience.