Stressful situations alter brain activity in regions responsible for self-control, like the prefrontal cortex, but these changes in brain activity do not necessarily cause binge eating in people with eating disorders, according to new research, suggesting for the first time that the triggers for this behavior are more complex than previously thought.
In a paper published Monday in the Journal of Neuroscience, a team of researchers studied the neurobiology behind binge eating in British women with the eating disorders anorexia nervosa and bulimia nervosa. The researchers measured the women's brain activity using magnetic resonance imaging while the women completed inhibitory-control tasks, and the team discovered that self-inhibition is preserved in the face of stress.
Inhibitory control is a cognitive process that regulates impulses and behavioral responses to stimuli; it includes what is known as self-control. A common theory in eating-disorder research is that stress impairs the inhibitory-control function, which can lead to a lack of self-control and binge eating, Margaret L. Westwater, a postdoctoral researcher at the University of Cambridge and lead author of the paper, told The Academic Times.
"As failed self-regulation in response to stressors has gained traction as a putative mechanism of binge-eating, it has become increasingly important to characterize the precise self-regulatory deficits associated with binge-eating disorders," the authors reported in the paper.
Their experiment tested a sample of 85 adult women in the U.K., some of whom had bulimia nervosa or anorexia nervosa bingeing/purging subtype, a specific kind of anorexia. The sample also included women with a healthy body mass index who did not have any mental health conditions.
The women spent two days eating a semicontrolled diet in a research lab, where they performed a series of inhibitory-control tasks and other cognitive tests. They received breakfast and a snack in the morning and then fasted for six hours in the middle of the day. The main experimental task took place during the fasting period.
While the participants' brain activity was being measured with fMRI, they were instructed to push a button to stop a moving bar when it reached a specific point on a screen. In about 25% of the trials, the bar stopped early. The participants were told to prevent themselves from pushing the button if this occurred, and that it was equally important to stop the moving bar at the target and to withhold their response if the bar stopped early.
Following the task, the women were randomly assigned to either a stress-inducing cognitive test or a neutral cognitive test. Both tests involved mental math. In the stress test, the women were electrically shocked in the abdomen while solving math problems and receiving negative feedback from the researchers, in order to create a pressured and stressful environment.
"They were told that they had to meet a performance criterion for their data to be used in the study, so there was quite a bit of pressure to do really well in the mental math task," Westwater said, noting that the test was manipulated so all the participants failed various performance criteria.
The neutral test included similar math problems, but without the pressure to perform and with less stimulation to the abdomen. If the participants completed the stress-inducing test on day one of the experiment, then they completed the neutral test on day two, and vice versa.
After the stress-inducing or neutral cognitive tests, the women again completed the same inhibitory-control task involving the bar on a screen. Westwater said this allowed the researchers to measure how the brain responds during different self-control tasks within the same person and then compare how stress changes brain activity relative to each participant's baseline performance.
The participants then had more freedom in their food consumption at a large dinner buffet. There were no restrictions on portion size in order to measure if they binged from stress.
"The idea was to mimic the circumstances under which they might find themselves in their daily lives when they're stressed, to see if this stress was actually leading to increased food intake," Westwater said. "Being in a research study is, of course, not the same as your daily life, but we did try to make it as adaptable as possible."
Ultimately, the stressful task altered brain activity associated with inhibitory control in both groups of women with eating disorders, but it had no effect on their task performance — they still had the ability to stop their actions while stressed.
Women with bulimia nervosa showed worse task performance during proactive inhibition, which is a cognitive process that enables us to slow down a motor response in anticipation of having to stop. Although this process is important for self-control broadly, it may be particularly important for binge-eating disorders, because binge-eating episodes are characterized by difficulties in sensing when to stop eating, Westwater explained.
During that same bar task, the same group of women also had increased activation in their brains' superior frontal gyrus, a part of the prefrontal cortex. This could suggest that patients with bulimia nervosa are not recruiting an efficient brain network to execute the cognitive processes needed to perform the task, Westwater said.
"Being able to slow down what you're doing is actually quite important when we're making decisions," she explained. "We have to be able to adjust to both our internal and external environment to make a decision. So it might be that impairments and proactive inhibition have a role in how women with bulimia nervosa respond to their environment when making decisions in relation to eating."
Though stress influenced brain responses during self-control in both patient groups, it did so in different ways. The two eating disorders that were examined in the study had very similar symptoms but were differentiated by the fact that those with anorexia were significantly underweight. The patients with anorexia showed stress-induced reductions in brain activation in the prefrontal cortex during the task, whereas stress increased brain activation in patients with bulimia. Westwater said this may be due to the extra physiological stress that being underweight puts on the body, resulting in a change in brain activity following acute stress.
Some of the observed brain changes were able to explain how much food the women consumed at the dinner buffet. For example, the more the ventromedial prefrontal cortex region was being activated, the less food was consumed. This could mean that activation of this brain region during self-control has some relevance to dietary control.
"Even though we found interesting changes in brain activity and some associations with their food intake, we actually did not find that stress affected their behavior in any significant way," explained Westwater.
"And we think that given the theory that we've tested, which suggests that stress should really impair someone's ability to control themselves, you would need to see that in the behavior — you would [expect] to see that people are performing badly on the task. But that's actually not what we found," she continued.
The authors of the paper suggested that more nuanced models of eating disorders are needed, rather than models that simply assume stress causes people to struggle with self-control. The findings of the current paper indicate that there are much more complex processes going on within eating disorders that include binge-eating behavior.
"We can recognize that our mood definitely can have an effect on what we're eating, but the question is, how does this happen? And I think what our study suggests is that it's not that stress all of a sudden makes it impossible for you to stop what you're doing, but it might be that it's changing how you're making decisions," Westwater said.
The study, "Prefrontal responses during proactive and reactive inhibition are differentially impacted by stress in anorexia and bulimia nervosa," published April 12 in the Journal of Neuroscience, was authored by Margaret L. Westwater, University of Cambridge and the National Institute of Mental Health; Flavia Mancini, Hisham Ziauddeen and Paul C. Fletcher, University of Cambridge; Adam X. Gorka, Christian Grillon and Monique Ernst, National Institute of Mental Health; and Jane Shapleske and Jaco Serfontein, Cambridgeshire and Peterborough NHS Foundation Trust.