A closer look at binge-eating
University of Cambridge
People who experience bulimia nervosa and a subset of those affected by anorexia nervosa share certain key symptoms, namely recurrent binge-eating and compensatory behaviours, such as vomiting.
The two disorders are largely differentiated by body mass index
(BMI): adults affected by anorexia nervosa tend to have BMI of less than 18.5
kg/m2. More than 1.6 million people in the UK are thought to have an eating
disorder, three-quarters of whom are women.
One prominent theory of binge-eating
is that it is a result of stress, which causes individuals to experience
difficulties with self-control. However, until now, this theory has not been
directly tested in patients.
To examine this theory, researchers at the University of Cambridge, working with clinicians at Cambridgeshire and Peterborough NHS Foundation Trust, invited 85 women -- 22 with anorexia nervosa, 33 with bulimia nervosa and 30 healthy controls -- to attend a two-day stay at Wellcome Trust-MRC Institute of Metabolic Science Translational Research Facility (TRF).
The facility, which includes an Eating Behaviour Unit,
is designed so that a volunteer's diet and environment can be strictly
controlled and their metabolic status studied in detail during a residential
status. The setting is intended to be as naturalistic as possible.
During their stay, each morning the
women would receive controlled meals provided by a nutritionist. The women then
underwent a fasting period during which they were taken to the next door
Wolfson Brain Imaging Centre, where they performed tasks while their brain
activity was monitored using a functional MRI scanner.
The first tasks involved stopping
the progression of a bar rising up a computer screen by pressing a key. The
main task involved stopping the moving bar as it reached the middle line. On a
minority of trials, stop-signals were presented, where the moving bar stopped
automatically before reaching the middle line; participants were instructed to
withhold their response in the event of a stop-signal.
The women then performed a task
aimed at raising their stress levels. They were asked to carry out a series of
mental arithmetic tests while receiving mild but unpredictable electric shocks,
and were told that if they failed to meet the performance criterion, their data
would be dismissed from the study. They were given feedback throughout the
task, such as 'Your performance is below average'.
The women then repeated the
stop-signal task again.
Once the tasks had been completed --
but while the volunteers might still be expected to be in a heightened state of
stress -- they returned to the Eating Behaviour Unit, where they were offered
an 'all you can eat' buffet in its relaxing lounge and were told they could eat
as much or as little as they would like.
On the second day of their study,
the volunteers carried out the same tasks, but without the added stress of
unpleasant electric shocks and pressure to perform. (For some participants, the
order of the days was reversed.)
Dr Margaret Westwater, who led the
research while a PhD student at Cambridge's Department of Psychiatry, said:
"The idea was to see what happened when these women were stressed. Did it
affect key regions of the brain important for self-control, and did that in
turn lead to increases in food intake? What we found surprised us and goes
counter to the prevailing theory."
The team found that even when they
were not stressed, those women with bulimia nervosa performed worse on the main
task, where they had to stop the rising bar as it reached the middle bar -- but
this was not the case for those women affected by anorexia nervosa. This
impairment occurred alongside increased activity in a region in the prefrontal
cortex, which the team say could mean these particular women were unable to
recruit some other regions required by the brain to perform the task optimally.
Interestingly -- and contrary to the
theory -- stress did not affect the actual performance in any way for either of
the patient groups or the controls. However, the patient groups showed some
differences in brain activity when they were stressed -- and this activity
differed between women with anorexia and those with bulimia.
While the researchers observed that
the patients in general ate less in the buffet than the controls, the amount
that they ate did not differ between the stress and control days. However,
activity levels in two key brain regions were associated with the amount of
calories consumed in all three groups, suggesting that these regions are
important for dietary control.
Dr Westwater added: "Even
though these two eating disorders are similar in many respects, there are clear
differences at the level of the brain. In particular, women with bulimia seem
to have a problem with pre-emptively slowing down in response to changes in
their environment, which we think might lead them to make hasty decisions,
leaving them vulnerable to binge-eating in some way.
"The theory suggests that these
women should have eaten more when they were stressed, but that's actually not
what we found. Clearly, when we're thinking about eating behaviour in these
disorders, we need to take a more nuanced approach."
In findings published last year, the
team took blood samples from the women as they performed their tasks, to look
at metabolic markers that are important for our sense of feeling hungry or
feeling full. They found that levels of these hormones are affected by stress.
Under stress, patients with anorexia
nervosa had an increase in ghrelin, a hormone that tells us when we are hungry.
But they also had an increase in peptide tyrosine tyrosine (PYY), a satiety
hormone. In other words, when they are stressed, people with anorexia nervosa
produce more of the hunger hormone, but contradictorily also more of a hormone
that should tell them that they are full, so their bodies are sending them
confusing signals about what to do around food.
The situation with bulimia nervosa
was again different: while the team saw no differences in levels of ghrelin or
PYY, they did see lower levels of cortisol, the 'stress hormone', than in
healthy volunteers. In times of acute stress, people who are chronically
stressed or are experiencing depression are known to show this paradoxical low
cortisol phenomenon.
Professor Paul Fletcher, joint
senior author at the Department of Psychiatry, said: "It's clear from our
work that the relationship between stress and binge-eating is very complicated.
It's about the environment around us, our psychological state and how our body
signals to us that we're hungry or full.
"If we can get a better
understanding of the mechanisms behind how our gut shapes those higher order
cognitive processes related to self-control or decision-making, we may be in a
better position to help people affected by these extremely debilitating
illnesses. To do this, we need to take a much more integrated approach to
studying these illnesses. That's where facilities such as Cambridge's new
Translational Research Facility can play a vital role, allowing us to monitor
within a relatively naturalistic environment factors such as an individual's
behaviour, hormone levels and, brain activity."
The research was funded by the
Bernard Wolfe Health Neuroscience Fund, Wellcome, the NIH-Oxford-Cambridge
Scholars Program and the Cambridge Trust. Further support was provided by the
NIHR Cambridge Biomedical Research Centre.