Earlier on
in this month of November, a BBC report revealed findings from researchers
about how obesity affects not only the body but the mind as well which could
lead to memory loss and dementia.
Lucy Cheke
and her colleagues at the University of Cambridge recently invited a few
participants into her lab for a kind of ‘treasure hunt’ which involved the
participants having to navigate a virtual environment on a computer screen,
dropping off various objects along their way thereafter answering a series of
questions to test their memory of the task, such as where they had hidden a
particular object.
When examining what might have influenced their performance,
you might expect that Cheke would have been more concerned with the
participant’s IQ – not their waistline. Yet she found a clear relationship
between their Body Mass Index – a measure of your weight relative to your
height – and apparent memory
deficits: the higher a
participant’s BMI, the worse they performed on the Treasure Hunt task.
In doing so, Cheke has contributed to a small but growing
body of evidence showing that obesity is linked to brain shrinkage and memory
deficits. This research suggests that obesity may contribute to the development
of neurodegenerative conditions such as Alzheimer’s Disease.
Surprisingly, it also seems to show that the relationship
between obesity and memory is a two-way street: being overweight or obese not
only impacts on memory function, but may also affect future eating behaviour by
altering our recollections of previous eating experiences.
Cheke’s interest in the subject began unexpectedly. “At the
time I was looking at the ability to imagine a future state, particularly in
terms of making decisions about food,” says Cheke. “If you’re hungry, you’ll
imagine your future self as being hungry, too, but obese people seem to make
such decisions on fact-based judgements rather than imagining” she was quoted
by the BBC.
One possibility was that the obesity might have been damaging
their capacity for “mental time travel”. Scientific research has long shown
that memory and imagination are intimately linked, as we piece together
fragments of past recollections to predict how future events might pan out.
The link made sense, she says, with some signs that obesity
affects areas of the brain known to be used in memory and imagination. In 2010,
for instance, researchers at Boston University School of Medicine reported that
healthy, middle-aged adults with increased abdominal fat tend to have slightly lower overall brain volume. In particular, the hippocampus, a
deep brain structure sometimes called the brain’s printing press thanks to its
role learning and memory, was significantly smaller in obese people compared to
leaner individuals.
There were also some hints from animal studies. “In studies
focusing on weight changes and eating behaviours in rodents, the animals were
terrible at learning tasks such as the Morris water maze,” explains Cheke. “The
more I looked into it, the more I expected to see memory deficits, but that
question was still very much open.”
Hence her experiment with the treasure hunt. Sure enough the
obese participants found it particularly difficult to remember the location of
the different objects – adding some important evidence for her hypothesis, and
supporting earlier findings that indirectly linked obesity to
impairments of cognitive function.
More recently, a brain scanning study including more than 500
participants confirmed that being overweight or obese is associated with a
greater degree of age-related brain degeneration. These effects were biggest in
middle-aged people, in whom the obesity-related changes corresponded to an estimated increase in 'brain age' of
10 years.
Obesity is a complex condition with many contributing
factors, however; so exactly how it might affect brain structure and function
is still unclear.
“Body fat is the defining feature of obesity, but you’ve also
got things like insulin resistance, hypertension, and high blood pressure,”
says Cheke. “These can go hand in hand with behavioural factors [such as
overeating and lack of exercise] and they can all potentially cause changes in
the brain.”
“For example, insulin is an important neurotransmitter, and
there’s a lot of evidence that diabetes
is associated with changes in learning and memory,” she adds, “but there’s also evidence that high body
fat on its own leads to inflammation in the brain, which can also cause
problems.”
Inflammation is another potential culprit. Psychologists from
the University of Arizona examined data from more than 20,000 participants in
the English Longitudinal
Ageing Study, in which
measures of memory, BMI, and blood plasma levels of an inflammatory marker
called C-reactive protein were collected every 2 years between 1998 and 2013.
They found that greater body mass was associated with a
decline in memory function, and also with higher levels of the inflammatory
protein. Although these links are indirect, the results suggest that brain
inflammation is one plausible mechanism by which differences in body mass might influence cognitive
function in otherwise
healthy, aging adults.
This should be of particular concern, given recent evidence
that the path between memory and obesity may go both ways, as attention and
memory control our appetite and eating behaviour. In other words, a deficit in
your memory could cause you to gain weight.
Early evidence that memory plays an important role in eating
behaviour came from a 1998 study showing that patients with severe amnesia will readily eat multiple meals one
after the other, because
they could not remember that they had just eaten.
“This shows that when we’re deciding how much to eat we’re
not just basing those decisions on physiological signals about how much food
there is in our stomach, but also on cognitive processes like memory,” says
experimental psychologist Eric
Robinson of the
University of Liverpool.
“If your memory’s impaired or just not very good then you
might overeat,” he adds. “I wanted to know if this could be reversed. If you
improve a person’s memory, could that be a useful way of getting them to eat
less?”
Robinson and his colleagues recruited 48 overweight or obese
people and invited them to eat lunch in the lab. The participants were randomly
divided into two groups, and given audio recordings to listen to while they
ate.
Those in one group listened to audio instructing them to pay
attention to their food, while those in the other listened to an audio book
with non-food related content.
The researchers then invited them back the following day,
presented with some high-energy snacks, and measured how much they ate. They
found that those who had been instructed to focus on their lunchtime meal the
previous day ate nearly one third
less of the snacks than
those who had been distracted by the audio book.
A larger follow-up study confirmed these findings. This time,
Robinson and his colleagues randomly assigned a total of 114 women to one of
two groups, and tried to manipulate the extent to which they were aware of
their eating behaviour.
Again, they gave all participants the same lunchtime meal,
consisting of a ham sandwich, mini sausage rolls, a packet of crisps, rice
cakes, chocolate chip cookies and seedless grapes.
Before sitting down to eat, the participants in one group
were told that they were taking part in a study of eating behaviour, and that
the amount of food they ate would be measured. The rest were told that they
were taking part in a study of how their thought processes and moods change
during the course of the day.
The researchers found no overall difference between how much
participants in both groups ate. Those who had been told that they were taking
part in a study of eating behaviour tended to eat fewer cookies than those in the other group,
however, apparently because their awareness of their own food consumption was
heightened.
Attention and memory are independent of each other, but they
are closely linked – we cannot remember something that we did not pay attention
to and, by the same token, our memories of something tend to be more vivid the
more we attend to it.
It’s therefore possible that a vivid memory of lunch could
reactivate the body’s physiological state, so that we do not feel so hungry,
and consequently eat less at dinner. On the other hand, someone who was
distracted during lunch would form weak memories of the meal, and so thinking
about it at dinner might make them feel hungrier and eat more.
In one 2011 study, for instance, half the participants played
Solitaire on a computer while eating their lunch. Sure enough, they had hazier
memories of their lunch and went on to eat significantly more biscuits later on than those who did not.
This is particularly interesting, given the evidence that
over-eating can impair your memory, with both the over-eating and the memory
problems reinforcing each other, pushing you down a slippery slope. “Our
research suggests that you might eat more if you have an impaired memory,” says
Robinson, “so you end up in a vicious cycle where memory’s impaired by an
unhealthy lifestyle, and then that impairment promotes over-consumption.”
He points out that we still have to be careful not to draw
firm conclusions, though, until we have stronger proof that this vicious cycle
exists and has a real effect on people’s health. “This idea makes sense
intuitively, but there’s still no direct evidence for it.”
In the meantime, the finding that food memories and awareness
can influence eating behaviour does at least suggest a novel approach to helping people lose
weight and maintain a
healthy BMI, and Robinson and his colleagues have developed a smartphone app that encourages people to eat more
attentively.
“There’s now convincing evidence that attention and memory affect
how much people eat, but this comes from laboratory studies,” says
Robinson.“We’re trying to see if the lab findings translate to the real world.
Our app encourages people to take photos of what they’re eating and answer
questions about their meals, the idea being that creating vivid memories will
make them less likely to overeat during the day.”
Cheke and her colleagues are now following up their initial
findings by trying to pick apart the various factors that contribute to
obesity, in order to try to determine which are likely to influence brain
structure and function.
They are also using a smartphone app to collect information
about people’s lifestyles and behaviour, and are recruiting volunteers in and
around Cambridge to help them gather the data they need.
“One person may be obese because they don’t do any exercise
and eat a lot of junk food,” says Cheke. “Another might be obese for genetic
reasons but actually eat really well and do lots of exercise, and yet another
may be obese because they have insulin problems.”
“We’re trying to get all these different variables to see the
relative contribution, so we’ve got people out wearing activity monitors and
filling food diaries for us. Doing studies like this is the only way we’ll be
able to tease these things apart.”
