Brian Wansink of the University of Cornell,
in his excellent book “Mindless Eating – Why we eat more than we think[1]”
describes many experiments from his laboratory where food intake is found to be
quite often “mindless”, that is, the consumption of food occurs without any
real thought as to the reasons for either starting or stopping eating. Here is a simple example. Bright MBA students
are offered a chance to watch the Super Bowl in a bar where soft drinks are
free and where the students can go and collect as many chicken wings as they
like as often as they like and enjoy the game with their friends. The bones get
chucked into a bowl on the table. They are absorbed in the game and so they
don’t notice that for half the tables, the bowls of chicken bones are emptied
regularly throughout the evening. For the other half, the bowls fill up with
chicken bones. Those in the latter group whose bowls were not refreshed ate
just under 30% less chicken wings than those who had their bowls refreshed. The
latter were into mindless eating but for those whose bowls piled up with
chicken bones, they had a visual cue to halt mindless eating. Many, many other
marvellous examples are to be found in this excellent book, which everyone in
food and health should read.
The opposite to mindless eating brings us
into the sphere of self control which seeks to override impulses and habits and
it represents a conscious and thus effortful form of self-regulation involving
the prefrontal cortex, that part of the human brain that utterly distinguishes
us from all other species including our nearest primate relatives.
In some respects, this mindless and mindful
eating pattern ties in with the theory of decision making proposed by the Nobel
laureate in economics, Daniel Kahneman in his book “Thinking fast and slow”[2].
He identifies two distinct systems in the brain for decision-making. System 1
“operates automatically and quickly, with little or no effort and no sense of
voluntary control” (Mindless eating?). System 2 “allocates attention to the
effortful mental activities that demand it. The operations of System 2 are
often associated with the subjective experience of agency, choice and
concentration” (Self control or mindful eating?). Kahneman sees System 1 as the
default where most decisions are made and system 2 then operates to support
System 1. It’s fascinating, relevant but let’s turn to nutrition.
The brain occupies about 2% of our body
weight and for an adult, the brain accounts for 20% of total caloric intake. It
is a majorly expensive organ in terms of energy just as super-computers are
also massive energy consumers. Under normal circumstances, the brain only uses
glucose as a fuel and it will slightly deviate from this after a fast of a day
or so. Now when we are sitting around with friends having a coffee and chatting
about life and loves and whatever, the amount of glucose by the brain is at its
lowest, ticking over liked a car in park or neutral. Now give the subjects a mental
task, which requires a serious usage System 2 decision-making and the brain,
starts to consume significant amounts of glucose. Now some time after
completing this demanding mental task, give the subjects what is known as “the
marshmallow choice”. You can take a marshmallow now or you can hold on and
restrain yourself and then have two marshmallows later, “smaller and sooner” as
opposed to “larger but later”. How does the mental task influence self-control?
Lets turn to some experiments with human
volunteers[3]. Participants were asked to watch a 6-minute
video. One group watched it without any interference in a relaxed manner. In
the second group, certain stimuli appeared on the screen and this required an
extra mental effort to follow the video. For the first group, blood glucose
levels didn’t change. For the second group, their blood glucose levels fell. Numerous
other studies support this. Now lets go one step further and have two limbs to
the experiment. Participants were first exposed to a thought-suppression task
such as suppressing frightening thoughts of a white bear. Another group were
not given any thought-suppressing tasks (control). The theory here is that by
forcing the brain to use System 2 (worrying about a white bear), glucose would
be used by the brain in significant quantities. Half the subjects were given a
glucose drink and the other half relaxed reading magazines (the groups were
separate). Now all subjects were given a task, which was quite frustrating and
actually impossible to achieve. Those who were given the thought-suppressing
tasks imagining a white bear gave up quicker than the control group. Their
self- control was cut short by the prior use of glucose by the brain. However,
the glucose drink obliterated this effect. The glucose drink replenished the brain’s
glucose supply and now they fared as well as those that did not have any
thought-suppressing exercises. Again, the literature abounds with such
examples. In summary, forcing subjects to focus on some mental task uses up
blood glucose and subsequent self-control falls. However, drinking glucose
reverses this.
There is a final twist in the tail from a
recent paper in the Proceedings of the National Academy of Sciences[4].
Subjects were classified according to their beliefs that will power is either a
limited trait and easily depleted (limited resource belief) or that will power
is plentiful and not easily depleted (non-limited resource belief). The
subjects then consumed a drink, half of the drinks contained sugar and half an
artificial sweetener. The subjects were then given a demanding task such as
deleting the letter “e” from a text but with rules about which ones to drop out
and which ones to leave in. This required a lot of glucose consuming brain
concentration. Then they were given a standard psychological test (The Stroop test), which measures
self-control. Among those who believed that will power was plentiful, the sugar
drink had no effect. Among those who believed that will power was a limiting
and easily depleted, the glucose drink did the trick. They performed better in
the self-control test than those given the drink with the artificial sweetener.
All in all, these data tell us that
cognitive and mental capacity can be readily influenced by exhaustive mental
tasks. This causes a decline in the brain’s supply of glucose. The net effect
of that deficiency is to reduce self-control. Apparently, among those who
believe that self-control is weak and limited, a sugary drink will restore
mental performance. So, the next time you hear some guru bashing sugar,
remember that glucose alone is the fuel of the brain and that that fuel is
precious for every day decision making.
[1] “Mindless Eating – Why we eat more than we
think” by Brian Wansink and published by Hat House, London. Available on
Amazon
[2] “Thinking fast and slow” by Daniel
Kahneman, published by Penguin and available on Amazon
[3] Gailliot MT & Baumeister RF (2007) Personality and
Social Psychology Review,11, 303-7
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