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Sugar, the brain and self control

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
[4] Job V et al (2013) PNAS, 110, 14,837-42


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