It’s a Thursday night. You’re working on your physics problem set that is due the next day. You’ve been in the library for what feels like an eternity. You’ve been focusing on nothing but your problem set, inhibiting the urges to check your phone that is buzzing in your backpack, talk to your friends, and ask your classmates for help. After a grueling few hours of effort on your problem set, you pack up your things and trudge across campus back to your dorm room. You enter your room and your three roommates are sitting in the common room of your quad. You sit down as one of your friends is in the middle of a long story. She mentions that she has gotten in another argument with your boyfriend and she seems pretty frazzled. You respond, “Well that stinks, but I’m honestly not surprised, he’s just not a great guy.” Your friends all stare at you, surprised by your cold, somewhat hostile comment. Why did you lash out and act aggressively toward your friend? Normally you are so kind in these situations. What happened to your self-control? The answer to these questions lies within the frontal cortex has a lot to do with glucose.

When we think about the brain, we probably think about the long list of tasks it completes for us on a day to day basis. The brain controls our thoughts, beliefs, goals, and impulses. It helps us focus, move, feel, think, and remember. Without a steady supply of energy, our brain is unable to function at its maximum capacity. The brain relies heavily on glucose as its major source of energy [1]. Glucose is a sugar molecule made up of six carbon atoms. Our brains absolutely love sugar – they need sugar. The brain makes up only 2% of our body weight, yet it consumes 20% of our glucose-derived energy [reviewed in 1]. What this means is that the brain consumes more sugar than any other part of the body. Talk about a sweet tooth!

When we consume glucose, our body breaks down the sugar during three processes: glycolysis, the citric acid cycle, and oxidative phosphorylation. For more information about the breakdown of glucose, click here!  The end result of the breakdown of glucose is energy in the form of adenosine triphosphate (ATP). ATP is cellular energy, and it is used to sustain many important cellular processes including the production of macromolecules like proteins, carbohydrates and fats, the transport of ions across cell membranes, and the contraction of our muscles [reviewed in 2].  Essentially, our brain depends on sugar to make energy so we can carry out all of our cellular processes.

Thinking about our brain’s relationship with glucose and energy, we may be wondering what processes will be affected if our brains do not have enough glucose. We might also be wondering which brain tasks need the most glucose. There is a solid body of literature concerning glucose and cognitive functioning. Researchers have found that controlled, effortful cognitive processes that rely on executive function are impacted by changes in blood sugar [reviewed in 3]. Controlled processes are effortful and require lots of attentional resources, while automatic tasks are fast, error prone, and do not require many attentional resources. When we have low blood sugar and are faced with a challenging cognitive task, we will do worse on the task when we have low blood sugar than we will when we have higher blood [reviewed in 3].

Controlled, effortful process are involved in many behaviors and functions, including self control [3]. Self-control is a specific type of self-regulation that involves consciously and effortfully overriding automatic, dominant responses to certain stimuli and in certain situations [4]. When we’re in a situation dealing with people, our self-control prevents us from saying rude or inappropriate things. Self-control is hard work; it takes serious effort to override impulses, emotions, and automatic responses [4]. The relationship between effort and self-control prompted Roy Baumeister and colleagues to study the importance of glucose in self-control. They conducted a series of studies to determine whether or not self-control depended on glucose. They found that, in college students, blood glucose dropped significantly in individuals who completed acts of self control. In particular, the acts-of self control included emotion regulation and an interracial interaction. Studies have shown that people work hard and use self-control in an interracial interaction because it takes self-control to suppress stereotypes [3]. They also found that blood glucose after one self-control act was related to poor self-control on a follow up test [3].

Think back to your physics problem set. While you were doing your work, you were exerting self-control because you were focusing on your work and inhibiting the urges to talk to your friends, use your phone, get up, or get the answers from a friend. You were working hard and using a lot of your attentional resources to complete your homework. When you got back to your dorm room right after your work you were immediately placed in a social situation. During frontally demanding tasks, blood sugar drops, and when your blood sugar drops your self-control is not where it needs to be. This could explain your less than friendly response to your friend’s problems. Immediately after our frontal cortices work hard on a cognitively demanding task, we show less empathy, are less honest, and more aggression toward others [reviewed in 5, pages 91-92]. The next time you’re working on a cognitively demanding task, try having a sugary drink right after. This will revamp your frontal lobe and hopefully help you act a little nicer toward the people around you.  

References

1. Mergenthaler P, Lindauer U, Dienel GA, Meisel A. Sugar for the brain: the role of glucose in physiological and pathological brain function. Trends Neurosci. 2013;36(10):587-97.
2. Bonora M, Patergnani S, Rimessi A, et al. ATP synthesis and storage. Purinergic Signal. 2012;8(3):343-57.
3. D.M., Brewer, L.E., & Schmeichel, B.J. (2007). Self-control relies on glucose as a limited energy source: Willpower is more than a metaphor. Journal of Personality and Social Psychology, 92, 325-336.
4. Vandellen MR, Hoyle RH, Miller R. The Regulatory Easy Street: Self-Regulation Below the Self-Control Threshold Does not Consume Regulatory Resources. Pers Individ Dif. 2012;52(8):898-902.
5. Sapolsky, R. M. (2017). Behave: The biology of humans at our best and worst. New York, NY: Penguin Books.