Robert Kurzban Ph.D.

Mind Design

Glucose Is Not Willpower Fuel

Is the muscle model of self-control less then a metaphor?

Posted Aug 29, 2011

Bright idea

Does sugar make your brain-bulb burn bright?

In my last post, I talked about the idea that there is a resource that is necessary for self-control. People discussing this idea have  used various metaphors – self-control is a muscle, willpower is a reservoir – but few suggestions have been made about what, exactly, the resource in question actually might be. Not long ago, one proposal was made to make the idea “more than a metaphor.”1 In this post, I want to talk a little bit about the candidate for this resource, glucose. Could willpower fail because the brain is low on sugar?

Let’s look at the numbers. A well-known statistic is that the brain, while only 2% of body weight, consumes 20% of the body’s energy. That sounds like the brain consumes a lot of calories, but if we assume a 2,400 calorie/day diet – only to make the division really easy – that’s 100 calories per hour on average, 20 of which, then, are being used by the brain. Every three minutes, then, the brain – which includes memory systems, the visual system, working memory, then emotion systems, and so on – consumes one (1) calorie. One. Yes, the brain is a greedy organ, but it’s important to keep its greediness in perspective.

The next question is the comparison between a brain exerting willpower and one that’s not. Suppose, for instance, that a brain in a person exerting their willpower – resisting eating brownies or what have you – used twice as many calories as a person not exerting willpower. That person would need an extra one third of a calorie per minute to make up the difference compared to someone not exerting willpower.

Over the years, a number of researchers have asked whether the brain uses more glucose during tasks that feel effortful compared to tasks that don’t. Here is a sample of the conclusions drawn by researchers reviewing the evidence. Does exerting “self control” burn more calories?

  • Clarke and  Sokoloff (1998) remarked that although “[a] common view  equates concentrated mental effort with mental work…there appears to be no increased energy utilization by the brain during such processes” (p. 664), and “…the areas that participate in the processes of such reasoning represent too small a fraction of the brain for changes in their functional and metabolic activities to be reflected in the energy metabolism of the brain…” (p. 675).


  • Gibson and Green  (2002), talking about a possible link between glucose and cognition, wrote that research in the area “…is based on the assumption that, since glucose is the major source of fuel for  the brain, alterations in plasma levels of  glucose will result in alterations in brain levels of glucose, and thus neuronal function. However, the strength of this notion lies in its common-sense plausibility, not in scientific evidence…” (p. 185).

  • Lennie (2003) concluded that “[t]he brain’s energy consumption does not change with normal variations in mental activity” and that “overall energy consumption is essentially constant” (p. 495).

  • Messier (2004) concluded that it is “unlikely that the blood glucose changes observed during and after a difficult cognitive task are due to increased brain glucose uptake” (p. 39).

  • Gibson (2007), concluded that “task-induced changes in human peripheral blood glucose are unlikely to reflect changes in relevant areas of brain glucose supply” (p. 75).

Now, there is some evidence that might be taken to pull the other way, but these summaries suggest that there is more or less a consensus among researchers who investigate brain metabolism about this issue.

Nonetheless, as I indicated above, some have suggested that exerting self control drains the brain of glucose, which in turn leads to a diminished ability to exert self control. Specifically, I want to address a paper on this topic that is supposed to provide key evidence. A careful reading of this manuscript, which has been heavily cited as providing evidence in favor of the model, actually undermines the claim. In one study in this paper,1 roughly 100 subjects fasted for three hours prior to the experiment. Glucose was measured before and after these subjects were asked to watch a film for five minutes; half were told not to look at words appearing on the screen. (This was the “self control” task.) Recall that in five minutes, the whole brain consumes about 1.5 calories. The paper reports that subjects who watched the film without looking at the words did show the glucose drop, while those who didn’t saw a (non-significant) rise in glucose. This seems to demonstrate that exerting “self control” does consume glucose.

Given that the experts on brain metabolism find that blood glucose doesn’t change depending on what task subjects do, this finding seems striking. There are, however, two reasons to worry. First, the subjects in the self-control condition (N=52) had initial blood glucose readings well above those of the other group’s as well as above the level found in other studies. In other words, the reason these subjects’ blood sugar went down might have to do with the fact that they were abnormally high.2 Second, when I asked for the raw data, I was told that the data were “corrupted,” and so – like Joseph Smith’s Golden Plates –we just have to take the authors’ word for it.

The same paper reported a series of additional studies in which subjects (who had not fasted for three hours) had their glucose reading taken before and after doing a “self-control” task. In these studies, glucose didn’t go down at all. In fact, it went up (though not statistically significantly). (See Table 1 in this paper.)

In sum, the paper frequently touted as the best evidence in favor of the glucose-as-resource-for-self-control model shows 1) a drop in glucose for people with abnormally high levels of glucose and 2) no drop in glucose among a larger sample of subjects.

In short, the idea that more sugar is consumed when exerting “self control” is wrong. Actually, Hockey (2011) would say that the emphasis on glucose as the fuel for self control is not just wrong, but destructive of progress, writing:

Remarkably, given that fatigue has been studied formally for well over 100 years, there is still no scientifically mature theory of its origins and functions.

One part of the reason for this lack of progress, he suggests, is that there is an “irresistible tendency to think of it in terms of a loss of energy resources,” but that there is “no evidence” for the claim that “fatigue is the result of glucose depletion.” Hockey concludes that “there is little doubt that the energy-depletion perspective has been a source of distraction in the search for a theory of fatigue” (p. 167). Van den Berg (1986) pointed this out more than a quarter of a century ago. The right answer to the question about why exerting willpower is hard is not going to be packets of sugar:

Descriptions of behavioral acts or activities using a terminology in which concepts like energy play a major role are very likely based on a false analogy. The assumption that the brain is a machine like the muscles, albeit more complex, is ill-founded. Once we have developed a theory of the dynamics of behavior in which no traces remain of nineteenth century conceptual frameworks of energetics many of our current problems will vanish. We will not then be misled by the term mental energy, nor will we advise someone to take sugar when he or she complains of being tired after writing a paper! (p. 134)

1 Gailliot et al., 2007

2 For those who care about such things, the average was reported as 107.1 mg/dL. As one comparison from an entirely different group of undergraduates, Dvorak and Simons (2009) also had subjects fast and obtained a mean of 97.7 mg/dL (SD=11, N=180). So, the mean reported in the paper in question is around 10 standard errors above this mean.

References

Clarke, D. D., and Sokoloff, L. (1998) Circulation and energy metabolism of the brain. In  G. Siegel, B. Agranoff, R. Albers, S. Fisher, and M. Uhler (Eds.), Basic neurochemistry: Molecular,  cellular, and medical aspects (6th Ed.) (pp. 637-669). Philadelphia, PA: Lippincott Raven

Dvorak, R.D., and Simons, J.S. (2009). Moderation of resource depletion in the self-control strength model: Differing effects of the two modes of self-control. Personality and Social Psychology Bulletin, 35, 572-583

Gailliot, M. T., Baumeister, R. F., DeWall, C. N., Maner, J. K., Plant, E. A., Tice, D. M., Brewer, L. E., and 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.

Gibson, E. L. (2007). Carbohydrates and mental function: Feeding or impeding the brain? Nutrition Bulletin, 32, 71-83.

Gibson, E.L., and Green, M. W. (2002). Nutritional influences on cognitive function:  Mechanisms of susceptibility. Nutrition Research Reviews, 15, 169–206. Hockey, G. R. J. (2011). A motivational control theory of cognitive fatigue. In P. L. Ackerman (Ed.), Cognitive fatigue: Multidisciplinary perspectives on current research and future applications (pp. 167-187). Washington, DC: American Psychological Association.

Lennie, P. (2003). The cost of cortical computation. Current Biology, 13, 493-497. Messier, C. (2004). Glucose improvement of memory: A review.  European Journal of Pharmacology, 490, 33-57

Van den Berg, C. J. (1986). On the relation between energy transformation in the brain and mental activities. In R. Hockey, A. Gaillard, & M. Coles (Eds), Energetics and Human Information Processing (pp. 131-135). Dordrecht, The Netherlands: Martinus Nijhoff.

Copyright Robert Kurzban, 2011. All rights reserved. 

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