The human brain is a gas-guzzler of an organ, accounting for some 20 percent of the body’s total metabolic activity. The high cost of keeping a big brain functioning is presumed by many to be the reason why our big noggins took so long to evolve, and why no other organism has bothered to cram such a big brain in such a relatively small body.
What was a hurdle in evolutionary terms could, however, prove to be a blessing for the obesity-challenged. Because if normal everyday thinking burns up 20 percent of our total calories, just imagine how thinking really hard — doing math homework, say, or trying to figure out the plot of Lost — could melt the pounds away! Right?
Well, unfortunately, no. Thanks to fellow PT blogger Kelly McGonigal, I’ve come across an absolutely fascinating paper in Evolutionary Psychology that deals with the question of how mental effort relates to energy metabolism. According to author Robert Kurzban, a careful reading of the literature suggests that thinking hard (or to put it another way, engaging in effortful mental activity) does not correlate to an increase in calories burned — specifically, to the metabolization of glucose:
Research on brain metabolism suggests that the relationship between blood glucose and mental function is complex, and not simply a matter of more “effortful” processes leading to the “soaking up” of more glucose by the brain… Indeed, evidence suggests that the sorts of tasks in which subjects are engaging in this literature have very little effect on overall brain metabolism and, specifically, glucose use by the brain. 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), arguing that “…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).
The paper specifically addresses the idea, which has become very popular in discussions of self-control, that willpower is an expendable resource that gets used up (in Roy Baumeister’s memorable formulation) just as a muscle becomes fatigued through use. Building on that idea, some have suggested that the reason for the fatigue is that the brain uses up glucose. To my mind, Kurzban demolishes that idea pretty effectively — though he is less forceful in attacking Baumeister’s “like a muscle” claim (which he also disparages).
The really fascinating take-away from all this, for me, is that it shows how the sense of mental effort, though so similar-feeling to the sense of physically effort, is really a very different thing. “Ugh, I don’t want to jog any more!” feels a lot like “Ugh, I don’t want to concentrate any more!” not because they are similar at a metabolic level, but because both trigger a similar (or perhaps the same) piece of mental machinery that registers in consciousness as an unpleasant feeling of aversion.
So, if you want to melt away the flab, go for a run, mow the lawn, or have sex — don’t volunteer to do your neighbor’s taxes.