New research just published (July 11, 2016) shows that the blood-glucose hypothesis about willpower is probably not true, at least not as evidenced by current scientific studies.
The idea -- previously held -- was that blood glucose mediated the propensity of people to exert willpower, with decreased blood glucose making it less likely that someone would persevere in a task.
I myself had read some of the previous research and shared the blood-glucose hypothesis. Apparently, current research doesn't back up this claim. Miguel A. Vadillo, Natalie Gold, and Magda Osman -- writing in the journal Psychological Science, utilized a "new meta-analytic tool, p-curve analysis, to examine the reliability of the evidence from" 19 studies focusing on blood glucose and willpower. They found that overall there was not a reliable effect of glucose.
Here are two quotes from the article:
The findings from the present study are a surprise in the context of the wide acceptance of the glucose hypothesis in general scientific research and its popularity, as evidenced by the number of citations of Gailliot et al. (2007) in the literature and the continued influence of this hypothesis in recent reviews on ego depletion (e.g., Baumeister, 2014; Baumeister & Alghamdi, 2015). Moreover, the hypothesis has intuitive and seemingly practical appeal. If one accepts that a failure of self-control in regulating actions contributes to the many personal and societal problems that people face (Baumeister et al., 2000), then glucose supplements would provide a simple means to enhance willpower and ameliorate these problems (Baumeister & Tierny, 2011). In light of our results, it is doubtful that such a recommendation will work in the real world. This conclusion converges with recent evidence that glucose might have little or no impact on domain-general decision-making tasks (Orquin & Kurzban, 2016) and with an intriguing series of meta-analyses and preregistered replications suggesting that the ego-depletion effect itself might be less robust than previously thought (Carter, Kofler, Forster, & McCullough, 2015; Hagger et al., in press).
Our results suggest that, on average, these studies have little or no evidential value, but they do not allow us to determine whether the significant results are due to publication bias, selective reporting of outcomes or analyses, p-hacking, or all of these. It is not impossible that some of these studies are exploring small but true effects and that their evidential value may be diluted by the biases that pervade the rest of the studies. Perhaps future research will show that glucose does play a role in ego depletion effects, but our conclusions are based on the analysis of the extant literature in this area. Thus, our contribution must be seen as an additional piece of information in the wider context of attempts to verify the reliability of the glucose model of ego depletion.
In the past, I used the glucose-depletion idea as a partial explanation why day-long training sessions were difficult for learners. I also used it as a rationale for plying my workshop participants with treats in the afternoon. Well, at least I tried to make them somewhat healthy! As the meta-analysis above reveals, it's likely that some other mechanism is involved in the difficulties learners have during intensive learning sessions. As trainers and instructional designers, we still have to figure out a way to support learners during long learning sessions to prevent attention-zapping fatigue...
Vadillo, M. A., Gold, N., and Osman, M. (2016). The bitter truth about sugar and willpower: The limited evidential value of the glucose model of ego depletion. Psychological Science, Published Online July 11, 2016. Available at: http://pss.sagepub.com/content/early/2016/07/08/0956797616654911.full