One recent line of study has found substantial correlations between willpower and blood glucose: Researchers Matthew Gailliot and Roy Baumeister started from the experientially derived premise that willpower seems to be a limited resource (similar to attention, as described in our previous blog post). A limited resource, they argued, implies an energy source that gets used up, is higher or lower at various points in time, and can be replenished in some way. They thought that glucose would be a logical candidate as the body's primary energy source. Furthermore, they noted based on multiple studies that "the influence of glucose seems most pronounced on effortful, controlled, or executive processes" (p. 306). This strongly describes self-control behaviors requiring willpower, like getting oneself to exercise or resisting a tempting dessert.
The authors noted that people are less able to pay attention when their glucose is low. This phenomenon is familiar to anyone prone to hypoglycemia, or anyone who has a family member with diabetes. The authors suggest that this occurs because attention naturally flits from one environmental stimuli to another, and that deliberate self-control is required to maintain attention on a single thing and to rule out competing stimuli. Furthermore, they conducted their own study in which they specifically monitored blood glucose while participants either did or did not try to maintain attention on a video and ignore experimentally-induced distractions. People who made an effort to stay focused showed a drop in glucose levels, while those who allowed themselves to be distracted did not. In another study, the researchers showed that maintaining attention on one task led to impaired performance on a subsequent task, with the effect statistically explained by the drop in glucose levels. And a third study showed that giving children a glucose drink as an experimental manipulation led to improvements in both attention and frustration tolerance during a demanding mental task.
Other research shows that glucose has the same type of effects on emotional self-regulation tasks. Low glucose predicts negative mood, and people with diabetes are often distressed by their mood fluctuations as a result. In another study, giving people a glucose drink improved their self-reported mood; this phenomenon is familiar to anyone who has gotten a boost in mood from a piece of chocolate. Mood effects alone could be explained by dopamine changes in response to the sugar, but other self-regulation variables such as impulsivity, risky sexual behavior, and alcohol use all seem to be linked to low glucose in a similar fashion. More seriously, aggressive behavior and even crime have been linked to poor glucose tolerance: In one striking longitudinal study, researchers found that 27% of the variance in recidivism could be predicted by glucose tolerance in an 8-year follow-up of people with a past imprisonment for violent crime. Finally, glucose varies over the course of the day and many people's glucose processing becomes less efficient in the evening. This might account for the dinnertime self-control problems seen in many households, including those with toddlers. Clinically, people with diabetes often describe difficulty concentrating or up-and-down mood when their glucose is too low (hypoglycemia), although the same effects are sometimes linked to overly high glucose (hyperglycemia) as well.
Our local expert on glucose, Dr. Teri Hernandez, has noted that brain glucose levels are tightly controlled: The brain's glucose supply is maintained at a constant level even at the expense of other body systems, so that even if blood glucose outside the brain fluctuates widely the brain's glucose level stays consistent. Brain glucose and blood glucose levels are therefore not identical. After starvation, for instance, glucose transporters significantly up-regulate their activity leading to a rapid restoration of brain glucose levels at the expense of the rest of the body. Because of this self-protective brain chemistry, Gailliot and Baumeister's model of glucose directly affecting executive control via the prefrontal cortex is too simplistic; a more complex model that incorporates up- or down-regulation of glucose transport is probably more accurate. Perhaps the effects are more gradual or situational, or self-control does not reside exclusively in the brain, or glucose has effects on parts of the endocrine system (e.g., the pituitary gland). Interestingly, the brain is one part of the body that does not require insulin to use glucose, so hypo- or hyperglycemia related to kidney function is probably not the explanation.
Even though the exact mechanisms remain to be worked out, Gailliot and Baumeister's findings about blood glucose levels and self-control do suggest that willpower is about more than just mental fortitude. These strong physiological effects on tasks that would normally appear to involve willpower mean that behavior is less under conscious control that we usually believe. In fact, Two Minds Theory suggests that behavior is never under conscious control, and that persuasion or self-directive thoughts are effective only to the extent that they have indirect future effects on the Intuitive System. Because glucose levels tend to vary in predictable ways based on mealtimes and time of day, people may be able to learn to prioritize self-management tasks at times when their glucose levels are stable. New sensor technologies for continuous glucose monitoring may provide interesting further data about the connections between glucose and behavior in everyday life. And if attention or willpower are worse when glucose is high or low, technology may also provide new avenues to help people to change their behavior, using augmented cognition approaches to present information at times or in ways that are more helpful based on a person's ambulatory glucose level.
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