Improved Self-Control Associated with Using Relatively Large Amounts of Glucose: Learning Self-Control Is Metabolically Expensive

doi:10.4236/psych.2012.311148

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Psychology

2012. Vol.3, No.11, 987-990

Published Online November 2012 in SciRes (http://www.SciRP.org/journal/psych) http://dx.doi.org/10.4236/psych.2012.311148

Improved Self-Control Associated with Using Relatively Large

Amounts of Glucose: Learning Self-Control Is Metabolically

Expensive

Matthew T. Gailliot

Psychology Department, Stephen F. Austin State University, Nac o g doches, USA

Email: [email protected]

Received August 15th, 2012; revised September 14th, 2012; accepted October 12th, 2012

The current study examined whether changes in glucose during a self-control task would predict changes

in self-control performance later on. Participants attended two experimental sessions, spaced two weeks

apart. During each session, they had their glucose measured, completed the Stroop task as a measure of

self-control, and then had their glucose measured again. Larger decreases in glucose (from before to after

the Stroop task) during the first session predicted larger increases in improvement on the Stroop task dur-

ing the second session, in the form of increased speed. Learning self-controlmight benefit from using lar-

ger amounts of glucose. Learning self-control is metabolically expensive. These findings raise the possi-

bility that self-control fatigue occurs because metabolic energy is depleted during the learning of self-

control.

Keywords: Self-Control; Self-Regulation; Glucose; Metabolism; Learning; Fatigue; Energy; Memory

Introduction

Humans are metabolic organisms. Every thought, behavior,

and biological process requires the use of metabolic energy.

Some processes are metabolically expensive, however, and

consume more energy than do others (Aiello, 1997; Aiello,

Bates, & Joffe, 2001; Aiello & Wheeler, 1995; Gailliot, Hilde-

brandt, Eckel, & Baumeister, 2009). The current study tested

whether learning self-control is metabolically expensive, such

that improved learning would be associated with using larger

amounts of one metabolicenergy source—glucose.

Past work suggests that cognitive learning is metabolically

expensive. Children use a relatively large amount of glucose for

the brain, disproportionately more than adults do (Haymond,

1989), and childhood is a time of learning large amounts of

information. Several studies demonstrate that memory is im-

proved when glucose levels are optimal, such as after having a

glucose drink (for reviews, see Riby, 2004; White, 1991).

Memory is one form of learning, as the individual encodes,

stores, and retrieves information for later recall or recognition.

If glucose benefits learning among children and memory, then

glucose might benefit learning self-control.

Learning can be viewed as a form of growth, because learn-

ing occurs partly through neuronal spreading or the growth of

neurons. Growth is metabolically expensive.

Cancer cells are one form of growth, and they use a relatively

large amount of metabolicenergy, relative to non-cancerous

cells (Schoen et al., 1999; Weber et al., 2003; Younes, Lechago,

Somoano, Mosharaf, & Lechago, 1996). Other forms of bio-

logical growth, such as sperm production (Pitnick, Jones, &

Wilkinson, 2006), can also be metabolically expensive. To the

extent that learning self-control entails growth, such as among

neurons, then glucose would be expected to improve learning

self-control.

Using self-control is metabolically expensive, suggesting that

learning self-control might be also. Several studies have found

that, after using self-control in one domain (e.g., thought sup-

pression), self-control is impaired in any other domain (e.g.,

emotion regulation) (for reviews, see Baumeister, Gailliot,

DeWall, & Oaten, 2006; Baumeister, Vohs, & Tice, 2007;

Gailliot, 2009; Muraven & Baumeister, 2000). This effect ap-

pears to occur because self-control is impaired when glucose is

low, and using self-control reduces glucose in the bloodstream

(De- Wall, Baumeister, Gailliot, & Maner,2008; Fairclough &

Hous- ton, 2004; Gailliot et al., 2007; Gailliot & Baumeister,

2007; Gailliot, Peruche, Plant, & Baumeister, 2009; Masicampo

& Baumeister, 2008). Other work shows that self-control is im-

paired by other metabolic problems, such as low brainglycogen,

diabetes, glucose-6-phosphate dehydrogenase deficiency, and

glucose intolerance (DeWall, Gailliot, Deckman, & Bushman,

2009; Gailliot, 2008; Gailliot & Baumeister, 2007).

Thus, people might be better at learning self-control when

they use larger amounts of glucose while exerting self-control.

Using more glucose might indicate more effective learning or

growth.

Participants in the current study attended two experimental

sessions, spaced 2 weeks apart. During each session, partici-

pants completed the Stroop task as a measure of self-control,

before and after which their blood-glucose levels were assessed.

If glucose reductions benefit learning, then using more glucose

during the first session should predict greater improvements on

the Stroop during the second session.

Method

Participants

The final sample included 50 college undergraduates (30

women, 20 men) who participated in exchange for credit to-

M. T. GAILLIOT

ward a course requirement. The final sample excluded 11 par-

ticipants who did not return for the second experimental ses-

sion and 3 participants for whom procedural errors by an ex-

perimenter precluded analyzing their data.

Procedure

Participants attended two experimental sessions, spaced 2

weeks apart. During each session, participants first completed a

questionnaire packet unrelated to the current investigation.

They then had their blood-glucose levels measured, completed

the Stroop task, and then had their glucose levels measured

again.

Blood samples were taken with single-use blood sampling

lancets. Blood glucose levels were measured (mg/dL) using an

Accu-chek compact meter.

Participants completed the Stroop task for 20 minutes. For

the Stroop task, participants were given a list in which the

words red, blue, and green appeared in an incongruent font

color (either red, blue, or green). Participants were instructed to

statealoud the color of the ink and to ignore the meaning of the

word, and to do this as quickly and accurately as possible. The

experimenter recorded the number of errors and speed (number

of trials completed).

Results

Speed on the Stroop

The change in glucose (from the first to the second glucose

reading) during the first session predicted change in speed on

the Stroop (number of trials completed) from the first to the

second session, r(50) = –.30, p < .05. Larger decreases inglu-

cose during the first session predicted larger increases in the

number of trials completed. Thus, larger drops in glucose were

associated with greater improvements in performance.

Errors on the Stroop

The change in glucose (from the first to the second glucose

reading) during the first session did not predict change in errors

on the Stroop from the first to the second session, p > .5 8. The

relationship between glucose changes during the first session

and changes in speed across sessions did not appear attributable

to errors, such as if a speed-accuracy tradeoff existed. Specifi-

cally, glucose changes during the first session and changes in

speed remained significantly correlated when controlling for

changes in errors, r(47) = –.29, p < .05.

Glucose Changes during the Second Session

Changes in glucose (from the first to the second glucose

reading) during the second session did not predict change in

performance across the two sessions for either speed or accu-

racy, ps > .80. This is inconsistent with the alternative explana-

tion that people prone to experience glucose drops during the

Stroop task also are prone to improve their perfo rmance.

Discussion

The current study found that larger decreases in glucose

while performing the Stroop task predicted larger increases in

improvement in speed on the Stroop during a second experi-

mental session. Presumably, glucose is used during the Stroop

as participants learn how to perform the task, and larger

amounts of glucose consumption indicate greater learning.

These findings are consistent with past work on glucose and

metabolism. Glucose enables learning (e.g., among children,

during memory tasks) and growth (e.g., in cancer cells), and is

strongly linked to self-control. Glucose seems to enable effect-

tive learning of self-control, perhaps through neuronal growth

occurring while learning how to perform well on the Stroop.

The exertion of self-control in specific ways is difficult at

first but may eventually become automatized (Bargh, 1994;

Bargh & Chartrand, 1999). Initial attempts at quitting smoking,

for instance, can be effortful, but eventually people might learn

how to avoid smoking automatically. If so, then the initial de-

velopment of self-cont rol is metabolically costly but eventually

might require less metabolic energy as the capacity is auto ma-

tized.

It is plausible that people perform worse on self-control tasks

after having usedself-control (Baumeister et al., 2007; Gailliot,

2009; Muraven & Baumeister, 2000) because glucose decreases

(Fairclough & Houston, 2004; Gailliot et al., 2007) during the

initial self-control task as people learn how to perform it. Hence,

self-control impairments after initial self-control might be con-

sidered as harmful aftereffects of learning. People who learn the

self-control task most effectively might be the most likely to

show impairments on later self-control tasks. Learning self-

control could be metabolically expensive because neurons must

not only fire as usual but also expand and grow.

Studies on the aftereffects of using self-control have found

little or no evidence that general individual differences moder-

ate the effects of self-control on later self-control. Individual

differences that have emerged tend to be related to the relevant

self-control domain, such as differences in eating restraint mod-

erating eating consumption (Kahan, Polivy, & Herman, 2003;

Vohs & Heatherton, 2000), the motivation to respond without

prejudice moderating effects of stereotype suppression (Gailliot,

Plant, Butz, & Baumeister, 2007), the temptation to drink alco-

hol moderating alcohol consumption.

(Muraven, Collins, & Nienhaus, 2002), sex drive moderating

sexual restraint (Gailliot & Baumeister, 2007), and attachment

style moderating interpersonal functioning (Vohs, Baumeister,

& Ciarocco, 2005). It is possible that individual differences in

learning might moderate self-control fatigue effects across dif-

ferent self-control domains, because learning tendencies can be

domain general.

Gailliot et al. (2007) found (but did not report) that decreases

in glucose predicted impaired performance on later self-control

tasks, though glucose levels more reliably predicted self-control

performance. Whereas past work thus highlights the link be-

tween glucose levels and self-control (Fairclough & Houston,

2004; Gailliot et al., 2007), the current work suggests that

changes in glucos e levels also are influential.

People with diabetes and other metabolic problems tend to

exhibit impaired self-control (e.g., DeWall et al., 2009; Gailliot

& Baumeister, 2007). Their problems with self-control could

stem from difficulties learning to effectively exert self-control.

They might form self-control intentions and act upon them

much like individuals without metabolic deficits, yet experience

problems with learning.

Metabolic energy use is limited (Kleiber, 1961), suggesting

that metabolites used for one processes can be diverted away

from others (Gailliot et al., 2009). If so, then metabolism used

988

M. T. GAILLIOT

for learning might at times be diverted from other processes and

viceversa. Immune functioning and reproduction are metaboli-

cally expensive, and so learning might exhibit metabolic trade-

offs with health and reproductive capacity, for instance.

Demands to learning self-control could be related to metabo-

lism in ways aside from causing decreases in glucose. For ex-

ample, increased demands to learn self-control might lead to

increased metabolic energy intake, such as gaining weight in

novel environments.

Future work could focus on why learning self-control might

be a metabolically costly part of self-control. Learning might

entail greater changes in neuronal firing and connections, thus

requiring more energy than more simple neuronal activity.

Change requires growth and the additional metabolites to sup-

port that growth, which is above the typical rate of glucose use

during that same amount of time.

Failures in self-control might stem from failures in learning

how to exert self-control as much as they stem from failures in

exerting self-control. People might lack the glucose needed to

effectively learn superior self-control strategies, for instance,

such as a dieter failing to learn how to walk away from the

kitchen and engage in an activity other than eating. The indi-

vidual might experience thoughts helpful toward successful

self-control, but without sufficient glucose, fail to learn from

them.

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