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Review Article
Chew the Pain Away: Oral Habits to Cope with Pain and
Stress and to Stimulate Cognition
Roxane Anthea Francesca Weijenberg1and Frank Lobbezoo2
1Department of Clinical Neuropsychology, VU University Amsterdam, Van der Boechorststraat 1, 1081 BT Amsterdam, Netherlands
2Department of Oral Kinesiology, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and
VU University Amsterdam, MOVE Research Institute Amsterdam, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, Netherlands
Correspondence should be addressed to Roxane Anthea Francesca Weijenberg; r.a.f.weijenberg@vu.nl
Received September ; Accepted December
Academic Editor: Jian-Hua Liu
Copyright © R. A. F. Weijenberg and F. Lobbezoo. is is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
e acute eects of chewing gum on cognitive performance, stress, and pain have been intensively studied in the last decade. e
results have been contradicting, and replication studies proved challenging. Here, we review some of the recent ndings of this
topic and explore possible explanations for these discrepancies by incorporating knowledge derived from studies into oral habits
and bruxism. Both stress and cerebral functional specialization (i.e., the involvement of specic brain structures in distinctive
cognitive processes) are hypothesized to play a major role in the underlying physiological mechanisms of the diverse eects of
chewing gum on cognition, stress, and pain.
1. Introduction
Mastication is essential for grinding our food into smaller
particles []. During chewing, saliva is added to the particles
to lubricate them and create a food bolus that can be
swallowed []. Recently, reports have been published in the
literature, stating that mastication might also serve other
purposes, such as countering negative eects of stress []or
aiding in cognitive function [,].Itcouldbearguedthatthe
positive eects of mastication are similar to those of physical
activity []. Although mastication is of course not the same
as an intensive workout, it does have properties similar to
exercise: it also increases heart rates [,] and cerebral blood
ow [–]. ere are also direct cardiovascular improve-
ments resulting from exercise []. Physical activity is a key
element of an enriched environment, so while exercising, one
is also experiencing an enriched environment [], which
has been shown to improve cognition []. Exercising can
also attenuate the negative eects of stress []. Several
studies have explored the relationship between mastication,
cognition, stress, and pain. ese studies will be discussed
in more detail below. e aim of this review was to explore
the relationship between mastication, cognition, and pain
and to hypothesize on possible explanations.
2. Oral Habits
2.1. Gum Chewing. In two recent reviews [,], the eects
of chewing a piece of gum on cognition and stress in human
volunteers are described. e outcomes of these reviews
are summarized in Tabl e . Although the papers have great
overlap in the literature they included, the authors sometimes
come to dierent conclusions. For example, while, in one
paper,theauthorsemphasizethatchewinggumcannotbe
seen as an aid for mental challenges [], in the other paper,
the authors conclude that chewing gum enhances alertness
and that it might very well improve cognitive performance
[]. is discrepancy can partly be explained by the obser-
vation that Allen and Smith [] are more lenient in their
conclusions, as they view the majority vote as convincing,
whereasTuchaandKoerts[] focus on the contradictions
and possible detrimental eects of chewing gum.
Nevertheless, both reviews agree that working memory is
positively aected by chewing a piece of gum. It is interesting
Hindawi Publishing Corporation
BioMed Research International
Volume 2015, Article ID 149431, 7 pages
http://dx.doi.org/10.1155/2015/149431
BioMed Research International
T : e outcomes of two reviews on the eects of mastication
on cognition and stress in healthy volunteers.
Vari a b l e
Allen and
Smith,
[]
Tucha and
Koerts,
[]
Cognitive outcomes
Academic performance +
Alertness
Subjective +
Attention
Divided 00
Selective ±±
Shiing ±
Sustained/vigilance ±±
Executive functioning 0
Memory
Context dependent 0±
Recall ±±
Recognition −
Working + +
Test p e r f o r manc e −
Speed + ±
Spatial skill ±
Stress related outcomes
Biomarkers (i.e., pupil
dilation, heart rate) ±
Acute, self-reported 0
Chronic, self-reported +
Salivary cortisol ±
e outcomes of two reviews on mastication, cognition, and stress. + = the
authors report a positive eect; −= the authors report a negative eect; =
the authors report no eect; ±= the authors report contradicting results in
the literature.
that both also agree in their conclusion that divided atten-
tion is unaected. Some earlier papers mentioned that the
distracting (novelty) eect of chewing gum while performing
a task might have inuenced test results, but the fact that
divided attention is not aected argues against this.
As there are still many unexplained and contradictory
ndings, a nal conclusion cannot yet be made on the acute
eects of chewing a piece of gum on cognitive performance
and stress. Underlying physiological mechanisms remain to
be identied, and the time on task might be of inuence,
for example, how long participants chewed and whether they
chewed only prior to examination or also during the test. It is
possible that chewing gum has a transient positive eect, but
only aer cessation of chewing, since chewing while on task
can have a negative eect [,]. It would be interesting to
seewhateectotheroralhabitshaveonstressandcognition.
2.2. Bruxism. In a sample of children (– years), with
attention decit hyperactivity disorder (ADHD), it was
observed that levels of oral habits such as nail or pencil
bitingandbruxismwereallelevated[]. Bruxism is “a
repetitive jaw-muscle activity characterized by clenching or
grinding of the teeth and/or by bracing or thrusting of the
mandible” []. e activity is involuntary and can occur
during sleep (sleep bruxism) or during waking moments
(awake bruxism) []. Nail biting is an oral habit that is com-
monlyseeninthegeneralpopulation,andtriggersforitare,
amongst others, anxiety, stress [], boredom, and frustration
[]. Anxiety, stress, boredom, and frustration are also the
most common triggers for other (pathological) body-focused
repetitive behaviors, such as skin picking and hair pulling
[]. e need that is being met by engaging in these self-
damaging behaviors is thought to be “relief from negative
aective states” [].
ose suering from their habits, for example, brux-
ers with complaints of pain or other temporomandibular
disorders (TMD), usually seek the help from their dentist
rather than a psychologist [].isisperhapsreected
in the current popular treatment options for bruxism: it
is typically treated with oral splints, which is reected in
a publication bias with regard to bruxism therapies [].
Behavioral therapies are the least popular therapies to be sci-
entically explored, while pharmaceutical approaches have
been gaining popularity []. Current therapies seem to
be focused on preventing damage, rather than nding and
treating the cause []. is is most unfortunate,of course, as
options for treatments are disregarded this way. Risk factors
for bruxism were dened as peripheral, such as malocclusion,
or central (pathophysiological or psychosocial) [,]. It
was concluded that peripheral causes are not likely to play
a signicant role in the etiology of bruxism [,–]. It
can be argued that bruxism is a disorder from the central
neurotransmitter system since the basal ganglia (part of
the extrapyramidal system) and the thalamic pathways are
implicated in the origin of bruxism, with a crucial role to play
by the neurotransmitter dopamine []. Experienced stress
plays a mediating role, at least in awake bruxism [], while
others argue that stress might even be the main cause for
bruxism [].
is latter theory is tting with the observations that
experienced daily life stress is related to daytime clenching
[] and that self-reported bruxers have higher anxiety levels
and more oen experience severe stress compared to healthy
controls [].
3. Mastication, Stress, and Pain
3.1. Mastication and Stress Relief. Chewing and clenching
have been implied as a way to relieve stress and provide
relaxation []. e chewing force needed to chew a piece of
gum correlated to the amount of salivary cortisol reduction
aer performing a stressful task []. In restrained rats, the
length of stress-induced bruxism activity correlated inversely
to physiological parameters of stress, such as blood cortisol
and adrenaline levels []. Whether this stress relieving eect
is robust over prolonged periods of time is not yet clear. A lon-
gitudinal study examined the eect of regular gum chewing
during days (leisurely chewing a piece of gum for at least
minutes, twice per day) in young adults []. Aer these two
BioMed Research International
weeks, chewing gum was associated with decreased scores
for anxiety, depression, fatigue, and confusion compared to a
control group. is benet of chewing was transient; however,
as aer weeks (i.e., two weeks aer stopping with the
intervention), there was no longer a dierence between the
groups []. Another experiment showed a transient cerebral
response to changes in the masticatory domain: using fMRI, it
was shown that aer being tted with a new dental prosthesis,
adaptive brain activation in the right and the le precentral
and postcentral gyrus occurred during oromotor tasks like
jaw clenching, but only in the rst three months, even though
the prosthesis was worn continuously [].
3.1.1. Salivary Cortisol. Some comments with regard to cor-
tisol assessments need to be made. ere are two types
of human cortisol that can be assessed: total cortisol and
free cortisol. e latter can be sampled in blood, urine,
or saliva []. Levine and colleagues []emphasizethat
despite current popularity of these “biomarkers of stress,”
the question remains unanswered whether these assessments
reect actual metabolic and cerebral functioning, namely,
activity of the hypothalamic-pituitary-adrenal (HPA) axis,
as the hypotheses on which these assessments rely are, in
fact, still hypotheses []. Salivary cortisol assessments can
provide some useful information, but the sampling technique
is prone to false results, for example, due to pH changes aer
eating or drinking or due to contamination by blood from
oral lesions []. Furthermore, only small correlations have
been found between salivary cortisol and plasma free cortisol
measurements (the latter being the gold standard) []. In
another comprehensive review discussing salivary cortisol, it
is shown that there is in fact little scientic support for the
popularly assumed correlation between psychological stress
and the endocrine response []. Hellhammer and colleagues
[] emphasize that although salivary cortisol can be used as
a biomarker for perceived stress, this can only be done with
great caution, and one must be aware that there will only
be a moderate association with perceived, or task-induced,
stress []. e salivary cortisol response is inuenced by
many factors, such as estrogens (gender, menstrual phase,
and oral contraceptives), certain drugs, presence of chronic
stress, long-term exercise [], and overall physical tness
[]. Sleep and the circadian rhythm also inuence the HPA
axis activity [,]andnallyonemustkeepinmindthat
cortisol levels also are not stable during the waking hours but
exhibit ultradian oscillations []. Clearly, reports relying on
salivarycortisolassessmentshavetobeviewedwithsome
caution.
3.2. Mastication and Pain. It is known that, in newborn
babies, rhythmic oral motions, such as during breastfeed-
ing or sucking on a pacier, but also sweet taste such as
that from breast milk, or glucose or sucrose solutions, are
nonpharmacological approaches for pain relief []. Building
on this knowledge, it was investigated whether sweet taste,
chewing gum, or a combination of both could relieve pain
in –-year-old children, undergoing venipuncture or vac-
cination []. e authors did not nd an overall eect of
theinterventiononpainresponses.Inboys,continuously
chewing unsweetened gum reduced pain scores and ratings of
unpleasantness. In girls, however, the opposite was observed:
chewing sweetened gum reduced pain scores, but chewing
unsweetened gum increased them []. It should be added
that since the control group was also given chewing gum,
prior to the procedure, it is possible that some carry-over
eect of this chewing took place. On the other hand, the chil-
dren chewed gum for only a brief period: the control group
chewed for minute, and the intervention group chewed for
minutes and during the short medical procedure. It is possible
that the overall time was too brief to evoke a strong response
to chewing.
In another experiment to investigate the eect of chewing
on pain and possible underlying neural mechanisms, partici-
pants were submitted to nociceptive exion reex (NFR) pro-
tocol []. e NFR protocol encompasses a painful electro-
cutaneous stimulation of the lower leg, aer which the muscle
activity in the upper leg on the same side is measured [].
Blood samples were taken and brain perfusion was assessed
with near-infrared spectroscopy measuring (de)oxygenated
hemoglobin []. e subjects chewed a piece of mint-
avored gum leisurely for minutes. Assessments (applying
the NFR and taking blood samples) were made at baseline,
immediately aer chewing and minutes aer chewing.
Gum chewing decreased the NFR both immediately aer and
minutes aer chewing []. Serotonergic blood levels were
increased aer chewing, and signicant cerebral perfusion
was increased in the ventral part of prefrontal cortex (PFC)
[]. e authors conclude that chewing a piece of gum
apparently has analgesic eects, with the PFC mediating
this eect through serotonergic neurons of the dorsal raphe
nucleus []. e same dorsal raphe nucleus is implicated
in the origin of disordered eating [], in a hypothesis that
by changing the eating behavior (e.g., adhering to a food-
restricted diet) one also changes the serotonergic pathways
betweenthePFCanddorsalraphenucleusandthusalters
mood. is theory seems to t with the other observations
that stimulation through chewing might have benecial
eects on aect.
In an animal experimental study, rats were fed a so diet
for days []. Subsequently, they were injected in one of the
paws with complete Freund’s adjuvant (CFA) to temporarily
increase their sensitivity to pain (hyperalgesia). In the fol-
lowing – days, they were fed hard food (intervention) or
continued on so food (control). en, a heat stimulus was
separately applied to both paws, and the reaction time (with-
drawal latency) was measured. e dierence in reaction
speed between the injected and the control paw was taken
as a measure for the induced hyperalgesia. Rats on the hard
diet showed less CFA-induced hyperalgesia. is protective
eect of hard food was gone aer injection with the opioid-
antagonist naloxone []. Hard food was also protective
against inammation, as was shown by a decrease in activity
in immunoreactive cells. Inhibiting sensory pathways, by
cutting the inferior alveolar nerve or removing the primary
somatosensory cortex, reduced but not completely reversed
this eect []. e authors conclude that a protective eect
from hard food might involve the opioid system, which is
aected by sensory pathways, but perhaps also by other
BioMed Research International
neural pathways, such as the brainstem reticular formation
[].
A coupling of the endogenous opioid system and the
stress response has been widely studied and is well established
[,]. For example, it was found that endogenous opioid
systems attenuate the stress response in pregnancy [].
Underlying mechanisms are being studied: the attenuating
eect of suppression of kappa-opioid receptors on the stress
response has been shown in a wide range of animal models
[], and endogenous opioids were found to negate the
detrimental eects of stress hormones by protecting the
endothelial function, a condition which is thought to underlie
cardiovascular disease []. e locus coeruleus is thought to
play a key role in the interaction of the opioid system and the
stress response [].
4. Discussion
e literature discussed above has shown that there currently
is an interest in the relationship between mastication, cogni-
tion, stress, and pain. e acute eects of chewing gum on
cognitive measures and biomarkers of stress, such as salivary
cortisol, have not yet generated univocal results. However,
a closer examination of cerebral functional specialization of
cognitive functions might provide insight into the ambiguous
results that are currently being reported.
4.1. Cerebral Functional Specialization. e subcortical basal
gangliaarehypothesizedtoplayaroleinbruxism[].
Interestingly,theyarealsoinvolvedincognitivefunctions,
such as the nondeclarative memory (also known as the pro-
cedural memory) []. Basal ganglia are also known to play a
role in habit learning, most notably through feedback based
learning (rewards and/or punishment) regardless of whether
this learning is implicit or explicit []. is concurs with the
nding that chewing a piece of gum can enhance working
memory [], which was assessed by having participants
performing a routine that had to become habituated, and the
nding that spatial memory (learning the route to an escape
platform) was impaired in mice by removing the upper molar
crowns [].
e working memory enhancement of chewing gum
would indicate a positive stimulus of chewing for the PFC
and basal ganglia [] and also for the medial temporal lobe
and hippocampus for longer tasks []. Interestingly, short-
term working memory function was not negatively impacted
by physiological stress, but it was observed that task diculty
(which could be considered psychological stress) negatively
inuenced performance []. Participants showed elevated
levels of activation in the PFC while maintaining their
performance levels, a cerebral response which is thought to
compensate for the distraction of the stressor []. If chewing
reduces stress, this would facilitate the compensatory process
and thus positively aect working memory performance.
Both the hippocampus and the PFC are known for their
sensitivity to stress [], and a reduction of stress due to
chewing would explain the positive eect of gum on the
behavioral outcomes.
Declarative memory function (which can be subdi-
vided into episodic/autobiographic and semantic memory)
is involved in recall and recognition tasks []. is type
of memory is localized throughout the brain, including the
medial and temporal lobes [], but key areas for recall
memory are the frontal and parietal lobes and the cerebellum,
while learning and storing new memories (i.e., reproduction
and recognition) involve the hippocampus and the parahip-
pocampal gyrus []. e fact that these kinds of memory
are not enhanced by chewing a piece of gum is therefore
perhapsnotsurprising,astheydonotrelyprimarilyon
the basal ganglia. Most of the studies included in the two
reviews discussed here [,] did not use giving feedback
(punishment or rewards) in their paradigm, and thus, they
did not activate the basal ganglia []. Recently, Bos et al.
showed that recall and recognition of newly learned words
were improved in a stress condition (i.e., putting the hand in
painfully cold water) [].isisalsottingwiththecurrent
observations of the eects of chewing gum: if chewing indeed
reduces stress, this would also negate any stress-induced
enhancement of the long-term memory.
e lack or even detrimental eect of chewing on vigi-
lance is most likely a negative eect on the vigilance network
and the ascending reticular activating system (ARAS) [].
e ARAS has also been suggested to play a role in sleep
bruxism: minutes prior to the onset of sleep bruxism, arousal
responses such as increased heart rate and muscle tone are
observed in bruxers, which are indicators of ARAS activation
[]. It might be possible that active mastication reverses the
same pathway and thus downregulates the ARAS, causing
the negative eect on vigilance. Others reported that sleep
bruxers do not perform better or worse than controls in
neuropsychological tests for vigilance or motor response [],
which is tting with the current observations.
5. Conclusion
Mastication, as other physical activities, can most likely
relieve (acute) stress and even pain. Bruxers and nail biters
might unknowingly draw upon this eect, in order to alleviate
their commonly reported anxiety. Active mastication might
improvesomemeasuresofcognitiveperformance,suchas
working memory [,] or subjective alertness []. Bruxers
have not been shown to display cognitive advantages over
nonbruxers, with regard to vigilance []. is is perhaps
not surprising, as chewing has not been shown to enhance
vigilance either and, in fact, negatively eects it in children
[]. It would be interesting to see if bruxers outperform
nonbruxers in other cognitive domains, such as spatial or
working memory. Treatment for bruxism typically shows a
dental focus, oering splints or other occlusal appliances.
Counselingbyapsychologistandaphysicaltherapistinorder
to learn relaxation can be complementary to this [].
Otherpopulationsthatmightbenetfromtheseinsights
are persons at risk for cognitive decline or mental insta-
bility, such as older persons suering from dementia and
psychiatric patients. ey might experience positive eects
from oral and dental care and eating a diet that consists of
hard, chewing-enhancing foods. Maintenance of masticatory
BioMed Research International
function should be endeavored for all clinical groups. How-
ever, the long-term use of chewing gum and engaging in other
habits should not be encouraged, as this increases the risk
for complaints of fatigue, tenderness, and even pain in the
musculoskeletal structures of the masticatory system [–
].
Highlights
(i) e heterogeneous eects of chewing gum on cogni-
tive performance can partially be explained by cere-
bral functional specialization and the involvement of
the basal ganglia.
(ii) Stress and relief of stress can play an important role
in the physiological mechanisms underlying these
eects.
(iii) Oral habits such as bruxism might draw upon the
same eects for stress relief.
(iv) Active chewing might relieve stress or pain, but long-
term engagement in oral habits increases the risk of
fatigue, pain, or temporomandibular disorders.
Conflict of Interests
e authors declare that there is no conict of interests
regarding the publication of this paper.
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