Neuroscience and Biobehavioral Reviews 37 (2013) 2445–2453
Contents lists available at ScienceDirect
Neuroscience and Biobehavioral Reviews
journal homepage: www.elsevier.com/locate/neubiorev
Review
Chocolate and the brain: Neurobiological impact of cocoa flavanols on
cognition and behavior
Alexander N. Sokolov a,∗ , Marina A. Pavlova b , Sibylle Klosterhalfen a , Paul Enck a
a
Department of Internal Medicine VI: Psychosomatic Medicine and Psychotherapy, Research Division, University of Tübingen Medical School, D 72076
Tübingen, Germany
b
Developmental Cognitive and Social Neuroscience Unit, Department of Pediatric Neurology and Developmental Medicine, Children’s Hospital, University
of Tübingen Medical School, D 72076 Tübingen, Germany
a r t i c l e
i n f o
Article history:
Received 18 February 2013
Received in revised form 17 June 2013
Accepted 18 June 2013
Keywords:
Cocoa flavanols
Chocolate
Antioxidant
Anti-inflammatory
Neurogenesis
Angiogenesis
Age- and disease-related decline
Neurocognition
Neuromodulation
Neuroprotection
a b s t r a c t
Cocoa products and chocolate have recently been recognized as a rich source of flavonoids, mainly flavanols, potent antioxidant and anti-inflammatory agents with established benefits for cardiovascular
health but largely unproven effects on neurocognition and behavior. In this review, we focus on neuromodulatory and neuroprotective actions of cocoa flavanols in humans. The absorbed flavonoids penetrate
and accumulate in the brain regions involved in learning and memory, especially the hippocampus. The
neurobiological actions of flavanols are believed to occur in two major ways: (i) via direct interactions
with cellular cascades yielding expression of neuroprotective and neuromodulatory proteins that promote neurogenesis, neuronal function and brain connectivity, and (ii) via blood-flow improvement and
angiogenesis in the brain and sensory systems. Protective effects of long-term flavanol consumption
on neurocognition and behavior, including age- and disease-related cognitive decline, were shown in
animal models of normal aging, dementia, and stroke. A few human observational and intervention studies appear to corroborate these findings. Evidence on more immediate action of cocoa flavanols remains
limited and inconclusive, but warrants further research. As an outline for future research on cocoa flavanol
impact on human cognition, mood, and behavior, we underscore combination of functional neuroimaging
with cognitive and behavioral measures of performance.
© 2013 Published by Elsevier Ltd.
Contents
1.
2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cocoa flavanols in the brain signaling cascades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Neuroprotective action of cocoa flavanols in aging and neurological disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Animal studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Human population studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Clinical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Neuromodulation of cognition, mood, learning, and memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Animal studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Human studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concluding remarks and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
∗ Corresponding author at: Department of Internal Medicine VI: Psychosomatic
Medicine and Psychotherapy, Research Division, University of Tübingen Medical
School, Osianderstr. 5, D 72076 Tübingen, Germany. Tel.: +49 7071 29 89118;
fax: +49 7071 29 4382.
E-mail address: alexander.sokolov@uni-tuebingen.de (A.N. Sokolov).
0149-7634/$ – see front matter © 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.neubiorev.2013.06.013
Cocoa products and especially, chocolate has taken a special
place in our daily life and culture. This food of the gods as tells its
Latin name Theobroma cacao given by the noted Swedish nosologist
Carl Linnaeus in 1753, has been ennobled in many countries around
the globe as a curative drug, a culinary delight, and even a currency
for commodity trading, retaining its appeal over the centuries. No
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other natural product but chocolate has ever been viewed as having
a positive effect on a wide variety of health conditions ranging from
intestinal and female complaints, fever, and cardiovascular diseases to promotion of strength before military and sexual conquests
(Wilson, 2010; Wolfe and Shazzie, 2005). Reports on chocolate’s
health benefits are dated back as far as Aztec and Maya medical
practice (e.g., Hurst et al., 2002) and ever since, anecdotal evidence
has been abundant on chocolate effects on health. Only by the end
of the 20th century, however, claims on supposed health benefits
of chocolate have increasingly drawn a scientific interest in cocoa
products and chocolate, which eventually resulted in an approval
by the European Food Safety Agency (EFSA, 2012) of a health claim
for dark chocolate with high flavanol content as to its impact on
“maintenance of normal endothelium-dependent vasodilation”.
Most studies so far have been conducted on the effects of
chocolate intake on the cardiovascular system, skin, cholesterol
concentrations, and the release of neurotransmitters anandamide
and serotonin, and on the health-related properties of highquality dark chocolate, containing the stimulants theobromine
and caffeine (Lamuela-Raventós et al., 2005; Katz et al., 2011).
Dark chocolate also comprises high concentrations of flavanols (a
flavonoid subgroup, mainly epicatechin; Whiting, 2001) known as
potent antioxidative agents. While some work has been done on
the influence of theobromine and caffeine on mood and cognition
(e.g., Smit and Rogers, 2000; Smit et al., 2004; Smit and Blackburn,
2005; Nehlig, 2010), the impact of cocoa flavanols on human cognitive and affective function, executive control and behavior has
yet to be determined. In accord with accumulating evidence for
enhancing effects of chocolate consumption on cognitive function,
Messerli (2012) reports, as an occasional note, a strong positive
correlation between chocolate intake per capita and the number of
Nobel laureates in various countries.
In contrast to potential effects on cognition and behavior,
evidence-based benefits of cocoa and chocolate consumption
for cardiovascular system are well established and include
endothelium-dependent vasodilation recently found to contribute
to normal blood flow (Engler et al., 2004; Hooper et al., 2012;
Kay et al., 2006; Grassi et al., 2005, 2008). Cardiovascular health
has been closely linked to cognitive performance (e.g., DeCarli,
2012). Animal studies have shown that the absorbed flavonoids
directly interact with a number of cellular and molecular targets in
the brain, exerting pronounced antioxidative effects and improving brain tissue and function in the regions mainly implicated
in learning, memory, and cognition (Andrés-Lacueva et al., 2005;
Passamonti et al., 2005; Vauzour et al., 2008). This suggests a potential neuromodulatory and neuroprotective role for cocoa flavanols
and their significance for cognitive and affective function, executive control and behavior. However, only few human studies so
far have specifically addressed neurobiological, cognitive, affective
and behavioral effects of flavanol-rich cocoa products. The present
review focuses on analysis of the existing evidence on potential
neuromodulatory and neuroprotective actions of cocoa flavanols
in humans. Our analysis highlights further ways in investigation of
the impact of flavanol-rich cocoa products on neurocognition and
behavior.
2. Cocoa flavanols in the brain signaling cascades
The flavanol monoisomers epicatechin and catechin are
the predominant flavonoid compounds in cocoa, with the
2-phenyl-3,4-dihydro-2H-chromen-3-ol as underlying skeleton.
These monomers represent the base molecules for concatenated
oligomers, the proanthocyanidins. Antioxidant properties of flavanols are chemically mediated through oxidation of two aromatic
hydroxyl groups to a quinone (Bors and Michel, 2002). In addition, flavanols foster antioxidant system through modulation of
enzymatic activity (Stevenson and Hurst, 2007; Mann et al., 2009).
Flavanols occur in high concentrations in beverages such as green
tea and red wine, fruits and berries (e.g., apple skin, grapes, pears,
blueberry), vegetables (tomatoes, soy, and olives), and, especially,
cocoa (Manach et al., 2004; Neveu et al., 2010; Scalbert et al.,
2011; Sies et al., 2012). The flavonoid contents in cocoa products and chocolate differ substantially depending on the cocoa
variety (in some beans, amounting up to 20%), geographic origin,
cultivating, agricultural and postharvest practices, and manufacturing (Wollgast and Anklam, 2000; Niemenak et al., 2006). In the
Dutch population, chocolate contributes to about 20% of the total
flavonoid intake in adults, with an even higher percentage in children (Arts et al., 1999). In American diet, chocolate represents the
third top source of antioxidants after fruits and vegetables (255,
233, and 104 mg/day, respectively; Vinson et al., 2006). Also in the
French adult population with the total daily dietary polyphenol
intake of 1.2 g, including 99 mg catechins, cocoa products account
for the third major source of epicatechin (17%) after green tea (28%)
and apples (24%; Pérez-Jiménez et al., 2011). Yet human and animal
studies on neuroprotective properties of flavonoids, especially in
preventing cognitive decline, have mostly examined plant-derived
substances other than cocoa flavanols (e.g., Macready et al., 2009).
Flavonoid-rich pine extracts such as Gingko biloba are reported to
delay the onset of memory loss, dementia, and Alzheimer’s disease
(Weinmann et al., 2010), but the evidence remains controversial
(Birks and Grimley Evans, 2009).
Animal studies show that flavanols and their metabolites can
cross the blood–brain barrier, inducing beneficial effects on the
brain tissue and function (angio- and neurogenesis, changes in neuron morphology) and stimulating widespread blood circulation in
the brain (Vauzour et al., 2008). The most common flavanol found in
cocoa, epicatechin (Whiting, 2001), is rapidly absorbed in humans
and is detectable in blood plasma already 30 min after intake. The
epicatechin levels peak 2–3 h after intake, exhibiting a strong positive correlation with the dose of ingested chocolate (Richelle et al.,
1999), and return to baseline by 6–8 h after consumption. The possibility of flavanols and metabolites to penetrate and accumulate
in the brain regions mainly related to learning and memory, suggests they may exert a direct positive impact on the brain, including
cognition and neuroprotection (Nehlig, 2013).
Neurobiological impact of flavanols on the brain, learning, memory, and cognition are believed to occur in two major ways (Fig. 1).
First, flavonoids can specifically interact within a number of cellular signaling pathways, primarily with mitogen-activated protein
(MAPK), extracellular-signal-regulated (ERK) and phosphoinositide 3-kinase (PI3-kinase/Akt) signaling cascades. These cascades
trigger gene expression and protein synthesis for maintaining
long-term potentiation (LTP) and establishing long-term memories (Kelleher et al., 2004). Flavonoids modulate the transcription
factors engaged in signal transduction through protein-kinase inhibition (Goyarzu et al., 2004), and promote the expression of brain
derived neurotrophic factor (BDNF) that is critical for neurogenesis, also in adult animals, synaptic growth and neuron survival,
especially in the learning- and memory-related brain regions such
as the hippocampus and subventricular zone (Kim et al., 2006;
Valente et al., 2009). Second, flavonoids facilitate production of
the signaling molecule nitric oxide, which inhibits the incidence
of atheromatous plaque adhesion molecules causing inflammation
(Gonzalez-Gallego et al., 2007), and importantly, improves vascular endothelial function by relaxing the smooth muscle tissue of
blood vessels (e.g., Heiss et al., 2003; Schroeter et al., 2006). In this
way, flavanol-rich cocoa can impose vasodilation in a nitric oxidedependent way both at the cardiovascular and peripheral levels.
This in turn results in enhanced cerebral blood flow and blood
perfusion throughout the central and peripheral nervous system
(Fisher et al., 2003, 2006; Hollenberg et al., 2009), affording better
A.N. Sokolov et al. / Neuroscience and Biobehavioral Reviews 37 (2013) 2445–2453
2447
flavonoids may effectively protect against neural and oxidative
damage eventually giving rise to neurological disease, cognitive and
functional decline.
Cocoa flavanols
3.1. Animal studies
MAPK, ERK, PI3 signaling
gene expression,
protein synthesis
expression of BDNF
neurogenesis, synapc
growth, neuron survival
nitric oxide synthesis
inflammaon
vascular endothelian
funcon, angiogenesis
vasodilaon (heart, CNS,
sensory systems)
LTP, memory formaon
oxygen, glucose supply
neurocognive funcons
neuroprotecon
Fig. 1. Schematic of putative neurobiological actions of cocoa flavanols; small upand downward inclined arrows show up and down regulation of function, respectively. The absorbed flavanols either directly interact with cellular cascades (left,
green), yielding expression of neuroprotective and neuromodulatory proteins that
promote neurogenesis, neuronal function and brain connectivity, or act by improving central and peripheral blood flow and angiogenesis in the brain and sensory
systems (right, yellow). Note the two action pathways may cross-talk, contributing to more than one function. For example, improved neurogenesis, synaptic
growth, and neuron survival may promote neuroprotection, while neurocognition
may benefit from increased oxygen and glucose supply due to vasodilation. LTP,
long-term potentiation; mitogen-activated protein (MAPK), extracellular-signalregulated (ERK), and phosphoinositide 3-kinase (PI3-kinase/Akt) cascades; BDNF,
brain derived neurotophic factor; CNS, central nervous system. (For interpretation
of the references to color in the figures throughout the article, please consult the
web version of this article.)
supply of oxygen and glucose to the neurons and removal of waste
metabolites in the brain and sensory systems (e.g., blood delivery
to the retina; Huber et al., 2006; Kalt et al., 2010). In addition, animal models indicate that cocoa flavonoid administration stimulates
angiogenesis in the hippocampus (van Praag et al., 2007).
In a nutshell, the multiple neurobiological actions of cocoa
flavonoids in enhancing cognitive function and behavior may be
attributed to the expression of neuroprotective and neuromodulatory proteins that increase the number and connectivity of neurons,
and improve neuronal function. The other class of putative mechanisms is related to the effects on vascular function that through
enhanced blood perfusion both in the brain and peripheral nervous system including sensory systems, may improve neuro- and
angiogenesis (Williams and Spencer, 2012), cognitive function, and
executive control. However, while the long-term actions of cocoa
flavanols in counteracting oxidative stress, neuroinflammation and
neurodegeneration appear to be better understood, the mechanisms of more immediate neuromodulatory effects on cognition
and behavior remain unclear.
3. Neuroprotective action of cocoa flavanols in aging and
neurological disease
A number of health conditions in typical and atypical aging
(Alzheimer’s disease, vascular dementia, Parkinson’s disease) and
acute neurological conditions such as stroke have been associated with disturbances of cerebral blood flow and oxidative stress
(McGeer and McGeer, 2003; Hirsch et al., 2005). Potent antioxidant action and endothelium-dependent vasodilation capacity of
Several animal models of normal and pathological aging examined the effects of high-flavanol cocoa on the onset of age-related
cognitive deficits. Rozan et al. (2007) assessed the preventive effects
of Acticoa powder, a cocoa polyphenolic extract, on free radical production by leucocytes in rats following heat exposure and
protective effects of the cocoa powder on subsequent cognitive performance. Either high-flavanol cocoa powder (22.9 mg/kg/day) or
vitamin E, as the antioxidant reference, was administered orally to
rats for 14 days prior to heat exposure. The day after heat exposure, free radical production in rats treated with cocoa powder or
vitamin E was significantly reduced compared to controls. Unlike
controls, cocoa powder and vitamin E-treated rats discriminated
between active and inactive levers in a light extinction paradigm.
Throughout testing, the treated rats also exhibited decreased
escape latencies for reaching the hidden platform in the Morris
water maze. The results suggest that the daily oral administration
of Acticoa powder or vitamin E counteract the overproduction of
free radicals, and thereby protect rats from cognitive impairments
caused by heat exposure.
Bisson et al. (2008) administered orally Acticoa powder to
rats (as they grew from 15 to 27 months of age) at the dose of
24 mg/kg/day. In aged rats, high-flavanol cocoa improved cognitive performance in light extinction and water maze paradigms,
increased lifespan and preserved high urinary free dopamine levels. Acticoa powder appears to retard age-related impairments,
including cognitive decline in normal aging and neurodegenerative
diseases.
Converging evidence comes from animal models of neurodegenerative diseases (such as Alzheimer’s disease), which make use
of polyphenol-enriched diets. Fernández-Fernández et al. (2012)
examined an ability of a diet rich in polyunsaturated fatty acids
and polyphenols from dry fruits and cocoa (called LMN diet, a
patented product known to induce hippocampal neurogenesis in
adult mice) for counteracting the age-related impairment and neuropathology in wild type and transgenic mice (Tg2576 genotype
with over-expression of the human APP gene carrying the Swedish
mutation, K670N:M671L), an animal model of Alzheimer’s disease.
At the age of 13 months, once the amyloid plaques (A␤) were
formed, both mice types received LMN diet for further five months.
In the last two months, they performed on a behavioral test battery. Overall, both typically aging wild and, to a greater degree,
Tg mice exhibited reduction in sensorimotor reflexes, exploratory
behavior in the hole board, activity in the elevated plus-maze,
ambulation in the home cage during the dark phase, and in spatial
learning in the Morris water maze. The diet did not impact the detrimental effects observed in sensorimotor reflexes, but did clearly
reverse the behavioral effects of both aging and Tg genotype. This
behavioral improvement correlated with a 70% increase in cellular
proliferation in the subventricular zone of the brain, rather than
with a decrease of amyloid plaques. In contrast, LMN diet administered at an age of 10 months (before the plaques occurred) led
to a decreasing tendency of soluble and fibrillar A␤ levels in the
hippocampus along with a decrease in A␤ plasma content, suggesting a putative role of the diet in delaying plaque formation.
This is the first evidence that LMN diet can prevent behavioral
deterioration caused by aging and Tg genotype, and delay the
A␤ plaque formation. The results also highlight the increasing
importance of polyphenols from dry fruits and cocoa as human
dietary supplements in amelioration of the cognitive and functional
impairment during aging and neurological disease. Arendash et al.
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(2007), however, administered a polyphenol-rich omega-3 fatty
acid diet of Greenland Eskimos to another type of Tg mice (another
animal model of Alzheimer’s disease), and did not find any diet
benefits, except for several behavioral measures such as open field
activity and maze entries. In both studies, Tg mice were similar,
with Arendash et al. (2007) using a second-generation cross of the
Tg2576 and the 6.2 lines that carried an additional mutation in the
presenilin gene 1, PS1. The distinct outcomes were likely due to the
difference in either diets or study protocols to which the mice were
exposed.
In an animal model of stroke, Shah et al. (2010) administered
orally mice with 5, 15, or 30 mg/kg epicatechin 90 min before
middle cerebral artery occlusion. Epicatechin-treated mice, compared to the control group, exhibited significantly smaller lesion
volumes and improved neurological scores. Mice that were posttreated with 30 mg/kg of epicatechin at 3.5 h after the occlusion
had also significantly smaller infarct volumes and improved neurological scores. Villarreal-Calderon et al. (2010) reported that a
treatment with dark chocolate prevents the inflammation of the
vagus nerve resulting from a 16-month exposure of mice to the
polluted air of Mexico City. Mice exposed to polluted air had a
significant imbalance in genes coding for antioxidant defenses,
apoptosis and neurodegeneration at the level of the dorsal vagal
complex and this imbalance was mitigated by chocolate administration.
3.2. Human population studies
Observational human population studies on neuroprotective
and neuromodulatory action of flavonoids, including high-flavanol
cocoa, have often been poorly controlled, whereas prospective longitudinal studies remain laborious and costly. To date, therefore,
evidence for beneficial neuroprotective and anti-inflammatory
effects of cocoa flavanols on cognitive and behavioral decline in
aging and neurological disease is rather limited. In a prospective
study of a large sample of men aged 69–89 years (Kalmijn et al.,
1997), risk of cognitive decline (assessed by the Mini-Mental State
Examination) tended to inversely relate to flavonoid intake, though
no association was found between the risk of cognitive decline
and vitamin C- or E-intake served as the antioxidant reference to
flavonoids. In a cross-sectional study of the elderly Norwegian population, Nurk et al. (2009) investigated the association between
cognitive performance and flavonoid intake from chocolate, wine,
and tea. Participants aged 70–74 years (n = 2031; 55% females) completed a comprehensive cognitive test battery consisting of the
Kendrick Object Learning Test, Trail Making Test (part A), versions
of the Digit Symbol Test, Block Design, Mini-Mental State Examination, and Controlled Oral Word Association Test. Habitual food
intake was assessed by a self-reported food frequency questionnaire. Chocolate, wine, or tea consumers yielded significantly better
mean test scores and lower prevalence of poor cognitive performance. Those consuming all three items had the best test scores
and the lowest risk for poor test performance. The association
between intake of the foods and cognition were dose-dependent,
with sharp improvements at intakes of ∼10 g/day chocolate and
∼75–100 ml/day wine, and a linear improvement for tea intake.
The effect was most pronounced for wine and modestly weaker
for chocolate intake. It appears that in the elderly, a diet high in
some flavonoid-rich foods is associated with better performance in
several cognitive abilities in a dose-dependent manner.
Participants of a prospective 10-year Personnes Agées Quid
(PAQUID) study (n = 1640; aged ≥65 years, free from dementia)
underwent testing of cognitive function at four consecutive time
points (Letenneur et al., 2007). At each visit, the cognitive functions
were assessed by test battery including Mini-Mental State Examination, Benton’s Visual Retention Test, and “Isaacs” Set. Flavonoid
intake from multiple food items including chocolate (assessed once
as the study begun) was associated with both better cognitive
performance at baseline and better evolution of performance over
time. The most positive evolution was found in participants in
the two highest quartiles of flavonoid intake compared to those
in the lowest quartile. After 10-year follow-up, participants with
the lowest flavonoid intake had lost on average 2.1 points on the
Mini-Mental State Examination, compared to the 1.2-point loss in
participants of the highest quartile. The study raises the possibility
of an association between dietary flavonoid intake and cognitive
aging.
3.3. Clinical studies
The known benefits of flavonoids for vascular health (Ried et al.,
2012) may represent a promising approach in treating cerebrovascular disorders and protecting cognitive and functional behavior
in the elderly. Age- and disease-related disturbances in cerebral
blood flow are thought to be commonly accompanied by cognitive
and behavioral decline. Dietary high-flavanol cocoa intake is associated with an increased cerebral blood flow velocity in the middle
cerebral artery, suggesting a promising role for high-flavanol cocoa
consumption in the treatment of cerebrovascular ischemic syndromes such as dementia and stroke. For instance, Sorond et al.
(2008) report that following two weeks of high-flavanol cocoa
intake, in 34 healthy elderly volunteers (aged 72 ± 6 years; 16
males), mean blood flow velocity in the middle cerebral artery measured by transcranial Doppler ultrasound significantly increases by
8% at one week and 10% at two weeks.
Potent antioxidative and anti-inflammatory properties of
flavonoids have been proposed to play a role in preventing mild
cognitive impairment, a precursor of dementia, and Alzheimer’s
disease. In Alzheimer’s disease, an increased production and accretion of A␤-peptides activates microglia, resulting in release of
inflammatory mediators that further enhance A␤ production, giving rise to neuronal dysfunction and cellular death. While ␤and ␥-secretase facilitate A␤ production, ␣-secretase inhibits it.
Recent work in cultured human neuroblastoma cells shows that
low concentrations of nitric oxide up-regulate the expression of ␣secretase and down-regulate that of ␤-secretase. This suggests the
cerebrovascular nitric oxide might inhibit A␤ production (McCarty,
2006; Pak et al., 2005). Cocoa flavanols, especially epicatechin, act
directly on the endothelium of brain vessels, stimulating activity of
the endothelial nitric oxide synthase that in turn induces vasodilation and improves cerebrovascular perfusion (Fisher et al., 2006;
Schroeter et al., 2006; Patel et al., 2008). So far, there does not
seem to be any proven association between intake of antioxidants
and vitamins and Alzheimer’s disease (see Luchsinger and Mayeux,
2004 for review), however, several studies did report a diminished
cerebral blood flow in dementia patients (Ruitenberg et al., 2005;
Nagahama et al., 2003). Cerebrovascular atrophy is also known to
lead to mild cognitive impairment and subsequently to Alzheimer’s
disease. It is therefore conceivable that beneficial properties of
cocoa flavanols may slow down the transition from mild cognitive impairment to Alzheimer’s disease (Nagahama et al., 2003).
Commenges et al. (2000) conducted a clinical trial with 1367 participants aged over 65 years, 66 from which developed dementia.
The relative risk of developing dementia adjusted for age for the two
highest consumptions of flavonoids was 0.55 (95% confidence interval, 0.34–0.90). After further adjustment for sex, education level,
weight and vitamin C intake, the relative risk decreased to 0.49 (95%
confidence interval, 0.26–0.92). Thus, it appears that antioxidant
flavonoid intake is inversely related to the risk of dementia. However, in this study flavonoids came mainly from fruits, vegetables,
wine and tea rather than cocoa.
A.N. Sokolov et al. / Neuroscience and Biobehavioral Reviews 37 (2013) 2445–2453
Most recently, Desideri et al. (2012) studied 90 elderly individuals with mild cognitive impairment (mean age, 71 years; 43 males)
who consumed once daily for eight weeks a drink containing either
∼990 mg (high), ∼520 mg (moderate), or ∼45 mg (low-flavanol)
of cocoa flavanols. Cognitive function was assessed by the MiniMental State Examination, Trail Making Test A and B, and verbal
fluency test. At the end of the follow-up period, Mini-Mental State
Examination scores were similar in the treatment groups. In the
high- and moderate-flavanol compared to the low-flavanol groups,
time required to complete both Trail Making Tests was significantly
lower. Verbal fluency score was significantly better in the highcompared to low-flavanol group. The high- and moderate-flavanol
groups also exhibited decreased insulin resistance, blood pressure,
and lipid peroxidation. This appears to be the first dietary intervention study to demonstrate the efficacy of regular consumption
of cocoa flavanols for improving cognitive function in the elderly
with mild cognitive impairment.
The ability of flavonoids to improve and maintain vascular
function offers a further possibility to investigate the relationship between cocoa flavanol intake and neuronal and functional
loss after stroke (for recent animal data, see, e.g., Shah et al.,
2010; Section 3.1). In the meta-analysis of three human studies comprising 114,009 participants, Buitrago-Lopez et al. (2011)
reported a 29% reduction of stroke risk in high chocolate consumers compared to low consumers. Buijsse et al. (2010) found
an even stronger inverse correlation between chocolate consumption and stroke risk than for myocardial infarction. Rautiainen et al.
(2012) examined the association between the total antioxidant
capacity (including fruits, vegetables, tea, coffee, and chocolate)
and stroke risk in women, aged 49 to 83 years, from the Swedish
Mammography cohort. The study included 5680 women with
and 31,035 women without a history of cardiovascular disease.
Diet was assessed with a self-reported food frequency questionnaire, and dietary total antioxidant capacity calculated using
oxygen radical absorbance capacity values. Stroke cases were
subdivided into cerebral infarctions, hemorrhagic and unspecified strokes. Using multivariate analyses with hazard ratios, the
dietary antioxidant capacity was found to be inversely associated
with total stroke in disease-free women (17% risk reduction) and
hemorrhagic stroke in women with disease history (45% risk reduction).
In Parkinson’s disease, abnormal action of the neuromodulator adenosine, which fails to suppress unwanted motor activity
in the basal ganglia via striatopallidal neurons, has been linked to
impaired motor function (Jankovic, 2008). Adenosine antagonists,
including caffeine and chocolate, have therefore been considered for ameliorating parkinsonian motor dysfunction. Parkinson’s
patients do report an increased chocolate consumption, independent of concomitant depressive symptoms (Wolz et al., 2009).
However, in an investigator-blinded, placebo-controlled, crossover
trial in 26 patients with moderate Parkinson’s disease (Wolz et al.,
2012), a single acute dose of dark chocolate failed to improve
motor function (assessed by Unified Parkinson’s Disease Rating
Scale motor score) over white flavanol-free white chocolate. The
outcome might likely be due to lacking patient blindfolding, the
dose of chocolate used, its flavanol content, and the time frame of
treatment.
To the most part, the neuroprotective potential described above
is attributable to cocoa and chocolate flavanols rather than other
ingredients such as caffeine that has been widely implicated in
counteracting age- and disease-related cognitive decline such as
in Alzheimer’s and Parkinson’s disease (Costa et al., 2010; Santos
et al., 2010). Unlike in coffee, tea and soft drinks, the caffeine (and
theobromine) concentration in chocolate and cocoa products is
much lower than that of flavanols to account for potential effects
of chocolate (Benton, 2004).
2449
4. Neuromodulation of cognition, mood, learning, and
memory
4.1. Animal studies
Flavonoids are believed to trigger expression of neuromodulatory proteins in the brain regions implicated in learning, memory,
and cognition, suggesting cocoa flavanols can exhibit immediate
and short-term action on neurocognition, mood, and behavior. Surprisingly, only a few animal studies have addressed these issues.
Mice treated with one of the major chocolate flavanols, epicatechin, at the dose of 500 ␮g/g (daily supply of 2.5 mg) showed
pronounced angiogenesis in the hippocampus (van Praag et al.,
2007). Epicatechin treatment combined with exercise (running
a wheel) improved retention of spatial memory and increased
dendritic spine density in the dentate gyrus of the hippocampus. Moreover, epicatechin treatment facilitated gene expression
associated with learning in the hippocampus but did not affect hippocampal adult neurogenesis. Yamada et al. (2009) compared the
effects of short-term versus long-term (two-week) oral administration of cocoa mass in large amounts (100 mg/100 g body weight) in
the rat elevated T-maze test, an animal model of anxiety. Shortterm administration significantly abolished avoidance behavior
during immediate test performance, suggesting a reduced fear conditioning. Long-term administration enhanced brain concentration
of emotion-related neurotransmitter serotonin and its turnover.
The findings indicate short-term cocoa intake shows an anxiolytic effect, whereas long-term intake affects brain monoamine
metabolism. This suggests flavanol impacts on the amygdala underpinning regulation of anxiety and encoding of affective valence (e.g.,
Morrison and Salzman, 2010). Flavanol action therefore may occur
in the brain regions outside the hippocampus and subventricular
zone, in which it has already been established.
4.2. Human studies
In humans, several studies have aimed to identify immediate
and short-term action of cocoa flavanols on mood and cognitive
performance with as yet inconclusive outcome. Crews et al. (2008)
had healthy older adults (n = 101; 41 males; age, ≥60 years) to
consume daily for 6 weeks either a 37-g bar of dark chocolate
and 8 ounces (237 ml) of artificially sweetened cocoa beverage or
similar placebo products. Participants underwent hematological,
blood pressure, and pulse rate measurements, and accomplished
several cognitive tests: Selective Reminding, Wechsler Memory
Scale-III Faces I and Faces II subtests, Trail Making Test, Stroop Test,
Wechsler Adult Intelligence Scale-III Digit Symbol-Coding subtest,
and General Activation subscale of the Activation-Deactivation
Adjective Check List (A-DACL). The only effect observed was a
significantly higher, compared to the placebo group, pulse rate,
with no effects found on blood pressure, hematological, and cognitive variables. In a randomized, double-blind, controlled, balanced,
three period crossover study, 30 healthy young adults (mean
age, 22 years; 13 males) consumed high-flavanol (520 mg and
994 mg) cocoa drinks and a matched control drink, with a three-day
washout between drinks (Scholey et al., 2010). Over a 1 h testing
period, participants repeatedly performed 10-min cycles of a Cognitive Demand Battery (two serial subtraction tasks, Serial Threes and
Serial Sevens), a Rapid Visual Information Processing (RVIP) task,
and a “mental fatigue” scale. High-flavanol cocoa intake improved
Serial Threes performance. The 994-mg beverage yielded speeded
RVIP responses, but also more errors during Serial Sevens. Only the
520-mg beverage attenuated self-reported “mental fatigue”. This
is the first report on immediate improvements of cognitive function following high-flavanol cocoa consumption in healthy young
adults.
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A.N. Sokolov et al. / Neuroscience and Biobehavioral Reviews 37 (2013) 2445–2453
In healthy young adults (n = 30, eight males; age range, 18–25
years) who in a crossover, order counterbalanced design consumed
once either 35 g dark chocolate (with 720 mg of high-flavanol
cocoa) or a matched quantity of flavanol-free white chocolate,
high-flavanol cocoa improved visual contrast sensitivity (assessed
by reading numbers varying in their luminance relative to background), working memory for location, choice reaction time, and
the time required to detect direction of coherent motion (Field
et al., 2011). This outcome extends the range of cognitive functions
affected by high-flavanol cocoa consumption and provides the first
evidence on the immediate effects of high-flavanol cocoa on visual
functions.
In 72 healthy participants aged 40–65 years, Pase et al. (2013)
examined immediate and sub-chronic effects of cocoa flavanols
on mood and cognition. Three separate groups were assigned an
Acticoa dark chocolate drink mix containing 500, 250, or 0 mg
(placebo) of polyphenols once daily for 30 days. At baseline, at
1, 2.5, and 4 h after a single acute dose, and again after 30-day
treatment, performance on cognitive tasks (Simple and Choice
Reaction Time, Digit Vigilance, Tracking, Spatial and Numeric
Working Memory, Immediate and Delayed Word Recall, Delayed
Word and Picture Recognition) was assessed with the Cognitive
Drug Research battery (Kennedy et al., 2002) and self-reported
mood with the Bond-Lader Visual Analogue Scale (Bond and Lader,
1974). While mood remained unaffected by the acute treatment,
no effects on cognition occurred at all time points. At 30 days, the
high-dose treatment significantly improved self-rated calmness
and contentedness compared to placebo. This is the first evidence
on the effects of cocoa polyphenols on mood in healthy participants. The outcome suggests a possibility for cocoa polyphenols
for ameliorating the symptoms associated with clinical anxiety and
depression. Although some recent studies do indicate a relationship between chocolate consumption and depressive symptoms
(e.g., Rose et al., 2010), the nature of the relationship remains to
be clarified.
Latest developments in modern non-invasive neuroimaging techniques such as functional magnetic resonance imaging
(fMRI), electroencephalography, and magnetoencephalography
(EEG/MEG), have made it possible to explore modulatory effects
of food constituents on neural activity in the human brain and its
association with behavior. Yet, with the exception of some work
done on the modulation of brain reward system by chocolate intake
(e.g., Small et al., 2001; Stice et al., 2008; Valentin et al., 2007), only
a few neuroimaging studies have examined the neural networks
selectively activated by cocoa flavanols.
Francis et al. (2006) used a paired letter-digit task in healthy
young female participants (n = 16; age range, 18–30 years)
while recording the blood oxygenation level-dependent (BOLD)
responses in an fMRI protocol following 5-day ingestion of 150 mg
of cocoa flavanols. Participants had to press a key, making either one
judgment in the “no switch” condition (“is the letter a vowel or consonant”, “is the digit even or odd”) or two judgments in the “switch”
condition (responding both to the letter and digit). Although an
overall BOLD signal increased during the cognitive task, no effects
were found in response time, switch cost (the difference in response
time between “switch” and “no-switch” conditions), and heart rate
after consumption of this moderate dose of cocoa flavanols. Ingestion of a single acute dose (450 mg of cocoa flavanols) yielded
an increased cerebral blood flow (Fig. 2), confirming a potential
of cocoa flavanols for treatment of vascular impairment such as
dementia and stroke (see Section 3).
Over a 30-day period, Camfield et al. (2012) administered a daily
chocolate drink (250 mg or 500 mg cocoa flavanols versus lowflavanol cocoa as placebo) to 63 volunteers (aged 40–65 years).
Neurocognitive changes associated with flavanol supplementation
during performance of a spatial working memory task at baseline
Fig. 2. Increased blood flow response (±SEM, standard error of mean) in the gray
matter of four female participants following ingestion of an acute dose of high
compared to low flavanol drink.
From Francis et al., 2006. The effect of flavanol-rich cocoa on the fMRI response to a
cognitive task in healthy young people. Journal of Cardiovascular Pharmacology 47,
Suppl. 2, S215–20. Copyright © 2006 Lippincott Williams & Wilkins with permission
of Wolters Kluwer Health/Lippincott Williams & Wilkins.
and at the end of the treatment were assessed using EEG (steady
state visually evoked potentials, SSVP). Behavioral measures of
accuracy and response time did not differ in a dose-dependent
manner, whereas average amplitude and phase of the evoked
potentials at a number of posterior parietal and centro-frontal sites
significantly differed between the groups during memory encoding,
the memory hold period, and retrieval (Fig. 3). The authors assume
the differences in brain activation, even in the absence of behavioral
effects, point to an increased neural efficiency in spatial working
memory associated with chronic cocoa flavanol consumption.
Table 1 summarizes the results of research on the neuroprotective action of flavanols in aging and neurological diseases on the
one hand, and the neuromodulatory effects on cognition, mood,
learning, and memory on the other hand. As seen from the table,
basic animal intervention studies on mechanisms of action of flavanols still outnumber human and clinical studies demonstrating
that these effects are of physiological and clinical relevance.
In summary, recent research has yielded encouraging albeit as
yet inconclusive outcome. For exploring possible effects of highflavanol cocoa on human behavior, cognitive and brain functions,
standardized psychometric tasks and neuroimaging protocols are
required along with administering proper high-flavanol cocoa
dosages. To date, there is still limited evidence for high-flavanol
cocoa effects on human cognitive and affective function, and behavior. Moreover, it remains unclear how high-flavanol cocoa modulates the brain networks underlying neural cognitive processing.
5. Concluding remarks and future directions
Flavonoids, potent antioxidant and anti-inflammatory agents,
represent up to 20% of compounds found in cocoa beans. Flavanols, and especially epicatechin, are the most common cocoa
flavonoids. The flavanol contents in cocoa products and chocolate
vary greatly depending on the bean variety and origin, agricultural and processing practices. In part, the variability of flavanol
contents in cocoa and chocolate may be responsible for the mixed
outcomes presently observed in research on the effects of cocoa
flavanols on neurocognitive and affective function, executive control, and behavior. This might be due to the fact that to date, only
few – especially, human – intervention studies have directly examined exposure to cocoa flavonoids, while the most data comes from
A.N. Sokolov et al. / Neuroscience and Biobehavioral Reviews 37 (2013) 2445–2453
2451
Fig. 3. Topographic differences in the average steady state visually evoked potentials SSVEP amplitudes at baseline and retest for a spatial working memory task (encoding,
hold interval, and retrieval) in the low, medium and high cocoa flavanol (CF) groups. Warm and cool colors show SSVP amplitude decreases and increases, respectively,
post-treatment compared to baseline. (For interpretation of the references to color in the figures throughout the article, please consult the web version of this article.)
From Camfield et al., 2012. Steady state visually evoked potential (SSVEP) topography changes associated with cocoa flavanol consumption. Physiology and Behavior 105,
948–57. Copyright © 2011 Elsevier Inc. with permission of Elsevier Ltd.
Table 1
Cocoa flavonoid effects on the brain, cognition, and behavior – overview.
Brain region/function
Animal models
Human studies
15
8, 9, 10, 19, 24, 25
Hippocampus
Neurogenesis
Improved neuronal connectivity
Angiogenesis
15, 26
27
27
28
Subventricular zone of the brain
Neurogenesis
6, 26
Whole brain
Improved cerebral blood flow (general
improvement of neural function due
to enhanced oxygen and glucose
delivery)
Vision
Improved blood delivery to the retina
Rapid visual processing
Visual contrast sensitivity
Coherent motion direction
14
Performance
Choice reaction time
7
Cognition, learning, and memory; mood
General
22
Mental calculation
Working memory for location
29
Fear conditioning (anxiety)
Calmness, contentedness
Aging
Reduced cognitive decline
Neurological disease
Alzheimer (reduced cognitive decline)
Stroke (reduced risk)
11
23
7
7
1, 6
16
23
7
20
13, 16, 18, 25
4, 5
2, 3, 12, 17, 21
Key to references: 1: Bisson et al. (2008), 2: Buijsse et al. (2010), 3: Buitrago-Lopez
et al. (2011), 4: Commenges et al. (2000), 5: Desideri et al. (2012), 6: FernándezFernández et al. (2012), 7: Field et al. (2011), 8: Fisher et al. (2003), 9: Fisher et al.
(2006), 10: Francis et al. (2006), 11: Huber et al. (2006), 12: Janszky et al. (2009), 13:
Kalmijn et al. (1997), 14: Kalt et al. (2010), 15: Kim et al. (2006), 16: Letenneur et al.
(2007), 17: Mink et al. (2007), 18: Nurk et al. (2009), 19: Patel et al. (2008), 20: Pase
et al. (2013), 21: Rautiainen et al. (2012), 22: Rozan et al. (2007), 23: Scholey et al.
(2010), 24: Schroeter et al. (2006), 25: Sorond et al. (2008), 26: Valente et al. (2009),
27: van Praag et al. (2007), 28: Vauzour et al. (2008), 29: Yamada et al. (2009).
research using dark chocolate as flavanol supply. Despite multiplicity of flavanol effects in the brain, neurobiological actions of
flavanols are believed to occur in two major ways: (i) via direct
interactions with cellular and molecular signaling cascades, especially in the brain regions dedicated to learning and memory,
and (ii) via central and peripheral blood-flow improvement and
angiogenesis, including the brain and sensory systems. Overall, evidence on the persistence of neuroprotective and neuromodulatory
actions of cocoa flavanols, albeit as yet limited and inconclusive,
suggests that cocoa flavanols may exert both long lasting and
immediate effects on neurocognition and behavior. The immediate effects can be attained with a single acute or subchronic (for
several weeks) administration of cocoa flavanols in appropriate
dosages. The lasting effects likely require chronic intake of flavanolrich cocoa products over an extended time frame. In both instances,
more controlled follow-up studies should be in place to determine
the effects’ duration.
While long lasting neuroprotective properties of flavonoid
intake for neurodegenerative and neuroinflammatory diseases of
the nervous system have been relatively well documented (Katz
et al., 2011; although more research specifically on cocoa flavanols
and greater, e.g. population-based, participant samples is required),
more immediate action on cognitive and affective function, executive control, and behavior remains largely unknown, as are sex
differences in both long-term and immediate actions (see Sokolov
et al., 2013 for a companion review). Sex-related variability in data
is likely to account for the lack of significant effects of cocoa flavanols on the brain responses and cognitive performance observed
in some of the studies. In females, distinct phases of the menstrual
cycle and the perimenopause may also add to data variability.
The main issues to resolve in the studies to come include (i)
blindfolding both the experimenter and participants, which is
important for reducing the well-known reward value of chocolate
intake, (ii) determining an appropriate (immediate vs. short-term
vs. long-term) time frame and dose of flavanol administration, (iii)
choice of simple tests of cognitive, executive and affective function
and performance that possess sufficient sensitivity and specificity
(Macready et al., 2009), and (iv) for examining neuroprotective
properties of flavanol consumption, the onset and duration of
consumption. Another important issue to consider is a so called
2452
A.N. Sokolov et al. / Neuroscience and Biobehavioral Reviews 37 (2013) 2445–2453
food matrix, or food composition, in which cocoa flavonoids appear
in food (e.g., Lamuela-Raventós et al., 2005; Manach et al., 2004).
For example, ingestion of 100 g dark chocolate along with 200 ml
milk results in a substantial reduction of both total antioxidant
capacity and (-)epicatechin content of human plasma, compared
to ingestion of 100 g pure dark chocolate, and the reduction is even
greater after ingestion of 200 g milk chocolate (Serafini et al., 2003).
Finally, other constituents of cocoa and plants rich in flavanols,
such as their relatively high content of tryptophan, a precursor of
neurotransmitter serotonin (e.g., Bertazzo et al., 2011), may either
contribute to the neurobiological effects of flavanols or, depending
on the constituents’ bioavailability (e.g., due to the food matrix;
Smit, 2011), exert different actions through different pathways.
Forthcoming studies should explore whether these actions are
complementary, antagonistic or synergistic.
In conclusion, future research has to combine functional
neuroimaging techniques such as fMRI, EEG and MEG with neurocognitive and behavioral correlates to uncover long lasting and
immediate effects of chocolate consumption on human cognition,
mood, and behavior.
Acknowledgements
We thank Otmar and Edelgard Knoll and the Willy Robert Pitzer
Foundation for support. MAP was supported by the Else Kröner Fresenius Foundation (Grants P2010 92 and P2013 127), the Reinhold
Beitlich Foundation, the Berthold Leibinger Foundation, and by the
Heidehof Foundation.
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