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Effects of walnut consumption on cognitive performance in young adults

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Walnuts contain a number of potentially neuroprotective compounds like vitamin E, folate, melatonin, several antioxidative polyphenols and significant amounts of n-3 α-linolenic fatty acid. The present study sought to determine the effect of walnuts on verbal and non-verbal reasoning, memory and mood. A total of sixty-four college students were randomly assigned to two treatment sequences in a crossover fashion: walnuts-placebo or placebo-walnuts. Baseline data were collected for non-verbal reasoning, verbal reasoning, memory and mood states. Data were collected again after 8 weeks of intervention. After 6 weeks of washout, the intervention groups followed the diets in reverse order. Data were collected once more at the end of the 8-week intervention period. No significant increases were detected for mood, non-verbal reasoning or memory on the walnut-supplemented diet. However, inferential verbal reasoning increased significantly by 11·2 %, indicating a medium effect size (P = 0·009; d = 0·567). In young, healthy, normal adults, walnuts do not appear to improve memory, mood or non-verbal reasoning abilities. However, walnuts may have the ability to increase inferential reasoning.
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Effects of walnut consumption on cognitive performance in young adults
Peter Pribis
1
*, Rudolph N. Bailey
2
, Andrew A. Russell
2
, Marcia A. Kilsby
3
, Magaly Hernandez
1
,
Winston J. Craig
1
, Tevni Grajales
2
, David J. Shavlik
4
and Joan Sabate
`
5
1
Department of Nutrition and Wellness, Andrews University, 8475 University Boulevard, Marsh Hall 313, Berrien Springs, MI
49104-0210, USA
2
Department of Educational and Counseling Psychology, Andrews University, 4195 Administration Drive, Bell Hall 159,
Berrien Springs, MI 49104, USA
3
Department of Clinical and Laboratory Sciences, Andrews University, 4270 Administration Drive, Halenz Hall 327, Berrien
Springs, MI 49104, USA
4
Department of Epidemiology and Biostatistics, School of Public Health, Loma Linda University, 11234 Anderson Street,
Nichol Hall 2005, Loma Linda, CA 92354, USA
5
Department of Nutrition, School of Public Health, Loma Linda University, 11234 Anderson Street, Nichol Hall 1102, Loma
Linda, CA 92354, USA
(Received 18 January 2011 – Revised 28 June 2011 – Accepted 5 July 2011)
Abstract
Walnuts contain a number of potentially neuroprotective compounds like vitamin E, folate, melatonin, several antioxidative polyphenols
and significant amounts of n-3 a-linolenic fatty acid. The present study sought to determine the effect of walnuts on verbal and non-verbal
reasoning, memory and mood. A total of sixty-four college students were randomly assigned to two treatment sequences in a crossover
fashion: walnutsplacebo or placebo walnuts. Baseline data were collected for non-verbal reasoning, verbal reasoning, memory and
mood states. Data were collected again after 8 weeks of intervention. After 6 weeks of washout, the intervention groups followed the
diets in reverse order. Data were collected once more at the end of the 8-week intervention period. No significant increases were detected
for mood, non-verbal reasoning or memory on the walnut-supplemented diet. However, inferential verbal reasoning increased significantly
by 11·2 %, indicating a medium effect size (P¼0·009; d¼0·567). In young, healthy, normal adults, walnuts do not appear to improve
memory, mood or non-verbal reasoning abilities. However, walnuts may have the ability to increase inferential reasoning.
Key words: Walnuts: Intelligence: Memory: Mood: Inference
In the last 20 years, nutritional neuroscience has emerged as a
recognised discipline with the potential to make significant
contributions to our understanding of the relationship
between nutrition and cognitive functions
(1)
. Some research
has been conducted on animals and humans indicating that
different neural systems might be affected by dietary manipu-
lations including fruits, vegetables, nuts and fish
(2 – 6)
.
For the central nervous system to function well, it needs suf-
ficient amounts of water, vitamins such as folate, thiamine,
vitamins B
6
and B
12
,a-lipoic acid, lutein and n-3 fatty acids
(FA)
(7,8)
. Walnuts contain a number of potentially neuropro-
tective compounds like vitamin E, folate, melatonin, numerous
polyphenolics and significant amounts of n-3 a-linolenic acid
(ALA; 18 : 3n-3)
(9 – 12)
. ALA is the precursor for EPA (20 : 5n-3)
and DHA (22 : 6n-3). DHA is important for neuronal mem-
brane stability, neuroplasticity, synaptic plasticity, gene
expression, cell migration and apoptosis. It increases the
speed of signal transduction, neurotransmission and increases
serotonin and dopamine concentrations. EPA regulates the
synthesis of arachidonic acid (20 : 4n-6) and modulates
important inflammatory and immune functions
(13 – 15)
. The
types of FA that are available for the composition of cell
membranes and subsequent cognitive processing depend
upon diet.
Cognition can be defined narrowly by mental processes like
perception, recognition, reasoning, deduction and intuition,
and broadly by aggregated concepts like intelligence,
memory or affect. Most of the research on the relationship
between nutrition and cognitive functions has been done
using animals or pathological populations
(2 – 6,16)
. As far as
*Corresponding author: Dr P. Pribis, fax þ1 269 471 3485, email pribis@andrews.edu
Abbreviations: ALA, a-linolenic acid; APM, Advanced Progressive Matrices; FA, fatty acid; POMS, Profile of Mood States; WGCTA, Watson-Glaser Critical
Thinking Appraisal; WMS-III, Wechsler Memory Scale-Third Edition.
British Journal of Nutrition, page 1 of 9 doi:10.1017/S0007114511004302
qThe Authors 2011
British Journal of Nutrition
the authors know, the effect of walnut consumption on
cognition has never been studied in healthy volunteers.
Thus, the present study was designed to investigate the
effect of dietary walnut supplementation on cognition and
mood in college-age students, using two different intelligence
tests a memory test and a mood test.
Methods
Subjects
Following an advertisement campaign on the campus of
Andrews University (Berrien Springs, MI, USA), 273 subjects
responded with interest to participate in the study. Subjects
were disqualified if they were not students, not between the
ages of 18 and 25 years, indicated that they would not be
present on the campus the whole school year 2009/2010,
did not complete a comprehensive lifestyle survey and
a short FFQ to assess n-3 FA intake
(17)
, had known food
allergies or if they did not participate in a comprehensive
screening interview (Fig. 1).
Experimental design
We have used a double-blind, randomised, placebo-
controlled, crossover design (Fig. 2)
(18)
. The sixty-four
selected subjects were assigned using random allocation
software
(19)
to one of the two study groups – walnuts pla-
cebo and placebowalnuts (each of which contained
thirty-two subjects). Participants in one experimental group
consumed the test meal (banana bread with walnuts) for the
first 8 weeks followed by 6 weeks of washout and another
8 weeks of placebo diet (banana bread without walnuts);
the other experimental group followed the diets in reverse
order. The participants were asked to maintain their usual
activities and other lifestyle habits throughout the duration
of the study. The study personnel performing measurements
and analyses were blinded to the subjects’ diet sequence.
This study was conducted according to the guidelines laid
down in the Declaration of Helsinki and all procedures invol-
ving human subjects were approved by the Institutional
Review Board of Andrews University. Written informed con-
sent was obtained from all subjects. Subjects were offered a
$200 honorarium for their participation.
Dietary intervention
Participants were asked to consume three slices of banana
bread every day for 16 weeks (8 weeks banana bread with
walnuts and 8 weeks banana bread without walnuts). For
the dietary supplementation, we used English walnuts
(Juglans regia L.). The two banana breads (walnut and pla-
cebo) were baked at the university’s kitchen and had very
similar appearance and taste. For the banana bread with wal-
nuts, fresh, finely ground walnuts were mixed into the dough
and baked at 1778C (3508F) for 1·5 h. Three slices of banana
bread with walnuts contained two servings (1/2 cup or 60 g)
of ground walnuts (6·80 g of ALA). The daily ration of
banana bread without walnuts contained only 0·56 g of n-3
FA coming mainly from margarine.
Once per week, the subjects visited the Nutrition and Well-
ness Department and received fresh supplies (three loaves
twenty-one slices) of banana bread for the upcoming week.
Participants were asked to refrigerate the banana bread to pre-
vent spoilage. During the visit, each subject’s body weight was
measured (without shoes and heavy clothing).
Each week, a sample slice of the banana bread batches
(walnut and placebo) was collected and kept frozen until
the end of the study. Mixed samples were analysed for
levels of macronutrients and FA (Covance Laboratories, Madi-
son, WI, USA). The composition of each of the banana breads
is presented in Table 1.
Adherence to dietary intervention
Adherence was measured by self-report and biological indi-
cators. At the weekly clinics, subjects reported in a form
how many slices of the banana bread they consumed daily
and at what times during the day. Fasting whole blood was
collected at the end of each dietary intervention period.
Plasma was separated and stored at 2808C for later analysis.
At the end of the study, linoleic acid (LA; 18 :2n6) and
ALA were measured as FA methyl esters through the use of
GC (Columbia Analytical Services, Kelso, WA, USA)
(20,21)
.
Cognitive testing
The cognitive and mood functioning of the students were
tested at baseline and at the end of each of the two 8-week
treatment periods. One university academic semester is 15
weeks long. Each intervention was designed to run between
weeks two and ten of the university semester calendar to
avoid the stress associated with the beginning of a new seme-
ster or final exams, thus providing a stable environment for
the cognitive testing. Students who tested first at the baseline
were also tested first and at the same time of the day at the
end of each intervention period, to ensure that all subjects
had similar testing conditions.
Three different instruments were used to assess memory,
non-verbal reasoning and verbal reasoning. Raven’s Advanced
Progressive Matrices (APM)
(22)
was administered without
time restrictions, and was used to measure general intellectual
capacity, and specifically, the ability to solve complex non-
verbal problems. Raven’s APM is composed of Set I and
Set II; Set I was given at the first testing point as a practice
test, but only the data from Set II were included in the
analysis. The reliability and validity of the test have been
established across a range of populations, including African
Americans and other non-Whites
(23)
.
The Watson-Glaser Critical Thinking Appraisal (WGCTA)
was used to assess students’ verbal reasoning faculties, which
incorporate the attitude of enquiry, the need for evidence
to support conclusions, knowledge of the rules of argument,
and skills in integrating these components. The WGCTA
is designed to measure different, though interdependent,
aspects of critical thinking through its subtests: inference,
P. Pribis et al.2
British Journal of Nutrition
Fig. 1. Participant flow diagram.
273 participants applied online
128 participants completed Lifestyle
survey + FFQ
Reason for disqualification Number
Not students
Not between the ages 18 and 25
Not being present on campus in 09/10
Did not complete Lifestyle survey + FFQ
Had food allergies
Disqualified participants
15
9
12
98
11
145
Sixty-four randomised
Thirty-two participants constituted the
Walnut – placebo group
Thirty-two participants constituted the
Placebo – walnut group
Twenty-three participants completed
the study
Six participants withdrew from the study
Three owing to lack of interest during first
testing period
Three contracted H1N1 flu, unable to eat the
banana bread
Two transferred to different college
during washout, but provided data at 8 weeks
One dropped for non-compliance, but provided
data at 8 weeks
Twenty-four participants completed
the study
Eight participants withdrew from the study
Three owing to lack of interest during first
testing period
One contracted H1N1 flu, unable to eat
the banana bread during first testing period
Four contracted H1N1 flu, unable to eat the
banana bread during second testing period,
but provided data at 8 weeks
Sixty-four participants were excluded
Thirty-two due to incomplete interview
Thirty-two due to other reasons
Effect of walnuts on cognition and mood 3
British Journal of Nutrition
recognition of assumption, deduction, interpretation, and
evaluation of arguments. Content on the WGCTA test reflects
both ‘neutral’ items and ‘controversial’ items, which relate to
social, economic and political subjects. In this way, the
WGCTA also reflects critical thinking abilities in the presence
of emotional and cognitive biases. The WGCTA is distin-
guished with a long history of development and use in
different countries and different settings
(24)
. The WGCTA was
found to have adequate content validity, construct validity,
reliability over time and estimates of internal consistency
(25)
.
Wechsler Memory Scale – Third Edition (WMS-III) was used
to comprehensively evaluate brain behaviour relationships
involved in learning and memory. WMS-III recognises
immediate, delayed and working memory. Each domain
is tested in two modalities, auditory and visual, and in two
formats, recall and recognition. Data were derived from
scores on subtests and these eight indexes: immediate
memory, auditory immediate, auditory delayed, auditory rec-
ognition delayed, visual immediate, visual delayed, general
memory and working memory. All core subtests were admi-
nistered; optional subtests were excluded from administration.
WMS-III has adequate internal-consistency reliability and
validity properties
(26)
.
The Profile of Mood States (POMS) questionnaire was used
to estimate the intensity of mood disturbance in participants.
The POMS is one of the most widely used and accepted
mood scales in studies of cognition
(27,28)
. The POMS covers
six mood domains: tensionanxiety, depression dejection,
angerhostility, vigour activity, fatigue inertia and con-
fusionbewilderment. The total mood disturbance score is
computed by summing the five negative domain scores and
subtracting the vigour score. Thus, higher scores indicate a
greater degree of mood disturbance.
Statistical analysis
The sample size determination was based on 5 % significance,
95 % power and measurable medium to large effect size of
0·5 –0·7 SD. Allowing for a 30 % dropout rate, the total
sample size required for the study was sixty-four individuals.
Primary analysis was carried out as per-protocol analysis.
Descriptive values are expressed as means and standard devi-
ations or percentages. All analyses were carried out for both
sexes combined. For the crossover analysis, the SAS statistical
software (version 9; SAS institute, Cary, NC, USA) used the
mixed models procedure (PROC MIXED). Sequence, treat-
ment and period were fixed effects and subject was treated
as a random effect. We tested for possible interactions
between the dietary treatment and the sequence of the testing
period. When differences reached significance, effect sizes
(Cohen’s d
(29)
) were calculated for determination of the clini-
cal relevance of the observed effects. A dbetween 0·20 and
0·49 was considered to represent a small effect, a dbetween
0·50 and 0·79 was considered moderate and a dgreater than
or equal to 0·80 was considered a large effect
(30)
. We used
the SPSS (version 18.0; SPSS, Inc., Chicago, IL, USA) to per-
form independent ttests and
x
2
tests for baseline character-
istics of participants, and paired ttests to compare the
changes in FA levels in plasma and weight change over
time. Two-sided Pvalues less than or equal to 0·05 were con-
sidered statistically significant.
Results
The characteristics of the study population are described in
Table 2. The participants were between 18 and 25 years old.
The mean BMI for males was 23·3 (SD 3·0) kg/m
2
, and it was
22·8 (SD 3·6) kg/m
2
for females. The mean n-3 FA intake was
0·4 (SD 0·6) g/d for males and 0·9 (SD 1·1) g/d for females at
baseline. Approximately half the number of participants
were Caucasians, followed by African Americans, Hispanics,
Asians and people of mixed ethnicity. The majority (90 %) of
participants were undergraduate students. The groups had
similar exercise habits. The majority (61 %) of students had
normal weight and were non-vegetarians (60 %). There were
no differences for all of the above characteristics among the
study group.
Walnut
Placebo
Walnut
Randomisation
8 weeks 6 weeks washout 8 weeks
CCCCCCCC CCCCCCCC
CCCCCCCC
CCCCCCCC
T+B T+B T+B
Placebo
Fig. 2. Experimental design. T, Cognitive testing; B, blood drawing; C, weekly clinic: weight, test meal distribution, compliance assessment.
Table 1. Composition of the test meal (bana na bread) per 100 g*
Walnuts Placebo
Energy (kJ/kcal) 1612/385 1202/287
Carbohydrate (g) 40·9 48·9
Protein (g) 7·0 5·2
Fat (g) 21·5 7·9
SFA (g) 2·5 1·5
MUFA (g) 3·8 1·9
PUFA (g) 14·1 4·0
a-Linolenic acid (18 : 3n-3) (g) 2·5 0·2
Linoleic acid (18 : 2n-6) (g) 12·2 4·0
* Values obtained by chemical analysis (Covance Laboratories, Madison, WI, USA).
P. Pribis et al.4
British Journal of Nutrition
Over the course of the study, seventeen (27 %) subjects
dropped out. Six withdrew during the first week of the
study because of lack of interest, two transferred to a different
college between the fall and spring semesters, eight were
excluded because they contracted the H1N1 flu and were
not able to consume the test meal for several weeks, and
one was dropped from the study because of non-compliance.
The dropout rate was very similar in both experimental
groups. According to the compliance sheets, participants con-
sumed 98 % of the test meals. Mean plasma concentrations of
ALA and LA increased 68 and 11 %, respectively, while partici-
pants were consuming the walnut test meal compared to pla-
cebo (Table 3).
The majority of the students (52 %) consumed one slice of
banana bread at a time, 26 % consumed two slices at a time
and 22 % consumed all the three slices on one occasion.
Most students consumed the banana bread for breakfast
(42 %), while the rest distributed it equally either for lunch,
dinner or snacks. Consumption of three slices of banana
bread daily modified the food intake of the participants.
Three slices of banana bread with walnuts represented
4354 kJ (1040 kcal), and three slices of banana bread without
walnuts 3245 kJ (775 kcal). Using an FFQ, we monitored the
food displacement and observed that during the dietary inter-
vention, males consumed less milk, cereals, pasta and bread
but drank more soya or rice milk; females consumed less
Table 3. Plasma concentrations (mg/ml) of linoleic and a-linolenic acids
at the end of each dietary intervention
(Mean values and standard deviations)
Walnuts
(n47)
Placebo
(n47)
Fatty acid (common name) Mean SD Mean SD P
18 : 2n-6 (linoleic acid) 248·3 88·3 223·6 66·4 0·013*
18 : 3n-3 (a-linolenic acid) 9·9 6·7 5·9 2·3 ,0·001*
* Paired ttest.
Table 2. Baseline characteristics of the participants
(Mean values, standard deviations, number of participants and percentages, n64)
Walnuts
placebo (n32)
Placebo
walnuts (n32)
Group n%n%P
Age (years) 0·872*
Mean 20·6 20·7
SD 2·0 2·1
Hours of sleep (h) 0·510*
Mean 7·2 7·0
SD 1·3 0·7
BMI (kg/m
2
)0·471*
Mean 22·6 23·2
SD 3·3 3·5
n-3 intake (g) 0·730*
Mean 0·7 0·6
SD 1·1 0·7
Ethnicity 0·255†
Caucasian 18 56·3 11 35·5
African American 5 15·6 7 22·6
Other 9 28·1 13 41·9
Marital status 0·162†
Single 31 96·9 28 87·5
Class standing 0·977†
Freshman 8 25·0 7 22·6
Sophomore 7 21·9 9 29·0
Junior 8 25·0 7 22·6
Senior 6 18·8 5 16·1
Graduate 3 9·4 3 9·7
Exercise (30 min/d per week) 0·722†
02d 1546·91238·7
34d 1237·51238·7
5– 6 d 5 15·6 7 22·6
Weight category 0·459†
Underweight (BMI ,18·4 kg/m
2
)39·4413·3
Normal weight (BMI 18·5 –24·9 kg/m
2
)2268·81653·3
Overweight/obese (BMI .25·0 kg/m
2
) 7 21·9 10 33·3
Vegetarian status 0·595†
Lacto-ovo-vegetarian/vegan 12 37·5 13 40·6
Pesco-vegetarian 1 3·1
Omnivore 19 59·4 19 59·4
* Independent means ttest.
x
2
test.
Effect of walnuts on cognition and mood 5
British Journal of Nutrition
milk, cheese, dried fruits and cereals. During the first interven-
tion period, the average weight increase for the walnut group
was 1·3 (SD 1·3) kg and it was 0·9 (SD 1·0) kg for the placebo
group (P,0·001 for both diets). During the second interven-
tion period, weight increased 0·4 (SD 1·4) kg for the walnut
group and 0·5 (SD 2·2) kg for the placebo group (P¼0·195
for walnuts; P¼0·279 for placebo).
Mean scores and the treatment effects for both sexes for the
APM and the WGCTA are presented in Table 4. For the APM,
all calculations are based on raw scores. No significant
changes in the APM were observed. All calculations for the
WGCTA scores are based on raw scores. The raw score is
the total number of correct responses. We observed a signifi-
cant, moderate effect size increase in inference, a subtest of
the WGCTA (11·2 %; P¼0·009; d¼0·567) on walnut diet.
Table 5 presents the mean scores and treatment effects for
the WMS-III memory test. All the calculations for the WMS-
III are based on scaled scores adjusted for age. No significant
changes in any of the subtests were observed. Mean scores
and the treatment effects for the POMS are presented in
Table 6. All the calculations for the POMS scores are based
on raw scores. No significant changes were observed for
any of the subsets or the total score.
Discussion
This randomised, crossover trial showed no effect of walnut
supplementation for 8 weeks in college-age healthy males
and females on memory, non-verbal reasoning and mood.
However, a selective effect was observed for verbal reasoning.
To our knowledge, the present study has been the first to
examine the effect of nutritional supplementation on non-
verbal reasoning and verbal reasoning in healthy adults.
As no effect was observed for the APM, the authors con-
clude that non-verbal reasoning seems to be stable enough
to not be altered by this nutritional intervention. This may
be partly explained by the fact that the study population,
college students, may be already having high cognitive
abilities and so further improvement would be difficult
to achieve. Using the 18 25 year olds US norms, mean
scores represent the 81st and 64th percentiles, yielding
intelligence quotient equivalent of 113 and 105 for males
and females, respectively
(22)
.
The ability to think critically is generally recognised as an
important educational objective and is considered a desirable
outcome. Inference is the ability to designate the degree of
accuracy of conclusions that have been drawn from certain
facts. Inferential reasoning is tested in such a way that subjects
are read a short narrative followed by five statements. Subjects
must make a decision whether the statements are true,
partially true, false, partially false or whether there is not
enough information to make a judgement. The results for
inference showed a significant improvement after walnut sup-
plementation (P¼0·009; d¼0·567; 11·2%), but we did not
observe any period-specific effect. The observed improve-
ment occurred already after 8 weeks of walnut supplemen-
tation; nevertheless, whether such effect might persist over a
longer period like years or at different life stages, we do not
Table 4. Mean scores and changes in non-verbal and verbal intelligence for both sexes*
(Mean values and 95 % confidence intervals)
Walnuts Placebo Walnutsplacebo
Mean 95% CI Mean 95 % CI Difference 95 % CI Difference (%) 95 % CI P
Raven’s Advanced Progressive Matrices (n51) 27·0 25·0, 29·0 26·8 24·9, 28·8 0·2 20·8, 1·0 0·6 22·7, 3·8 0·735
The Watson-Glaser Critical Thinking Appraisal (n49)
Inference 9·9 8·9, 10·7 8·9 8·0, 9·8 1·0 0·3, 1·7 11·2 2·9, 19·6 0·009
Recognition of assumption 12·5 11·5, 13·4 12·2 11·2, 13·1 0·3 20·4, 1·0 2·3 23·4, 8·1 0·419
Deduction 11·7 10·9, 12·5 11·7 10·9, 12·5 0·0 20·7, 0·7 0·0 25·6, 5·8 0·962
Interpretation 12·2 11·5, 12·9 12·2 11·5, 12·9 0·0 20·5, 0·6 0·0 24·2, 4·7 0·906
Evaluation of arguments 12·3 11·6, 12·9 12·1 11·4, 12·7 0·2 20·3, 0·7 1·6 22·8, 5·9 0·473
Total score 45·8 39·2, 54·4 44·2 37·6, 50·8 1·6 22·2, 5·5 3·7 25·0, 12·4 0·399
* The small differences in sample size by test are due to no-shows for certain tests.
P. Pribis et al.6
British Journal of Nutrition
know. The authors are unable to explain why inference alone
was affected by consumption of walnuts and not the other
‘critical thinking’ subtests recognition of assumption, deduc-
tion, interpretation, and evaluation of arguments. All the other
subtests either did not change or there was only a very small
increase; none of the subtest scores decreased. The WGCTA
has a strong internal consistency. Individual subtests correlate
highly (r0·50·8) with the total score. In our study, there was
a significant direct correlation between inference and the total
score (r0·8; P,0·001). This would support the idea that single
factor results are consistent with the overall data. n-3 FA can
modulate many of the signal transduction mechanisms operat-
ing at the synaptic level, influence pathways with different
neurotransmitters such as serotonin, dopamine or acetyl-
choline, and possibly be responsible for some of the reported
effects
(31,32)
. On a practical level, maybe students or young
professionals in social studies, mathematics, sciences or
other fields that involve a great deal of critical thinking or
decision-making could possibly benefit and gain a slight
advantage through regular consumption of walnuts. Consider-
ing these findings, it is important that further research be
conducted in order to gain a deeper understanding of this
differential impact of walnuts consumption on this component
of verbal reasoning.
Previous studies have been conducted around the effects of
n-3 FA on mood states with varying results. Most studies
appear to focus on depressed populations, and some have
found reduced levels of n-3 FA in plasma and cell membranes
of depressed patients
(33 – 35)
. Appleton et al.
(36)
evaluated ran-
domised controlled trials in various populations including
healthy samples and concluded, that trial evidence of the
effects of n-3 FA on depressed mood has increased but
remains difficult to summarise. The evidence available pro-
vides some support of a benefit of n-3 FA in individuals
with diagnosed depressive illness but no evidence of any ben-
efit in individuals without a diagnosis of depressive illness.
The present study was built around the premise that walnuts
are an available, excellent source of n-3 FA. As participants
in the present study scored around the 50th percentile at base-
line (not depressed)
(27)
, the results in our study confirm the
present consensus that n-3 FA supplementation in non-
depressed healthy adults has no effect on mood.
Table 5. Changes in memory for both sexes
(Mean values and 95 % confidence intervals)
Walnuts Placebo Walnutsplacebo
Mean 95 % CI Mean 95 % CI Difference 95% CI Difference (%) 95 % CI P
The Wechsler Memory Scale-III (n52)
Logical memory I 12·7 11·9, 13·5 12·5 11·7, 13·3 0·2 20·5, 0·9 1·7 23·7, 7·1 0·525
Faces I 13·1 12·2, 14·0 13·1 12·2, 14·0 0·0 20·9, 0·9 0·0 26·7, 6·5 0·976
Verbal paired associates I 14·4 13·8, 15·0 14·1 13·5, 14·7 0·3 20·2, 0·9 2·3 21·7, 6·4 0·255
Family pictures I 13·3 12·7, 14·0 13·5 12·9, 14·1 20·2 20·7, 0·4 21·2 25·3, 2·9 0·553
Letter-number sequencing 11·0 10·2, 11·8 11·0 10·2, 11·8 0·0 20·6, 0·7 0·0 25·5, 6·5 0·867
Spatial span 11·3 10·4, 12·1 11·7 10·9, 12·6 20·4 21·0, 0·2 23·8 28·9, 1·4 0·148
Logical memory II 13·5 12·7, 14·2 13·7 13·0, 14·4 20·2 20·7, 0·3 21·6 25·3, 2·2 0·405
Faces II 12·5 11·5, 13·5 12·5 11·5, 13·5 0·0 20·8, 0·9 0·0 26·7, 7·6 0·898
Verbal paired associates II 11·9 11·8, 12·1 12·0 11·8, 12·1 20·1 20·2, 0·2 20·3 21·9, 1·4 0·744
Family pictures II 13·4 12·7, 14·1 13·7 13·1, 14·4 20·3 20·9, 0·2 22·5 26·6, 1·7 0·238
Auditory recognition 12·6 11·9, 13·4 12·3 11·5, 13·0 0·3 20·3, 1·0 2·9 22·0, 7·8 0·245
Auditory immediate 27·1 26·0, 28·2 26·6 25·5, 27·6 0·5 20·3, 1·4 2·0 21·2, 5·2 0·221
Visual immediate 26·4 25·2, 27·6 26·6 25·4, 27·8 20·2 21·2, 0·8 20·7 24·3, 3·0 0·703
Immediate memory 53·5 51·5, 55·5 52·8 50·8, 54·8 0·7 20·7, 2·2 1·4 21·3, 4·1 0·308
Auditory delayed 25·4 24·7, 26·2 25·7 24·9, 26·4 20·2 20·8, 0·3 21·0 23·2, 1·3 0·396
Visual delayed 25·9 24·5, 27·3 26·2 24·8, 27·6 20·3 21·4, 0·9 21·0 25·4, 3·3 0·634
Auditory recognition delayed 12·6 11·9, 13·4 12·3 11·5, 13·0 0·4 20·3, 1·0 2·9 22·0, 7·8 0·245
General memory 62·9 60·1, 65·6 64·2 61·5, 67·0 21·3 23·9, 1·2 22·1 26·1, 1·8 0·288
Working memory 22·6 21·3, 24·0 22·8 21·5, 24·2 20·2 21·2, 0·8 20·9 25·2, 3·3 0·664
Table 6. Changes in mood for both sexes
(Mean values and 95 % confidence intervals)
Walnuts Placebo Walnuts–placebo
Mean 95 % CI Mean 95 % CI Difference 95 % CI Difference (%) 95 % CI P
Profiles of moods (n49)
Tension–anxiety 11·2 9·3, 13·2 11·9 9·9, 13·8 20·7 22·5, 1·2 25·2 220·8, 10·4 0·507
Depressiondejection 9·7 6·7, 12·7 12·1 9·1, 15·2 22·4 25·4, 0·5 220·0 244·2, 4·2 0·103
Anger–hostility 6·5 4·7, 8·3 7·5 5·7, 9·3 21·0 22·8, 0·8 213·3 236·8, 10·2 0·261
Vigour–activity 13·8 12·2, 15·4 13·1 11·5, 14·7 0·7 21·0, 2·4 5·3 27·5, 18·0 0·408
Fatigue–inertia 10·2 8·7, 11·7 10·1 8·6, 11·6 0·1 21·4, 1·7 1·1 214·3, 16·6 0·882
Confusion–bewilderment 9·5 8·2, 10·8 9·5 8·2, 10·8 0·0 21·2, 1·2 0·0 212·6, 12·1 0·969
Total score 33·5 24·7, 42·3 37·9 29·1, 46·6 24·4 212·3, 3·5 211·6 232·4, 9·2 0·267
Effect of walnuts on cognition and mood 7
British Journal of Nutrition
There is an unfortunate dearth of research on nuts and cog-
nition. One study by Willis et al.
(4)
reported on the effects of
walnuts on cognitive and motor functions in aged rats. Results
for the motor testing showed that a 2 % walnut diet improved
performance on rod walking, while the 6 % walnut diet
improved performance on the medium plank walk; the high-
est dose of the 9 % walnut diet did not improve psychomotor
performance. All of the walnut diets improved working
memory in the Morris water maze; nevertheless the 9 % diet
showed impaired reference memory. Nooyens et al.
(16)
recently reported that in the Doetichen Cohort Study, higher
intake of nuts was associated with significantly elevated cogni-
tive function (memory, speed, cognitive flexibility and global
cognitive function) at baseline. The difference in cognitive
functions between the lowest and the highest quintile of nut
consumption was equivalent to 5 8 years’ difference in age.
However, there was no significant change observed in cogni-
tive function at the second testing after 5 years. Both of these
studies were conducted on aged or ageing populations,
suggesting that the many neuroprotective compounds in
nuts possibly slow the cognitive declines that accompany
ageing. In our randomised trial, the results of the WMS-III
test showed no effects on memory after 8 weeks’ walnuts sup-
plementation. Initial cognitive functioning of our population
may have been too high for nutrients supplementation to be
effective. Using norms, the average score of the different
WMS-III subtest at baseline for both sexes ranged between
the 50th and 80th percentiles
(26)
. These results appear to indi-
cate that the effects of walnuts on healthy adults during a
period of short, intensive supplementation are limited.
A limitation of the present study is that the duration of the
intervention may have been too short. Another consideration
is that the relationship between walnut supplementation and
cognitive performance increases may be curvilinear rather
than linear. In such a case, future studies may benefit from
including experimental groups with varying levels of walnut
supplementation in order to identify the optimal amount of
walnuts necessary to enhance performance. This is difficult
to predict a priori, however, since the mechanisms by which
walnuts influence cognitive performance in both the short
term and the long term are not completely understood. An
additional constraint that we faced in this study was the drop-
out rate, which was elevated because of the flu epidemic. This
attrition rate reduced the level of statistical power available for
our analysis. However, the bias that would result from the
withdrawal of participants with excellent or poor cognition
would probably be negligible.
In conclusion, in this randomised walnuts feeding trial, two
different intelligence tests a memory test and a mood test
were used to assess the potential cognition changes in young
adults. Results seem to indicate that walnuts increase inferen-
tial verbal reasoning. Young, healthy adults involved in fields
that require inferential verbal reasoning faculties could poss-
ibly experience a modest improvement after regular walnut
consumption. Mood, memory and non-verbal reasoning do
not appear to be affected by walnuts. Further studies are
necessary to corroborate these findings.
Acknowledgements
The present study was supported in part by the California
Walnut Commission (Folton, CA, USA). The funders had no
role in the implementation, data collection, management, anal-
ysis or interpretation of the study, or in the preparation, review
and approval of the manuscript. We are indebted to the partici-
pants for their enthusiastic commitment to the study protocol
and to Benneth Chilson, Carol Burtnack, Jasmina Popovska,
Megan McLoughlin, Galiya Elias, Kristy Koeppe, Allison Ireland,
Matt Bonynge, Elizabeth Lundy, Ellen Klinedinst, Jamie Ely,
Nathanael Stephens and Lilian M. Correa for their technical
assistance. The authors’ responsibilities were as follows: P. P.
was the principal investigator, who also designed and planned
the study, applied for funding and drafted the manuscript. R. N.
B. and A. A. R. oversaw the psychological part of the study and
helped in the drafting of the manuscript. M. A. K. oversaw the
clinical part of the study. M. H. and W. J. C. oversaw the nutri-
tional part of the study. D. J. S. and T. G. were responsible for
statistical analysis. J. S. helped to design the study and drafted
the manuscript. The corresponding author (P. P.) had full
access to all data and had the final responsibility for the decision
to submit the manuscript for publication. J. S. is an unpaid emer-
itus member of the California Walnut Commission Scientific
Advisory Board. The authors declare no conflicts of interest per-
taining to this study.
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Effect of walnuts on cognition and mood 9
British Journal of Nutrition
... For instance, a study conducted on 4822 adults in China found that higher nuts consumption may contribute to a better overall cognition at older ages [9]. On the other hand, few studies showed there is no association between cognitive function and nuts intake [11,12]. For instance, a randomized cross-over trial showed no effect of walnut supplementation on memory, non-verbal reasoning, and mood in young adults [11]. ...
... On the other hand, few studies showed there is no association between cognitive function and nuts intake [11,12]. For instance, a randomized cross-over trial showed no effect of walnut supplementation on memory, non-verbal reasoning, and mood in young adults [11]. ...
... The findings of our study were consistent with the results of most of the previous studies in both humans and animals [7][8][9][10][11][12][13][14][15][16][23][24][25][26]. A cross-sectional study conducted in Figure 2. Interaction between nuts consumption and hypertension and diabetes in relation to cognitive function. ...
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The association between nuts intake and cognitive function is inconclusive. We aimed to investigate the association between habitual nuts consumption and cognition among Qatari adults. Data from 1000 participants aged >20 years who attended Qatar Biobank (QBB) were used. Nuts consumption was assessed by a food frequency questionnaire (FFQ). Blood samples were measured for magnesium, lipids and glucose. Mean reaction time (MRT) was used as an indicator of cognitive function. Linear regression was used to assess the association. A total of 21.1% of the participants reported consuming nuts ≥4–6 times/week (high consumption) while 40.2% reported consuming ≤1 time/month (low consumption). The mean MRT was 715.6 milliseconds (SD 204.1). An inverse association was found between nuts consumption and MRT. Compared to those with a low consumption, high consumption of nuts had a regression coefficient of −36.9 (95% CI −68.1 to −5.8) after adjusting for sociodemographic and lifestyle factors. The inverse association between nuts and MRT was mainly seen among those >50 years. There was an interaction between nuts consumption and hypertension. The association between nuts consumption and MRT was not mediated by hypertension, diabetes, or serum magnesium. Habitual higher consumption of nuts is positively associated with cognitive function, especially among old adults.
... Seven observational studies (6 cross-sectional studies [20,[29][30][31][32][33], 1 prospective cohort study [34]) and five RCTs [35][36][37][38][39] examining the associations between walnut consumption and cognition-related outcomes (cognitive function, brain MRI, stroke, mood) were identified. Cognition-related outcome domains, subdomains, and measurement tools were highly variable across the included studies. ...
... Two randomized (1 cross-over [35] and 1 parallel [37]) trials reporting the effects of walnut on cognitive function were included. One study was conducted in generally healthy young adults, while the other study examined older subjects with mixed baseline health (healthy or with type 2 diabetes [T2D], hypertension [HTN], and/or hyperlipidaemia [HLD]) ( Table 2). ...
... walnuts on subdomains of cognition and/or subgroups of the study population. In healthy young adults consuming 60 g/d walnuts or placebo for 8 weeks in a cross-over design, Pribis et al. [35] found no significant differences in total scores for memory (WMS-III), non-verbal reasoning (APM), or verbal reasoning (WGCTA). However, walnut consumption significantly improved performance on the inference subtest of the WGCTA relative to placebo (11.2% difference, p ¼ .009). ...
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Background Walnuts contain nutrients that are associated with improved cognitive health. To our knowledge, no review has systematically examined the effects of walnuts on cognitive function and risk for cognitive decline. Objective To conduct a systematic review and meta-analysis evaluating the effects of walnut intake on cognition-related outcomes and risk-factors for cognitive decline in adults. Methods Medline®, Commonwealth Agricultural Bureau, and Cochrane Central Register of Controlled Trials were searched for randomized controlled trials (RCTs) and observational studies published until April 2020 on walnut intake, cognition (e.g. cognitive function, stroke, and mood), and selected risk factors for cognitive decline (e.g. glucose homeostasis and inflammation). Risk-of-bias and strength-of-evidence assessments were conducted using standard validated tools. Random-effects meta-analyses were conducted when ≥3 studies reported quantitative data for each outcome. Results 32 RCT and 7 observational study publications were included. Meta-analysis of cognition-related outcomes could not be conducted due to heterogeneity of tests. None of the 5 cognition RCTs found significant effects of walnuts on overall cognition, although 3 studies found improvements on subdomains and/or subgroups. All 7 observational studies found significant associations and a dose-response relationship between walnut intake and cognition-related outcomes. Meta-analyses of 27 RCTs reporting glucose homeostasis and inflammation outcomes, selected risk factors for cognitive decline, did not show significant effects of walnut intake. Conclusions Due to the non-uniformity of tests for cognition-related outcomes, definitive conclusions regarding the effect of walnut consumption on cognition could not be reached. Additionally, evidence does not show associations between walnut intake and glucose homeostasis or inflammation, cognitive decline risk-factors. High-quality studies with standardized measures are needed to clarify the role of walnuts in cognitive health. • KEY MESSAGES • This is a systematic review and meta-analysis of 5 randomized clinical trials and 7 observational study articles of the impact of walnut intake on cognition decline and 27 randomized clinical trials of the effect of walnut intake on risk factors for cognitive decline including glucose homeostasis and inflammation. • The non-uniformity of tests performed to measure cognitive function in the various studies did not allow for a meta-analysis of these studies. A definitive conclusion could therefore not be reached regarding the effect of walnut intake on cognitive decline. • The evidence available does not show an association between walnut intake and glucose homeostasis or inflammation.
... A randomized controlled parallel clinical trial showed an enhancement of memory and executive functions in healthy adults aged 50e75 years consuming 84 g of almonds daily for six months [29]. Although Pribis et al. did not observe significant changes in nonverbal reasoning or memory, they reported a possible positive effect on verbal reasoning in a randomized controlled crossover study with college students consuming 60 g of walnuts for 8 weeks [30]. In the WAHA study of community-dwelling elderly men and women, carried out in two research centers (Loma Linda, CA, USA and Barcelona, Spain), a delay in cognitive decline was not observed after the intake of 30e60 g/day of walnuts for two years, but global cognition and perception scores improved in the Spanish cohort [31]. ...
... Similarly, Arab et al. reported lower depression scores among walnut consumers in the National Health and Nutrition Examination Survey [37]. However, the consumption of walnuts for 8 weeks, shorter than the period of the previous study, did not change the mood of college students [30]. Moreover, lower levels of cortisol in urine were observed after the SRP and PB interventions compared to baseline. ...
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... An observational cohort study assessed the association of various foods on the cognitive function of children and adolescents and reported a beneficial effect of nut consumption on visual attention and processing (17). One small double-blind, randomized, placebo-controlled crossover (8-week intervention and 6-week washout) trial with walnuts carried out with young college students aged 18 to 25 years showed improvements in inferential verbal reasoning (18). However, no experimental study focused on the effects of walnut consumption on adolescents' cognitive functions. ...
... The cognitive outcomes with mean values of 100 (SD 15) units corresponding to the standard values of neuropsychological scores in the general population (23) were considered, with a correlation between them of 0.25. The targeted intervention effect was a change of 3 units, based on previous studies in adult samples (16,18). We considered a type I error of 0.05 and corrected calculations for multiplicity using the Benjamini-Hochberg method. ...
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... Several bioactive substances and natural extracts are mentioned herein to cure and prevent AD. Until today, most natural chemicals investigated have been sourced mainly from plant sources, with only a few 10 Oxidative Medicine and Cellular Longevity In vivo [168][169][170][171][172] Almond (Prunus dulcis) Paste ...
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... In general, the study interventions ranged from 2 to 48 months, involved middleaged and older adults, and included between 30 and 85 g/ day of nuts. Results on cognition were mixed with some reporting improvements in various cognitive outcomes (i.e., short-term and visuospatial working memory, verbal fluency, processing speed, inferential verbal reasoning, and executive function) [17,19,22] and others not [18•, 20, 21] compared to the control diet. Data from Rakic et al. [22] suggests that the effect may be dose dependent as no discernable effect was observed at ~ 42 g/day, but was observed at ~ 85 g/day after 6 months. ...
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... 5,6 If one looks more closely, however, the literature has been mixed regarding evidence on dietary patterns and more general cognition. [7][8][9][10] The best evidence compiled from systematic reviews of the research supports the beneficial role that diets rich with plants, nuts, and berries have on cognition. While a recent review of randomized controlled trials found that the Mediterranean dietary pattern largely produced insignificant cognitive effects, 11 it was also reported that most robustly designed studies reviewed suggested cognitive benefit associated with the Mediterranean diet. ...
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Background and Aims Previous research has investigated the impact of diet on cognition, but the focus has often been on general cognition. This paper reports on a preregistered cross-sectional study aimed at testing for specific executive function differences across individuals who self-reported one of four distinct dietary patterns: No Diet, No Sugar, Vegetarian, or Mediterranean Diet pattern. Our hypotheses were aimed at testing whether adherence to a specialty diet improved decision making relative to those who reported following No Diet. Methods We administered an incentivized Bayesian choice task to all participants. The task involved multiple components of information—existing information (base rate odds) as well as new information (sample draw evidence)—to allow a test of how these information components were used in making probability assessments, and how this may differ by self-reported dietary pattern. Sample size, hypotheses, and analysis plans were all determined ex ante and registered on the Open Science Framework. Multi-variate linear and non-linear estimation methods were used to analyze the data. Results Our data failed to support our pre-registered hypotheses. In fact, we found some evidence that self-reported adherence to a specialty No Sugar Diet was associated with a reduced decision accuracy and was connected to an increased imbalance in how the participant weighted the two available sources of information when making choices. Conclusion Our results suggest that decision making is nuanced among dietary groups, but that short-term incentivized decisions in an ecologically valid field setting are likely not improved solely by following promoted dietary patterns such as the Mediterranean or Vegetarian diets.
... In particular, adults aged 20-59 years old required 16.4 ms less time to respond on the simple reaction time and 0.39 s less for the symbol digit substitution after consuming an average of 13.1 g walnuts per day. The effects of walnut consumption on cognitive performance in young adults has also been established by Pribis et al. through a randomized controlled trial: although no significant increases were detected for mood, non-verbal reasoning or memory as a result of the walnut-supplemented diet, inferential verbal reasoning increased significantly by 11.2% [76]. A recent study with a representative sample of 3632 US adults aged 65 years and older established an association between walnut consumption and cognitive function, with greater cognitive scores in volunteers consuming walnuts [77]. ...
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Long-term studies with regular tree nut consumption have indicated positive outcomes for multiple health benefits. Here, we review the beneficial effects of tree nuts, highlighting the impact on glucose modulation, body weight management, cardiovascular risk, inflammation, oxidative stress, cognitive performance, and gut microbiota. Nuts are important sources of nutrients and phytochemicals, which, together with a healthy lipid profile, could help prevent certain chronic diseases, protect against oxidative stress and inflammation, and improve cognitive performance, thus reducing the impact of aging and neurodegeneration.
... Almond and walnut supplementation for 4 weeks attenuated cadmium-induced memory impairment in rats, possibly through cholinergic and antioxidant activities [282]. A walnut enriched diet has been shown to improve cognitive and motor performance [283]. Its neuroprotective effects are believed to be associated with the remarkably attenuated expression of proinflammatory cytokines, decreased level of AChE, significantly restored levels of antioxidant enzymes, and reduced expression of NF-κB [284]. ...
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