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Acta Padiatrica, 2005; 94: 1667-1673
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The effect of voluntary dehydration on cognitive functions
of elementary school children
YAIR BAR-DAVID1, JACOB URKIN' & ELY KOZMINSKY2
'Prima?y Pediatrics Unit and the Division of Pediatrics, Soroka University Medical Center, Faculty of Health Sciences,
Ben-Gurion University of the Negev, Beer-Sheva, Israel, and 2Departmentof Education, Ben-Gurion University of the Negev,
Beer-Sheva, Israel
Abstract
Aims: (1) T o describe the occurrence of voluntary dehydration in two classes of elementary school students as expressed by
their morning and noon-time urine osmolality; and (2) to determine the relationship between the children's scores on
cognitive tests and their state of hydration. Methods: Group comparison among fifty-eight sixth-grade students (age range
10.1-12.4 y old) during mid-June at two schools in a desert town. Morning and noon-time urine samples were collected
in school, and five cognitive tests were scored in the morning and at noon-time. Main outcome measures: (1) morning and
noon-time urine osmolality; (2)scores of five cognitive tests (hidden figures, auditory number span, making groups, verbal
analogies, and number addition) that were applied in the morning and at noon-time. Results: Thirty-two students were
dehydrated (urine osmolality above 800 mosdkg HzO) in the morning. An individual's noon-time urine osmolality was
highly related to morning osmolality (r=0.67, p = O . O O O ) . The morning cognitive scores were similar in the hydrated and
dehydrated students (p=0.443).The adjusted mean scores of the noon-time tests, with the morning test scores as covariates,
demonstrated an overall positive trend in four of the five tests in favor of the hydrated group (p=0.025).The effectwas mainly
due to the auditory number span test @=0.024).
Conclusion: Voluntary dehydration is a common phenomenon in school-aged children that adversely affects cognitive
functions.
Key Words: Cognitivefunctions, fruid consumption, school-age children, voluntay dehydration
Introduction
Voluntary dehydration is a condition in which humans,
mainly children, while exposed to excessive insensible
water loss due to environmental conditions, d o not
drink appropriately in the presence of adequate fluid
availability [1,2]. This results in a state of dehydration
as evidenced by a high urine osmolality (Uosm).
Voluntary dehydration has adverse physiological effects
[3,4] and may have adverse cognitive effects [5-71.
Voluntary dehydration is highly prevalent in hot
or dry climates [8,9]. In previous studies, about 70%
of school-age children in southern Israel were found
to be in a state of chronic dehydration, expressed as
Uosm exceeding 800 m o s d k g HzO [8,9]. An important factor in the pathogenesis of renal stones
is the presence of concentrated urine for prolonged
periods due to chronic dehydration [3,4]. Following
dehydration, the kidneys concentrate urine in response
to antidiuretic hormone (ADH), which increases renal
water reabsorption, raising Uosm. T h e incidence of
renal stones in Beer-Sheva, the main city of the Israeli
Negev desert, is 8.5/1000 in contrast to 1.6/1000 in
Jerusalem and 0.95/1000 in the USA [3].
In adults, a significant deterioration in mental
functions (arithmetic ability, visuomotor tracking, and
short-term memory) was found at a level of less than
2% dehydration [5]. Rotshtein et al. [lo] have found
that mild degrees of dehydration cause fatigue, lack
of appetite, drowsiness, and reluctance to participate
in complex tasks. Heat exposure may have an independent role in the impairment of mental performance
[6,7,1 I]. There are very few reports on the effect of
chronic dehydration on mental abilities in children.
Children living in a hot climate are especially susceptible to developing voluntary dehydration because of
Correspondence: J. Urkin, Primary Pediatric Unit, Division of Health in the Community, Faculty of Health Sciences, Ben-Gunon University of the Negev,
PO Box 653,Beer-Sheva 84105,Israel. Tel: f972 8 6477480.Fax: +972 8 6477636.E-mail: [email protected]
(Received 6 December 2004; accepted 12 July 200.5)
ISSN 0803-5253print/ISSN 1651-2227 online
DOI: 10.1080/08035250500254670
I
2005 Taylor & Francis
1668
Y. Bar-David et al.
their high proportion of body surface to body mass.
Thus, their insensible water loss is increased. They are
also less likely to restrict their physical activity in the
hot hours of the day.
Previous studies in kindergarten children, age 2-6 y
[9], living in arid areas in Israel, have revealed that
Uosm exceeding 800 m o s d K g H20 was excreted by
60.5% of the children and over 1000 m o s d k g H2O
by 7% of the children. Average Uosm increased with
age, from 608 m o s d k g H 2 0 at 2 y to 832 m o s d
kg H 2 0 at the age of 6 y. In elementary school children, ages 9-11 y [8], Uosm exceeding 800 m o s d
kg H 2 0 was excreted by 72.6% of the children and
over 1000 m o s d k g H20 by 30% of the children.
Apparently the level of Uosm-the degree of voluntary
dehydration-increases with age.
It is well known that adverse physiological states,
such as sleep deprivation or malnutrition, affect cognitive performance. Even though it is reasonable to
assume that prolonged dehydration may result in impaired cognitive functions, we could not find reports
on this subject.
Study objectives
The purpose of this study was (1) to describe the
occurrence and prevalence of dehydration among
school children, and (2) to explore the relationship
between voluntary dehydration and cognitive functions
in elementary school children.
Methods
Participants
Fifty-eight sixth-grade students (age range 10.112.4 y) from two classes in two elementary schools in a
city in the desert region of southern Israel participated
in the study.
Environment
The climate of the schools’ geographic area is characterized by high daytime temperatures, temperate
nights, low humidity, and about 150 mm annual precipitation. The schools were not air-conditioned. The
ambient outdoor temperature on the day of the study
was 35°C and the schools’ indoor temperature was
30°C with a relative humidity of 30%.
Measures and procedures
Urine osmolality. Urine samples were collected in the
schools at 08.00 and 13.00 during one day in June by
pre-trained medical students. The urine samples were
kept frozen in numbered and sealed tubes and were
later thawed for osmolality measurements. Previous
studies [9] had demonstrated that this procedure does
not affect the results. Osmolality was measured by
one technician with a Fiske Osmometer calibrated
with standard solutions of 500 mosdkg H 2 0 and
900 m o s d k g H20.
Cognitive tests. Five cognitive tests were administered
twice in the following order to an entire class, immediately after the morning and the noon urine sampling,
by pre-trained psychology student majors. An explanation and examples were given for each test.
(1) Hidden Figures. Identifymg a given figure in
patterns that contain additional lines (destructing visual information). Fifty percent of the
occurrences include the figure. Testing time:
3 min. Maximal number of patterns is 100,
testing components of visual attention, perceptual speed; and flexibility. The score is the
number of correctly identified patterns minus
the number of incorrectly identified patterns.
(2) Auditory Number Span. Immediate memory of
a sequence of dictated digits. Ten sequences
of 4-8 digits per sequence, testing immediate
memory recall. Testing period is about 5 min.
The score is the number of correctly recalled
sequences.
(3) Making Groups. Constructing conceptual categories by selecting related items from a list of
seven objects. There are two lists, and total
testing time is 5 min, testing semantic flexibility.
The score is the number of legitimate formed
categories (maximal score is 12).
(4) Verbal Analogies. Matching a word from a list of
five words to a target, according to their analogy
to a given word pair. Testing analogous conceptions, identifying semantic relations between
pairs of words. Maximal number of pairs was
10. Testing time: 3 min. The score is the number of correctly identified pairs.
(5) Number Addition. Adding one- or two-digit
numbers in 2 min. Maximum number of adding
problems is 60. This test measures perceptual
speed and automaticity of applying arithmetic
operations. The score is the number of correctly
solved problems.
Total test administration time was 25 min. The
Hidden Figures, Auditory Number Span, Making
Groups, and Number Addition tests were adapted
from the Ekstrom et al. Kit of Factor-referenced
Cognitive Tests [12] with translation into Hebrew
of the instructions and the verbal terms. The Verbal
Analogies test was adapted from a Hebrew test
battery [ 131.
Study approval and confidentiality assurance
The study was reviewed and approved by the Israeli
Ministry of Education and the local school authorities.
Eflect of voluntary dehydration on cognitive functions
Table I. Age and gender distribution of the hydrated and the
dehydrated groups.
Table 11. Morning and noon-time osmolality by morning hydration
status.
Groups
Group
Hydrated
1669
Hydrated
Dehydrated
Subjects
Total
19
32
Gender
Male
Female
9
10
11
21
Age (years :months)
Mean
SD
11:Ol
0:05
11:Ol
0:06
n
Dehydrated
19
pvalue
32
Morning osmolaliry
Mean
SD
Noon osmolality
Mean
SD
619.63
164.45
997.09
127.84
0.00
607.37
240.96
935.22
253.78
0.00
differences were found (school A: M = 928, SD = 2 12,
n=17; school B: M=821, SD=236, n=34;
F( 1,49) = 2.47, p = 0.12).
Mean noon-time urine osmolality was 8 13 (SD 294)
m o s d k g H20. Noon-time Uosm levels were highly
related to the morning osmolality (r= 0.67, p = 0.000).
Statistical methods
The changes in students’ hydration status from the
Analyses were performed using SPSS version 10.1.3. morning to the noon urine samples are displayed in
Statistically significant results were considered at Table 111. Of the morning hydrated students, 68%
p<O.O5. MANCOVA, ANOVA, contingency, and remained hydrated at noon-time, and 81% of the
correlation analyses were applied.
morning dehydrated students remained dehydrated
at noon-time (x2 with continuity correction= 10.55,
df = 1,p = 0.00 1). The mean noon-time Uosm was 607
Results
(SD 241) mosm/kg H20 in the hydrated group and
935
(SD 254) mosdkg H20 in the dehydrated group
The final study group consisted of 51 sixth-grade
(F(1,49)
= 20.64, p=O.OOO).
students, 19 boys and 32 girls. Out of the original
There
was a slight but not statistically significant
58 students, seven were dropped from the study: six
decrease
in
noon-time Uosm compared to the morning
students did not provide one of the two urine samples
Uosm
(F(1,49)
= 1.34, p=0.25). The same trend was
and one did not complete the morning test battery.
observed for the two groups (no-time by hydration
group interaction: F(1,49) ~ 0 . 6 0p=O.44).
,
Osmolality level
Letters of consent were sent to the parents of the
children ensuring confidentiality of the results. Refusal
to take part in the study was fully honored without
any consequences.
Mean morning urine osmolality was 856 (SD 232)
mosdkg H 2 0 . Based on the definition of Katz et al.
[14] regarding the degree of dehydration, Uosm
above 800 m o s d k g H 2 0 is defined as a high degree
of dehydration. Based on this definition, the children
were divided into two groups: 19 children had morning Uosm below 800 m o s d k g H 2 0 (hydrated group),
and 32 children had morning osmolality above
800 mosdkg H20 (13 of them had osmolality above
1000 mosdkg H20) (dehydrated group; see Table I
for age and gender distributions). The mean morning
Uosm (see Table 11) was 620 (SD 164) mosdkg H 2 0
in the hydrated group and 997 (SD 128) m o s d
kg H2O in the dehydrated group (F(1,49) =83.78,
p=O.OOO). No age (F(1,49) =0.44,p=0.51) orgender
differences (x2 = 0.84, d€= 1, p = 0.36) were found
between the groups. Morning osmolality was not
related to gender (mean male Uosm was 804 (SD 278)
and mean female osmolality was 890 (SD 194),
F( 1,49) = 1.72, p = 0.20). Also, no school Uosm
Cognitive pe$ormance
The results of the morning cognitive tests are displayed
in Table IV. There were no statistically significant
differences between the groups on the tests (Wilks’
lambda = 0.90, F(5,45) = 0.98, p = 0.443), except for
a marginal difference on the Auditory Number Span
test in favor of the hydrated group (F(1,49)=3.51,
p=0.067).
Table 111. Changes in urine osmolality (hydration status) from the
morning to the noon-time measurement.
Noon osmolality
Morning
osmolality
Hydrated
Dehydrated
Total
Hydrated
Dehydrated
Total
13
6
19
6
26
32
19
32
51
1670
Y. Bar-David et al.
Table IV. Morning cognitive test means and standard deviations for the hydrated and the dehydrated groups.
Hydrated
( n = 19)
Groups
Tests
Hidden Figures
Number Span
Making Groups
Verbal Analogies
Number Addition
Dehydrated
(n = 32)
Mean
SD
Mean
SD
P
- 100.53
4.89
2.11
4.63
14.42
50.05
2.42
1.56
1.86
6.60
- 101.81
47.04
1.90
1.61
1.66
6.13
0.93
0.07
0.72
0.55
0.42
Noon-time cognitive test scores (Table V) were
higher than the morning test scores (Table IV)
(Wilks = 0.28, F(5,45) =23.29, p = 0.000; Hidden
Figures: F(1,49) = 114.72, p=O.OOO;
Auditory
Number Span: F( 1,49) = 4.24, p = 0.045; Making
Groups: F( 1,49) = 0.003, p = 0.958; Verbal Analogies: F(1,49) =6.19, p=O.O16; Number Addition:
F(1,49) =7.50, p=0.009). The adjusted mean scores
of the noon-time cognitive tests (see Table VI), with
the morning cognitive test scores as covariates for the
two hydration groups, demonstrated an overall positive
performance trend in four of the five tests (Wilks’
lambda = 0.73, F(5,40) = 2.90, p = 0.025) in favour
of the hydrated group. This effect was mainly due
to the Auditory Number Span test (F(1,44)=5.48,
p=0.024).
3.75
1.94
4.94
12.94
concentrated [ 16,171. The clinical relevance is that
probably most of the population living in hot climates,
and especially children, will have maximally concentrated urine during the summer months, thus becoming chronically dehydrated.
Children living in hot climates are prone to be in a state
of chronic voluntary dehydration
Two previous studies on this subject were conducted
during the summer by our group. Phillip et al. [9]
found an increased urine concentration among 200
Jewish nursery school children ages 2 to 6 y, living
in southern Israel. The mean Uosm found in this
study ranged between 600 f 192 mosdkg H20 €or
the 2-y-olds to 832 f 175 mosdkg H 2 0 for 6-y-old
children. In the second study conducted by BarDavid et al. [8], urine samples were collected from 354
children, 264 Jews and 90 Bedouins, aged 8 to 10 y,
Discussion
living in southern Israel. The mean Uosm value among
Normal water balance is dependent upon two Jewish children was 906 f 183 mosdkg H 2 0 commechanisms: the secretion of antidiuretic hormone pared with 771 k 196 m o s d k g H20 in the Bedouin
(ADH) and the sense of thirst. At plasma osmolality group. Over 80% of the Jewish group excreted urine at
(Posm) below about 280 m o s d k g H 2 0 , ADH is concentrations exceeding 800 mosdkg H20, with
essentially zero and the urine is maximally diluted 30%at concentrations exceeding 1000 mosdkg/H20,
with a urine osmolality as low as 50 m o s d k g H20. At compared with 50% and 6.7% in the Bedouin group,
Posm >280 m o s d k g H 2 0 , ADH is secreted in direct respectively. Two possible explanations for the differproportion to the rise in Posm, until Uosm reaches a ences found between the groups are: (1) it is possible
maximum of 1200 m o s d k g H20. However, in a that the Bedouins, who have lived in a very hot climate
hot climate, the threshold for thirst occurs at Posm of the desert for many generations, have adapted and
of 290-295 mosm/kg H 2 0 [15], corresponding to a developed a mechanism that lowers the thirst threshUosm of about 800 m o s d k g H20, i.e., at a level far old; or (2) the Bedouins are more aware of the
higher than the threshold for ADH secretion; we importance of drinking water even without the feeling
only begin to feel thirsty when our urine is highly of thirst.
Table V. Noon cognitive test means and standard deviations for the hydrated and the dehydrated groups.
Groups
Tests
Hidden Figures
Number Span
Making Groups
Verbal Analogies
Number Addition
Hydrated
(n=19)
Dehydrated
(n= 32)
Mean
SD
Mean
SD
P
-28.10
5.89
2.26
5.89
16.05
57.20
1.89
1.48
1.91
7.91
-46.09
4.28
1.75
5.12
14.78
43.53
1.85
1.30
1.26
7.24
0.21
0.01
0.20
0.09
0.56
Effect of voluntary dehydration on cognitive functions
1671
Table VI. Noon cognitive tests least square means (adjusted by morning cognitive scores) and standard errors for the hydrated and the
dehydrated groups.
~
Hydrated
( n = 19)
Groups
Tests
Hidden Figures
Number Span
Making Groups
Verbal Analogies
Number Addition
Dehydrated
(n = 32)
Mean
SE
Mean
SE
P
-32.71
5.66
2.21
5.75
14.84
8.92
0.42
0.31
0.35
1.04
-43.36
4.39
1.78
5.21
15.50
6.78
0.32
0.24
0.27
0.79
0.36
0.02
0.29
0.24
0.62
The high Uosm values in these two studies were
recorded from samples obtained once a day at noontime. Since very similar climatic conditions exist in
our area for many hours during the day, we assumed
that this mean Uosm reflects the dehydration state
during the day. In the present study, conducted in
the summer with 51 sixth-grade Jewish students, ages
10-12 y, we obtained urine samples twice, the first
at 08.00 and a second at 13.00. Based on Uosm, we
divided the study population into two well-defined
subgroups: about one-third of the children were well
hydrated with morning Uosm <800 mosdkg H 2 0 .
Most of the students in this group (68%) kept a
well-hydrated state at noon-time as reflected by Uosm
<800 mosdkg H20. None of the students in this
group excreted maximally concentrated urine at noontime. These are probably children with better drinking
habits. About two-thirds of the students started the
morning in a dehydrated state reflected by mean
Uosm >800 mosdkg H20. They were defined as
dehydrated with mean morning Uosm of 997 m o s d
kg H 2 0 . Half of the students in this group excreted
urine at a concentration exceeding 1000 m o s d
kg H 2 0 . Eighty-one percent (26 out of 32 students)
continued to excrete urine at a concentration exceeding 800 mosm/kg H 2 0 at noon-time. We assume that
these are the children with poor drinking habits.
The results support our assumption that most of
the children in our geographic zone are in a state of
dehydration during many hours of the day. We propose
that these children living in a hot climate apparently
are in a state of chronic voluntary dehydration. This
state may also be common for children living in other
climatic-geographic locations, where their fluid intake
is not sufficient (e.g., Germany [18] United Kingdom
[19], USA [20]).
during both light and heavy exercise in hot conditions
[2 13. A 2% loss of body fluids was estimated to cause a
20% decrease in physical performance [22]. Children
exercising in the heat are at risk of the adverse effects
of dehydration more readily than adults because
children have greater surface area to body mass than
adults [23].
However, even though it seems logical that academic and cognitive performance might be impaired
by dehydration as well, very few studies had been
conducted on this issue, especially in children.
Gopinhathan et al. [5] found that, after recovery
from exercise in the heat, soldiers demonstrated significant and progressive reduction in the performance
of arithmetic ability, short-term memory, and visuomotor tracking at a body fluid deficit of 2% or more
compared to the euhydrated state. Sharma et al. [6,7]
detected the effects of dehydration at various levels
(1, 2, and 3% of body weight deficit) on mental functions of different complexities in adults. They found
that, at a 1% dehydration level, there was only a marginal decline in the mean score of the coordination
function, but at the 2 and 3% dehydration level, the
performance score was significantly lower compared
to the eu- and 1% dehydration level. Cian et al. [24]
have shown that the response to dehydration, whether
following exposure to heat or exercise-induced, was
a significant reduction in cognitive performance for
various abilities, such as decisional or perceptual
tasks. This reduction seemed to be proportional to the
degree of dehydration, becoming significant after a loss
of 2% of body weight. When dehydration was followed
by fluid ingestion, corresponding to 100% of weight
loss (euhydration), significant improvement in cognitive performance (long-term memory) was observed.
They also reported a significant improvement in shortterm memory following hyper-hydration, compared
with euhydration. All the effects described above
Influence of dehydration on physical and
are based on dehydration induced by heat exposure or
mental pe?$ormance
exercise.
In our study we have not induced dehydration
The effect of dehydration on physical performance and
but
observed it in young students, and, for the first
exercise has been demonstrated in many studies in
athletes and in people working in hot climates [20]. time, demonstrated a direct correlation between their
Even a minor loss of 2% of body fluids negatively hydration state and their achievements in five cognitive
affects heart rate, tolerance time, and stroke volume tests aimed to evaluate concentration ability, visual
1672
Y. Bar-David et al.
attention, immediate memory span, semantic flexibility, and automatic application of arithmetic operations. The adjusted mean scores of the noon-time
cognitive tests for the two groups, with the morning
cognitive test scores as covariates, have demonstrated
an overall positive performance trend in four out of
five of the tests in favor of the hydrated group. The
main contributor of this effect was the Auditory
Number Span test. In other words, young students,
who were dehydrated at the beginning of the school
day, performed inferiorly to their well-hydrated peers
at noon-time on several cognitive tests, especially on
a short-term memory task (auditory number span).
Procedural memory tasks (number addition), perceptual tasks (hidden figures), long-term memory retrieval (verbal analogies), and higher thinking (making
categories) were less affected. It seems that the performance on mental tasks with a high cognitive load,
such as continuous rehearsal of digits for an immediate
recall, tends to deteriorate as a function of being in
a voluntary dehydrated state during the school day.
Mental tasks that are based on automated skills,
perceptual or procedural, long-term memory retrieval,
and controlled thinking have lower cognitive demands,
and are therefore less affected by chronic dehydration.
Hydration and mental performance-explanato y
mechanisms
Several mechanisms have been proposed to explain
the beneficial effects of hydration during or after
exercise. These include a decrease in cortisol level [25],
an increase in serum arginine, increased vasopressin
release, and an increase in glycerol [24,26]. Cortisol
level is known to increase with dehydration and have
detrimental effects on declarative memory, and on
spatial and thinking tasks, and no effect on procedural
memory [27]. Fluid ingestion is related to cerebral
vasopressin levels, which have a positive effect on
memory, and to glycerol level, which positively affects
cognitive performance through glucose supply to the
CNS [24,26].
Other adverse effects of chronic dehydration
Other adverse effects of chronic dehydration include
urinary stone formation [28], risk for certain types of
cancer [29], especially that of the urinary tract [30],
colon [31], and breast [32], inducement of mitral
valve prolapse in susceptible women [33], salivary
dysfunction leading to bad oral hygiene [34], chronic
constipation, and obesity [35].
Physiological and cognitive recovery from chronic
voluntary dehydration
Recovery from a state of dehydration is accomplished
by maintaining a sufficient fluid intake regime, Simply
said, but not simply done. Spontaneous voluntary
water consumption is not enough to achieve a true
state of euhydration, especially in hot climate residences and in exercising children [36,37]. One way
of reversing the physiological consequences of dehydration is through drinking education programs,
as shown by Frank and De Vries [38]. Other ways
to increase water intake are through improving water
taste and temperature [37,39].
On the cognitive level, a primary school in Edinburgh [40] conducted an experiment called “brain
hydration.” Pupils were actively encouraged to drink
water during and between the lessons. The teachers
reported an improvement in concentration. The
children were more settled and more ready to learn.
Overall, the school has more than exceeded its targets
on the national tests [4 11.
Limitations of the study
This study was performed in the context of a hot and
dry summer day. Therefore, the effects of voluntary
dehydration may be compounded with climatic factors,
such as transient or prolonged heat stress. Also, we
could not distinguish in our study between children
who were dehydrated by chance on the particular
day of the study and those who were in a chronic
state of dehydration, i.e., dehydrated for a longer period prior to the study. In addition, this study did not
address questions about physiological (e.g., cortisol
level) and behavioral mechanisms (e.g., effects on
appetite) that may mediate the effects of dehydration.
Finally, this study used Uosm as a proxy for dehydration level. The classical measure of the degree of
dehydration is water loss, expressed as weight loss.
Since the design of this study did not induce dehydration, direct weight loss was precluded.
Conclusions
(1) Many elementary school children residing in hot
climate zones, and perhaps other climatic zones,
are prone to be in a state of chronic voluntary
dehydration, which can be related to their
drinking habits.
(2) Chronic dehydration has adverse effects on
cognitive and physical performance, as well as
on general health.
(3) A vigorous attempt should be made to educate
the population, and especially children, to improve drinking habits. In our opinion, schools
are the best place to start such programs.
(4) An attempt should be made to improve access
to water sources, especially at schools and working places in hot, dry environments.
(5) Further research is recommended on the exact
relations between chronic dehydration and
Effect of voluntary dehydration on cognitive functions
cognitive performance and the physiology
and brain function in a chronic dehydration
state.
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