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The glycemic index of pigmented potatoes is
related to their polyphenol content
D. Dan Ramdath,*Emily Padhi, Aileen Hawke, Theva Sivaramalingam and Rong Tsao
Polyphenol extracts from coloured fruits and vegetables inhibit a-glucosidase in vitro, however it is not
known whether this translates into an attenuation of blood glucose response in vivo. We examined this
relationship in a GI study by feeding coloured potatoes to 9 healthy volunteers. We also examined the in
vitro inhibitory activity of potato anthocyanin extracts on rat intestinal a-glucosidase. Potatoes (Purple
Majesty; Red-Y38; Yukon Gold and Snowden) were fed with skin after cooking in a convection oven,
using a random block design and 50 g available carbohydrate. Glucose was used as the standard and
venous blood collected at 0, 15, 30, 45, 60, 90, 120 min. Areas under the curve (AUC) for glucose and
insulin were calculated, and GI and Insulin Index derived. Neither AUC for blood glucose response nor
insulin was significantly different among the various potatoes studied. Although the mean GI (SE) values
for the potato types varied (purple ¼77.0 9.0; red ¼78.0 14.0; yellow ¼81.0 16.0; and white ¼
93.0 17.0), these differences were not significantly different. The mean (SE) polyphenol content
(mg GAE/100 g DW) was 234 28; 190 15; 108 39; 82 1 for purple, red, yellow and white
potatoes, respectively. There was a significant inverse correlation between polyphenol content and GI of
the potatoes (r¼0.825; p< 0.05; n¼4). In vitro, polyphenol extracts of red and purple potatoes
inhibited a-glucosidase by 37.4 2.2% and 28.7 3.2%, respectively. The GI of coloured potatoes is
significantly related to their polyphenol content, possibly mediated through an inhibitory effect of
anthocyanins on intestinal a-glucosidase.
Introduction
Type 2 diabetes mellitus (T2DM) is a signicant global health
problem, characterized by abnormal glucose tolerance and
insulin resistance, and is oen associated with signicant long
term complications, and poor quality of life.
1,2
T2DM is asso-
ciated with diets high in calories and reduced physical activity;
consequently many dietary interventions have been advocated
for the management and prevention of T2DM.
3
The glycemic
index (GI) of foods has been proposed as evidence based guid-
ance in choosing carbohydrate rich foods on the basis of their
post prandial blood glucose raising potential.
4,5
Potatoes have
been characterized as being a medium to high GI food,
although this varies among different varieties
6,7
and by cooking
and processing methods
8–10
as well as starch digestibility.
9,11
Potatoes are the third largest food crop worldwide following
rice and wheat, and as such play a signicant role in human
health.
12
Potatoes with pigmented esh are becoming more
readily available in the market place and there is growing
interest in their potential health benets. Pigmented red and
purple esh potatoes contain two to three times more
antioxidants than white-esh potatoes.
13,14
The antioxidant
properties of pigmented potatoes are accounted for by the
presence of polyphenols, specically anthocyanins, phenolic
acids and carotenoids.
13,15
Further, consumption of purple and
yellow pigmented potatoes has been associated with a reduction
in inammation and oxidative damage in healthy adult males
when compared to white potatoes.
16
Despite their nutritional
and antioxidant properties, potatoes have been implicated in
contributing to T2DM due to their higher GI values; however, the
GI of pigmented potatoes has not been previously determined.
Foods rich in polyphenols, especially anthocyanins and
condensed tannins, have been associated with reduced glyce-
mic response, with a negative correlation observed between
polyphenol content of leguminous foods and non-leguminous
cereals and blood glucose response.
17
Further, polyphenol-rich
extracts from fruits have also been shown to inhibit the activity
of a-glucosidase, the main enzyme responsible for intestinal
starch digestion.
18
The effect of polyphenol content on the
digestibility of starch in newer cultivars of pigmented potatoes
and blood glucose response has not been studied. We reasoned
that the GI of pigmented potatoes may be lower than that of
white potatoes and that this may be related to the inhibitory
effect of anthocyanins on intestinal a-glucosidase. Further, with
pigmented potatoes emerging in the market place with
increasing frequency, the availability of GI values would be
Guelph Food Research Centre, Agriculture and Agri-Food Canada, 93 Stone Road West,
Guelph, ON N1G 5C9, Canada. E-mail: dan.ramdath@agr.gc.ca; Fax: +1 226-217-
8181; Tel: +1 226-217-8082
Cite this: DOI: 10.1039/c3fo60395d
Received 6th September 2013
Accepted 25th January 2014
DOI: 10.1039/c3fo60395d
www.rsc.org/foodfunction
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Food &
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benecial in making informed dietary choices. The objectives of
this study were to determine the GI values of commonly avail-
able pigmented Canadian potatoes, to dene the relationship
between potato polyphenol content and GI, and to assess the in
vitro inhibitory effect of crude anthocyanin extracts from
pigmented potatoes on intestinal a-glucosidase activity.
Methods
Potatoes
Potato cultivars of Purple Majesty with purple skin and esh,
Y38 with red skin and esh, Yukon Gold with white skin and
yellow esh, and Snowden with white skin and white esh, were
obtained as mature potatoes at fall harvest from the Elora
Research Station, University of Guelph (Elora, Ontario, Canada
43410N, 80260W) on a Conestoga silt loam soil in 2009.
Food analysis
Detailed proximate and dietary bre analysis was performed
commercially (Maxxam Analytics International Corporation,
Mississauga, ON, Canada) using standard AOAC methods for
total fat (AOAC 922.06), protein (AOAC 992.15), total dietary
bre (AOAC 985.29) and moisture (AACC 44-15A). Energy and
total carbohydrates were derived and available carbohydrate
was calculated as the difference between total carbohydrate and
total dietary bre.
Test foods
Fresh whole potatoes were washed in tap water, air dried,
packaged into 50 g available carbohydrate portions and stored
at 20 C. On test days, the potatoes were quickly defrosted in a
microwave oven, cut into 2 cm
3
cubes and baked in a convection
oven for 40 min until sowhen prodded with a fork. Potatoes
were served with skin on, a pinch of salt and pepper and about 5
g of margarine, if desired. Flavourings remained constant for
each participant, for all test foods. The potatoes as well as the
standard glucose solution were served with 300 mL of water.
Participants consumed the foods within 15 minutes.
GI study
This study was conducted according to the guidelines set by the
Declaration of Helsinki and all procedures involving human
participants were approved by the Canadian Shield Ethics Review
Board. Written informed consent was obtained from all partici-
pants. The study protocol was based on the method of Wolever
et al. (1991),
19
was approved by the Canadian Shield Ethics
Review board and registered at http://www.clinicaltrials.gov
(#NCT01053793). The following exclusion criteria were applied:
BMI > 30 kg m
2
, history of drug abuse, pregnancy or lactation,
allergy to potatoes, and abnormal fasting glucose ($5.56 mmol
L
1
), and nine healthy Caucasian adults (3 males, 6 females)
were recruited. On the day prior to testing, participants were
asked to refrain from alcohol, intense physical activity and high
bre foods at dinner. On each study day participants arrived at
the human trial facility (Nutrasource Diagnostics Inc., Guelph,
ON, Canada) in a fasted state (12 h). On the rst visit participants
were given a 50 g standard glucose solution. At subsequent study
visits participants consumed one of the four potato varieties or a
second standard glucose solution in a randomized block design.
Before consumption of the test meal or standard a fasting blood
sample was obtained using an indwelling catheter and addi-
tional blood samples collected at 15, 30, 45, 60, 90, and 120 min
aer the rst bite was taken. Participants remained seated
quietly during the test period. Whole blood samples were
collected into lithium heparin tubes and an aliquot was stored at
80 C pending glucose analysis on a YSI model 2300 STAT
analyzer (Yellow Springs, OH), with fasting samples measured in
duplicate. The remainder of the whole blood was centrifuged to
obtain plasma which was stored at 80 C pending analysis for
insulin using a commercial ELISA kit and multi-level quality
control material (Invitrogen, CA, USA).
Plasma antioxidant status
The oxygen radical antioxidant assay (ORAC) and ferric
reducing antioxidant power (FRAP) assays were used to deter-
mine total antioxidant capacity of serum as outlined by Li
et al.
20,21
Polyphenol analysis
Total polyphenol content (TPC) of ground freeze dried potato
powders was determined in triplicate using an adapted 96-well
micro plate Folin–Ciocalteu method
20,21
and expressed as milli-
gram gallic acid equivalent (GAE)/100 g dry weight basis (mg
GAE/100 g DW). Crude anthocyanin rich extracts were prepared
from 50 g ground freeze dried potato powders that were double
extracted with 500 mL acidied methanol (85% MeOH: 14%
H
2
O: 1% HOAc) and partially puried using ash chromatog-
raphy (Isolera One, Biotage, Sweden, AB). Methanol was
removed by rotary evaporation and the resulting extracts were
freeze dried and stored at 20 C until use. Total anthocyanin
concentration (TAC) was determined in triplicate using a
modied pH differential method
20,21
and expressed as mg cya-
nidanin-3-glucoside equivalent/100 g (mg CyGE/100 g DW). All
general purpose chemicals were of analytical or HPLC grade and
were purchased from Sigma-Aldrich Chemical Co. (St Lois, MO).
a-Glucosidase inhibition assay
Experiments were carried out with crude a-glucosidase from rat
intestinal powder (Sigma I1630: 25 mg mL
1
in 0.1 M phosphate
buffer, pH 6.9) as previously outlined.
18
Anthocyanin extracts
were dissolved in buffer and serial dilutions prepared. Acarbose
(Sigma A8980) was used as a positive control. The assay was
carried out in a 96-well microplate and contained: 100 mLof
extract, acarbose or buffer and 100 mL of enzyme solution. The
reaction was initiated by the addition of 100 mL of enzyme
substrate (5 mM p-nitrophenyl a-D-glucopyranoside; Sigma
N1377), and monitored at 405 nm at 30 C for 10–20 minutes.
Assays were performed in triplicate on three separate days for
each extract. The reaction rates of the assays with and without
extracts were compared, and the a-glucosidase inhibitory
activity determined.
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Data analysis
The incremental AUC for blood glucose response, excluding
area below fasting, was calculated for each potato tested by each
participant using the trapezoid method.
19
GI values were
derived by expressing the AUC as a percentage of the mean AUC
for the reference food; the mean of the resulting values for each
participant was used to determine the food GI. The incremental
area under the blood insulin response curve was calculated and
the insulin index (II) derived in the same manner as for GI
values. Data were entered into Excel, 2007 (MicrosoCorpora-
tion, Washington, USA), veried and then imported to STATA
23
(version 10.0, STATA Corporation, Texas, USA). Differences in
AUC, II and glycemic index were compared by potato type and
time using two-way ANOVA, with post-hoc analysis when
applicable. Spearman rank correlation was used to determine
the relationship between potato GI and polyphenol content.
Differences were considered statistically signicant at p< 0.05.
Results
Table 1 shows the nutritional composition of 50 g available
carbohydrate portions of the test meals. As eaten, the poly-
phenol content of purple and red pigmented potatoes were at
least twice as high as that of yellow and white potatoes,
respectively. Corresponding total anthocyanin (TAC) for purple
and red pigmented potato powders were 152 and 179 (mg CyGE/
100 g DW); no signicant amount of anthocyanin was detected
in the yellow and white potatoes.
Participants (3 males; 6 females) involved in the GI study
were all Caucasian with ages ranging from 23 to 36 years with a
mean (SE) BMI of 25.2 1.1 Kg m
2
and normal fasting blood
glucose levels (4.8 0.2 mmol L
1
). The mean blood glucose
AUC following consumption of purple, yellow, red, and white
potatoes and standard glucose solution were (mmol min L
1
):
90 17, 100 30, 101 26, 116 27 and 125 23, respectively.
One way repeat measures ANOVA revealed that among the
potatoes, blood glucose response at different time points was
similar, but this differed signicantly from the glucose standard
(Fig. 1). At 15 min all potato types showed signicantly lower
blood glucose values (p< 0.005) than the glucose standard;
however at 120 min white, red, and yellow potato types had
signicantly higher blood glucose values than the glucose
standard (p< 0.001).
The blood insulin response aer consumption of potatoes,
shown in Fig. 2, was similar to the glucose response curve with
insulin levels peaking around 45 min. One way ANOVA on ranks
indicated a signicant difference in the blood insulin AUC
between the glucose standard and red potato (H¼10.2; df ¼4;
p¼0.039). However, there were no signicant differences in the
blood insulin concentration between the glucose standard and
potato types at any of the time points. The insulin index (%) for
red, purple, white, and yellow potatoes were: 52 7; 76 10; 78
14 and 81 10, respectively; these were not signicantly
different by one-way ANOVA.
The mean (SE) glycemic index values were 77 9, 78 14,
81 16 and 93 17 for purple, red, yellow and white potatoes,
respectively (Table 1). Neither blood glucose AUC nor glycemic
index differed signicantly among the potato types when
assessed either by repeat measures ANOVA or by one way
ANOVA on ranks.
Plasma antioxidant status (Fig. 3A and B) as measured by
ORAC remained relatively unchanged following the consump-
tion of the different potatoes however, the pattern observed with
FRAP suggested a trend towards decreasing antioxidant status.
Table 1 Nutritional composition of test meals (per 50 g available carbohydrate)
Food
Weight
(g)
Energy
(KJ)
Fat
(g)
Protein
(g)
Total
CHO (g)
Fibre
(g)
Moisture
(%)
Anthocyanin
(mg CyGE)
a
Polyphenol
(mg, GAE)
b
Glycemic
index (%)
Purple potato (Purple
Majesty)
289.0 1020 0.3 6.3 53.8 3.8 78.5 16.4 1.5 145 17 77 9
Red potato (Y38) 375.9 1056 0.5 7.8 54.1 4.1 82.8 15.4 1.2 123 10 78 14
Yellow potato (Yukon
Gold)
267.4 1027 0.3 7.2 54.0 4.0 76.3 nd 68 25 81 16
White potato (Snowden) 274.4 1029 0.3 7.4 53.5 3.6 76.8 nd 52 19317
a
Total anthocyanin (milligram cyanidanin-3-glucoside equivalent/100 g dry weight (mg CyGE)/100gDW).
b
Total polyphenol (milligram gallic acid
equivalent/100 g dry weight (mg GAE/100 g DW)), nd ¼not detected.
Fig. 1 Blood glucose response elicited from different colour potatoes.
Values are means with standard errors represented by vertical bars.
Standard glucose ¼C; yellow potato ¼; white potato ¼B; purple
potato ¼A; red potato ¼:,*- at 15 min, mean blood glucose after
consumption of glucose standard significantly different from all 4
potato types (p< 0.01). **- at 120 min, mean blood glucose after
consumption of glucose standard significantly different from red (p¼
0.007), yellow (p¼0.013) and white potatoes (p¼0.001).
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Polyphenol
Overall, the highest polyphenol concentration was found in
purple potato, whereas the glycemic index of this potato was
lowest compare to the other potato varieties (Table 1). Ranked
correlation showed that there was a signicant inverse rela-
tionship between the total polyphenol content and glycemic
index among different coloured potatoes (r¼0.825; n¼4;
p< 0.05). Crude anthocyanin-rich extracts from red potato and
purple potato, at a concentration of 1.67 mg mL
1
, inhibited the
in vitro action of rat intestinal a-glucosidase by (mean and SE)
37.4 2.2% and 28.7 3.2%, respectively. The same concen-
tration of acarbose, a synthetic a-glucosidase inhibitor used to
manage postprandial blood glucose levels, resulted in 100%
inhibition.
Discussion
Potato consumption is signicant in many different cultures
globally
12
so it is important to provide evidence based guide-
lines on the choice of potatoes. Potatoes have long been regar-
ded as being a high GI food, but more recently the increasing
availability of potatoes with pigmented esh has generated
interest in their potential health benets. Pigmented potatoes
contain signicant amounts of antioxidant due to the presence
of anthocyanins,
14,15,24
and their consumption is associated with
a reduction in inammation and oxidative damage.
16
Given that
anthocyanin extracts from fruits have been shown to inhibit
intestinal a-glucosidase in vitro,
18
it was hypothesized that pig-
mented potatoes would have lower in vivo GI values compared
to white potatoes.
In the present study, consumption of the various pigmented
potatoes resulted in blood glucose AUC that were not signi-
cantly different. However, compared with the glucose standard
all potato varieties showed signicantly lower blood glucose
values 15 min aer consumption. At 120 min white, red, and
yellow potatoes had signicantly higher blood glucose values
than the glucose standard, indicating that the potatoes were
digested slower, as expected. Among the potatoes studied the
resulting GI values were not signicantly different. Importantly,
the potatoes used in this study had GI values ranging from mid-
high (purple) to very high (white). Variations in the GI of potatoes
have been accounted for by differences in the starch fractions
and digestibility,
9–11
and by cooking and storage methods.
8,10,24
According to Foster-Powell et al. (2002),
25
differences in GI
values of the same type of foods may be explained by method-
ological differences in processing, in determining the digestible
carbohydrate content of the test foods and in GI testing, and
inherent botanical differences. The varieties of potatoes used in
the present study had similar total dietary bre levels so their
digestible carbohydrate content was similar. The GI testing
method used in the present study is widely used and utilized key
Fig. 2 Insulin response elicited from different colour potatoes*values
are means with standard errors represented by vertical bars. Standard
glucose ¼C; yellow potato ¼; white potato ¼B; purple potato ¼
A; red potato ¼O,*blood insulin AUC between the glucose standard
and red potato is significantly different (H¼10.2; df ¼4; p¼0.039).
Fig. 3 (A) Plasma ferric reducing antioxidant power (FRAP) values and
(B) oxygen radical absorbance capacity (ORAC) following consump-
tion of potatoes. Mean and SEM; n¼3.
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improvements in human GI studies as recommended by
Wolever et al. (2008).
26
Further, the GI values obtained in the
present study for Red, White and Yellow are similar to values
obtained for same-day cooked and eaten, peeled, boiled or
baked potatoes.
8,10
Most GI studies on potatoes have used boiling as the
preferred cooking method. In the present study frozen potatoes
were thawed via microwave, cooked with skin in a convection
oven and consumed immediately. Fernandes et al. (2005)
7
reported that the mean AUC elicited by day-cooked Russet
Burbank potatoes did not differ signicantly whether baked in a
microwave or conventional oven. In addition, Soh and Brand-
Miller (1999)
10
found no differences in GIs when they compared
cooking methods (peeled and boiled; peeled, boiled and
mashed; peeled and microwaved; peeled and baked). However,
the effect of freezing whole potatoes and microwave thawing
must be considered since ice crystal formation during initial
freezing could have disrupted cells walls and physically separate
starch from amylases through shrinking of gelled starch gran-
ules and cell distortion.
27
On the other hand, microwave heating
increases hydration and consequently reverses retrogradation
of starch. Mulinacci et al.
24
have shown that microwave heating
of potatoes results in a higher level of resistant starch in both
pigmented and white varieties. In the present study there could
have been some resistant starch formation in the cooked
potatoes, although this would have occurred in all potato types.
It would have been ideal to use fresh potatoes in this study and
it is likely that their GI values would have been lower, given the
cell wall disruption and starch separation associated with
freezing. However, this approach would have led to variation in
the batch of potatoes studied.
A study on GI values for commercially available potatoes in
Great Britain also showed that there was no signicant differ-
ence in GI values between different potato types, although the
GI values ranged from 56 to 94.
6
There were no pigmented
potatoes in that study and the variation was accounted for by
differences in the texture of the potato types: waxy texture
produced low GI, whereas oury potatoes had high GI values.
6
It
has been suggested that the latter could be partially explained
by an increased amylopectin content of the oury, more mature
potatoes.
10
Fernandes et al.
7
reported a GI of 89 for boiled red
potatoes and showed that precooking and reheating, or
consuming cold potatoes resulted in a lowering of the GI, which
was accounted for by an increased formation of resistant starch.
It is possible that the GI values obtained in the present study
may be even lower if the pigmented potatoes were consumed
cold or aer precooking and reheating. It has been suggested
that the glycemic responses to consumption of carbohydrate
foods is also inuenced by particle size and the presence of
other macronutrient components, including fat, protein and
dietary bre.
28
However, the role of bioactive minor food
components such as polyphenols, particularly anthocyanins, on
blood glucose response and glycemic index has not been
explored.
It is well known that anthocyanin-rich extracts from fruits
and vegetables exhibit varying but signicant in vitro inhibitory
activity towards a-glucosidase, the main enzyme responsible for
intestinal starch digestion.
18,22
However, few human studies
have explored the relationship between the polyphenol content
of foods and glycemic response in vivo. In a GI study that
examined various leguminous and non-leguminous foods,
Thompson et al.
17
reported a negative correlation between GI
and polyphenol content. Results from the present study show
that within a given food crop GI values are inversely related to
the polyphenol content. Further, results of in vitro studies show
that anthocyanin extracts from the pigmented potatoes display
signicant inhibition of a-glucosidase, and offers an explana-
tion for the lower blood glucose response and GI values
observed for pigmented potatoes. Many of these in vitro studies
have reported that cyanidin 3-glucoside is a potent inhibitor of
a-glucosidase.
17,29
In the present study, no attempt was made to
identify the individual anthocyanin(s) that may account for
a-glucosidase inhibition. However, Li et al.
21
have shown that
the major anthocyanins in Purple Majesty and Y38 varieties of
potatoes (used in the current study), were petunidin and
pelargonidin, respectively. As such, it is unlikely that the inhi-
bition of a-glucosidase by anthocyanin extracts from these
potatoes is due to cyanidin 3-glucoside.
Several studies have shown that purple potatoes contain
more total antioxidant capacity, total phenolics and total
anthocyanins content than yellow, and white potatoes.
13,15,24,30
Further, Kaspar et al.
16
have shown that consumption of purple
potatoes is associated with a reduction in inammation and
oxidative damage. In the present study, we found that there was
very little change in plasma ORAC and FRAP. This suggests that
the GI values obtained were independent of the antioxidant
effects of polyphenols. Kasper et al.
31
reported that although
consumers ranked the aroma and appearance of white and
yellow potatoes higher than purple there were no signicant
differences in overall acceptance of the potato cultivars. It
appears that consumers may be willing to consume pigmented
potatoes, which are perceived to be benecial to health.
31
Taken
together with the lower GI values, the available evidence
suggests that pigmented potatoes (purple and red) may be
healthier choices than white potatoes. These ndings could
lead to greater awareness of consumers and breeders to
increase the availability and usage of pigmented potatoes.
Conclusion
The GI values derived for the potatoes studied could be used to
guide choices of potatoes in order to lower the overall GI and
glycemic load of the diet. Further, with pigmented potatoes
emerging in the market place with increasing frequency the
availability of GI values would be benecial in making informed
dietary choices. Among the four potatoes studied, there was a
highly signicant inverse correlation between polyphenol
content and in vivo GI suggesting a possible inhibitory effect on
intestinal a-glucosidase. Compared to white potatoes, antho-
cyanin rich extracts from the pigmented potatoes displayed
moderate inhibitory activity towards intestinal a-glucosidase in
vitro. Further research is necessary to identify the mechanism
by which polyphenol rich starchy foods inuence blood glucose
response.
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Conflict of interest statement and
contributions
All authors declare no conict of interest. DDR conceptualized
the study, managed the human trial, analyzed the data and
prepared the nal manuscript. AW, EP contributed to the
human trial, laboratory analyses and data analysis; TS assisted
with data analysis and manuscript preparation. RT oversaw
analyses of polyphenols and contributed to the nal
manuscript.
Abbreviations
GI Glycemic index
AUC Area under the curve
II Insulin index
GAE Gallic acid equivalent
TAC Total anthocyanin
CyGE Cyanidanin-3-glucoside equivalent.
Acknowledgements
The authors thank Professors Al Sullivan and Mary Ruth
McDonald of the Department of Plant Agriculture, University of
Guelph for providing the potato samples, Dr Ronghua Liu for
assistance with polyphenol, anthocyanin and antioxidant
analysis, and the NDI staffand volunteers for their involvement
in the human trial. This work was supported in part by the
A-base research program of Agriculture & Agri-Food Canada
(RBPI #1343 and #109).
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