Nutrition 21 (2005) 372–377
www.elsevier.com/locate/nut
Basic nutritional investigation
High-methoxyl pectin has greater enhancing effect on glucose uptake in
intestinal perfused rats
Meehye Kim, Ph.D.*
Department of Risk Analysis, National Institute of Toxicological Research, Korea Food and Drug Administration, Seoul, Korea
Manuscript received January 23, 2004; accepted July 23, 2004.
Abstract
Objective: Pectins have been known to decrease blood glucose levels. However, the mechanism of
this effect is unclear. The direct action of various pectins (high- or low-methoxyl pectins) on the
intestinal absorption of glucose was investigated in gut-perfused rats.
Methods: After equilibrium, jejunal and ileal segments were simultaneously perfused with an
isotonic electrolyte solution (pH 7.4) containing glucose (10 mM/L) and high- or low-methoxyl
pectins (10 g/L). Each test or control solution was perfused in a random sequence, with perfusion
times of 30 min. Changes in glucose concentration of perfusate solution reservoir were determined
over the experimental period.
Results: High- and low-methoxyl pectins in the perfusate significantly inhibited jejunal uptake of
glucose compared with the control (P ⬍ 0.05). High-methoxyl pectins had greater inhibitive effect
on intestinal absorption of glucose than low-methoxyl pectins. The observed changes in glucose and
water absorptions caused by high- or low-methoxyl pectins were reversible by switching to a
pectin-free perfusate. In addition, net water absorption changed to secretion after addition of highor low-methoxyl pectins.
Conclusions: These results suggest that the decrease in intestinal absorption of glucose observed
after perfusion of high- or low-methoxyl pectins may be caused by viscosity-related increases in
mucosal unstirred layer thickness. © 2005 Elsevier Inc. All rights reserved.
Keywords:
Glucose uptake; High-methoxyl pectin; Rats; Low-methoxyl pectin; Small intestine
Introduction
Many studies have shown that soluble dietary fibers such
as pectin and guar gum decrease blood glucose levels and/or
insulin secretion after a sugar load [1–10]. It has also been
reported that soluble fibers improve glucose tolerance by
decreasing the peak of postprandial glycemia and/or by
preventing late hypoglycemia in normal subjects [11] and
diabetic patients [12–14]. Several factors may influence
glycemia after an oral load of glucose: rate of gastric emptying, rate of intestinal absorption, hormonal gastrointestinal response, hepatic glucose balance and cellular metabolism of glucose.
The magnitude of postprandial hyperglycemia in humans
after test meals that contain absorbable and non-absorbable
* Corresponding author. Tel.: ⫹82-2-380-1783; fax: ⫹82-2-380-1786.
E-mail address: [email protected] (M. Kim).
0899-9007/05/$ – see front matter © 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.nut.2004.07.006
carbohydrates is related to the viscosity of the substances.
The effect of increasing meal viscosity on gastric emptying
and on intestinal glucose absorption has been studied
[15,16]. Holt et al. [16] reported that relatively large doses
of guar gum (16 g/meal) and pectin (10 g/meal) in 400 mL
of orange juice markedly decreased the rate of gastric emptying in normal humans, suggesting that this alteration in
emptying may account for the decrease in glucose absorption.
However, the direct effects of viscous fibers on intestinal
glucose absorption remain controversial [17,18]. The mode
of action remains obscure. Because dietary soluble fibers
vary in origin and chemical properties, their physiologic
functions also differ.
Pectin occurs in the human diet in various forms. Pectin
is an important component of the water-soluble dietary
fiber. It is primarily a polymer of ␣-134, glycosidic-linked
D-galacturonic acid units. The extent of esterification of
these units with methanol depends on the source [19].
M. Kim / Nutrition 21 (2005) 372–377
Changes in pectin’s degree of esterification, molecular
weight, and/or mode of distribution of free carboxylic
groups along the polymer chain have been reported to
strongly alter the strength of binding of minerals to pectin
[20 –22]. The effects of pectin on absorption of cholesterol,
bile acids, and other lipids [23–25], on digestion and utilization of proteins [26], on iron bioavailability [27–30], and
on utilization of ␤-carotene [31,32] have been shown to
depend on pectin’s molecular weight and degree of esterification. Recently it was reported that low-methoxyl pectin
ferments faster than high-methoxyl pectin in vivo and in
vitro [33].
However, no previous studies of high- or low-methoxyl
pectins on glucose absorption within the gut have been
reported. The lower part of the intestine has been found to
be an important site for the absorption of nutrients such as
calcium and magnesium in rats fed fructooligosaccharides
[34]. Thus the jejunum and ileum were highlighted in this
study; as a consequence, intestinal absorption was measured
simultaneously in the jejunum and ileum of the same rat to
minimize interanimal variation. Intestinal absorption measurements obtained in the same animal allowed comparisons between segments to be made with greater confidence.
To my knowledge, this is the first study to simultaneously determine jejunal and ileal absorptions of glucose
with high- or low-methoxyl pectins in the same rat by using
an intestinal perfusion technique that excludes the influences of other factors such as gastric emptying on absorption rate.
373
Table 1
Composition of isotonic electrolyte perfusion solution
mmol/L
Sodium chloride
Sodium sulfate
Potassium chloride
Sodium bicarbonate
Glucose
PEG 4000
25
40
10
20
10
1.25 ⫻ 10⫺3
were obtained from Sigma Chemical Co. unless otherwise
specified. In addition, the perfusing solution was gassed
with 95% O2:5% CO2 (v/v) during gut perfusion.
Surgical procedures
Rats were anesthetized (80 mg/kg of body weight) by
means of an intraperitoneal injection of sodium pentobarbital (Entobar, Han Lim Pharmaceuticals, Seoul, Korea),
and body temperature was maintained at 37°C with an
electric heating pad. After laparotomy, the jejunum was
exposed and the ligament of Treitz was localized. A 20-cm
length of proximal jejunum distal to the ligament of Treitz
and a 20-cm length of terminal ileum proximal to the cecum
with intact mesenteric vasculature were cannulated at both
ends and returned to the peritoneal cavity without disrupting
blood flow. The length of each gut segment was measured
with a standard 20-cm silk thread. Anesthesia was maintained by administration of intraperitoneal pentobarbital
throughout the experimental period.
Materials and methods
Experimental protocol
Animals and diets
Male Sprague-Dawley rats (Experimental Animal Breeding Laboratory, Seoul National University, Seoul, Korea)
that weighed 200 to 300 g were fed a non-purified diet
(Rodent Laboratory Chow, Ralston Purina, St. Louis, MO,
USA). Rats were maintained at 22 ⫾ 2°C and 60 ⫾ 5%
relative humidity in a room with a 12-h light, 12-h dark
cycle and given free access to food and water at all times.
High- or low-methoxyl pectins
High-methoxyl pectin (citrus pectin, ⬃90% esterified) and
low-methoxyl pectin (citrus pectin, ⬃30% esterified) were
purchased from Sigma Chemical Co. (St. Louis, MO, USA).
Perfusate
The solution used for jejunal and ileal perfusions is
presented in Table 1. The isotonic electrolyte solution (pH
7.4) contained pectin (10 g/L) and polyethylene glycol 4000
(5 g/L) as a non-absorbable marker for fluid transport.
Polyethylene glycol 4000, glucose, and other chemicals
In each rat, the jejunum and ileum were perfused independently and simultaneously by using an in situ perfusion
technique. After initial perfusion with saline for 10 min,
both segments were perfused with an isotonic electrolyte
solution (Table 1) that contained glucose (10 mM/L) and
high- or low-methoxyl pectins (10 g/L). The following was
applied to each perfused section. The cannulae from the
intestinal loop were connected by Teflon tubing to the
perfusing solution reservoir that was maintained at 37°C. A
peristaltic perfusion pump (Gilson, Middleton, WI, USA)
recirculated solution through the intestinal loop at a flow
rate of 0.5 mL/min (a rate chosen based on preliminary
experiments). For experiments, five rats were perfused with
each test and control solution. The study consisted of two
perfusion periods. The different solutions were perfused in a
random sequence, with perfusion times of 30 min. Each perfusion period was preceded by a 10-min rinse. The adequacy of
this rinse period in removing remnants of the preceding test
solution was confirmed in preliminary experiments. A 50-␮L
perfusate sample was withdrawn from the reservoir every 5
min for 30 min. All perfusions were performed under a heat
lamp (ambient temperature ⬃37°C) while monitoring rectal
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M. Kim / Nutrition 21 (2005) 372–377
temperature, and the abdomen was covered with a gauze pad
moistened with saline. At the end of the perfusion rats were
killed by cervical dislocation, and segments were drained,
removed, and then dried to constant weight at 80°C. The
results were standardized per gram of dry tissue.
Measurements
Analyses
The content of glucose was determined by a commercial
diagnostic kit (no. 996-90901, Wako Chemicals, Osaka, Japan)
based on the reaction of glucose with glucose oxidase and
peroxidase. The amount of polyethylene glycol 4000 was determined by turbidimetric method of Hyden [35].
Viscosity
The relative viscosity of perfusate solution was determined at 37°C by using a Cannon-Fenske viscometer (size
no. 75, Cannon Insurance Co., State College, PA, USA). All
viscosity values are presented as millipascals per second.
Water-holding capacity
The water-holding capacity for high- or low-methoxyl
pectins was calculated as the weight of water held per gram
of dry material. It was determined by modification of a
dialysis method [36].
Statistical analyses
Paired t tests were used to determine whether significant
differences existed at each time point between high- or
low-methoxyl pectins and control groups and between jejunum and ileum within the same rat. Unpaired t test was
conducted on data obtained between high- and lowmethoxyl pectin groups from separate rats. Differences were
considered statistically significant at P ⬍ 0.05 [37]. Data
were expressed as means ⫾ standard error of the mean for
five observations.
Results
The effect of high-methoxyl pectin (10 g/L) on jejunal
and ileal glucose uptake in perfused rats is shown in Fig. 1.
High-methoxyl pectin significantly decreased the uptake of
glucose in the jejunum and ileum of rats (P ⬍ 0.05). The
effect of low-methoxyl pectin on intestinal glucose uptake is
presented in Fig. 2. Addition of low-methoxyl pectin decreased the absorption rate of glucose from the jejunum in
perfused rats (P ⬍ 0.05). Uptake of glucose was linear in
the presence and absence of high- or low-methoxyl pectins.
High- and low-methoxyl pectins significantly inhibited uptake of glucose from the jejunum and decreased the absorption rate of glucose in rats (P ⬍ 0.05).
The effects of high- and low-methoxyl pectins on the
intestinal absorption of glucose after recirculation for 30
Fig. 1. Effect of HP on (a) jejunal and (b) ileal glucose uptake during
intestinal perfusion of rats. Values are means ⫾ standard error of the mean,
n ⫽ 5. *Significantly different from control, P ⬍ 0.05. HP, high-methoxyl
pectin.
min in perfused rats are shown in Figs. 3 and 4 respectively.
High-methoxyl pectin significantly decreased glucose absorption by 56% (P ⬍ 0.05) in the jejunum and by 52% (P
⬍ 0.05) in the ileum compared with the control. Addition of
low-methoxyl pectin decreased glucose absorption from the
jejunum by 18% (P ⬍ 0.05) and from the ileum by 13% (P
⬎ 0.05). The inhibitive effect of high-methoxyl pectin was
much greater than that of low-methoxyl pectin (P ⬍ 0.05).
The decrease in glucose absorption caused by high- or
low-methoxyl pectins was reversible by switching to a pectin-free (control) perfusate. However, no significant difference in glucose absorption between the jejunum and ileum
for each group of rats was observed.
The effect of high- or low-methoxyl pectins on the net
absorption of water is presented in Table 2. About 9% net
absorption of water occurred with glucose control solutions.
When high- or low-methoxyl pectins were added to the
perfusate solution, it reversed the net absorption to a net
secretion of 18% to 19% of water (P ⬍ 0.05). The observed
water secretion disappeared after switching to a pectin-free
perfusate.
The water-holding capacity was 43.7 ⫾ 1.6 g of water
per gram of high-methoxyl pectin and 23.4 ⫾ 1.2 g of water
per gram of low-methoxyl pectin. Also, relative viscosities
were 141 ⫾ 3.2 mPa/s for high-methoxyl pectin and 107 ⫾
2.9 mPa/s for low-methoxyl pectin compared with the control (10 ⫾ 0.3 mPa/s).
M. Kim / Nutrition 21 (2005) 372–377
375
Fig. 4. Effect of LP on the percentage of intestinal absorption of glucose
after recirculation for 30 min in perfused rats. Values are means ⫾ standard
error of the mean, n ⫽ 5. *Significantly different from control, P ⬍ 0.05.
LP, low-methoxyl pectin.
Fig. 2. Effect of LP on (a) jejunal and (b) ileal glucose uptake during intestinal
perfusion of rats. Values are means ⫾ standard error of the mean, n ⫽ 5.
*Significantly different from control, P ⬍ 0.05. LP, low-methoxyl pectin.
Discussion
In the present study, the direct effects of high- or lowmethoxyl pectins on intestinal glucose uptake were examined.
The results clearly demonstrated that high- and low-methoxyl
pectins significantly decrease intestinal absorption of glucose.
The inhibitive effect of high-methoxyl pectin on glucose absorption was much greater, probably due to the higher viscosity compared with low-methoxyl pectin. The results from this
study are in agreement with reports of many investigators who
found decreases in absorption of glucose due to the presence of
soluble fibers [17,38,39]. Moreover, the uptake of glucose was
linear in the presence and absence of high- or low-methoxyl
pectins (Figs. 1 and 2).
Guar gum has been observed to significantly decrease the
net absorption of glucose from 74% to 41% in growing pigs
[17]. Blackburn and Johnson [40] found that the rate of
glucose absorption was significantly decreased in the jejunum of rats pre-perfused with 6 g of guar gum per liter of
solution compared with the control. In addition, highmethoxyl apple pectin (6, 10, and 15 g/L) decreased intestinal glucose absorption in humans by means of an increased unstirred layer resistance [39]. My colleagues and I
previously reported that addition of chicory water-soluble
extract from chicory root (10 g/L) or inulin (10 g/L) significantly decreased intestinal absorption of glucose in perfused rats [41]. Many studies have shown that soluble fibers
are effective in controlling blood glucose levels in humans
[13,16]. The principal effect of adding soluble fibers to the
diet is a decrease in postprandial hyperglycemia.
However, some studies have demonstrated that dietary
fibers have no effect on glucose absorption [18]. Foster and
Hoos [42] observed that neither pectin nor cellulose impaired jejunal glucose absorption, but that pectin decreased
Table 2
Effects of high-methoxyl (HP) or low-methoxyl (LP) pectins on the net
absorption of water in perfused rats
Fig. 3. Effect of HP on the percentage of intestinal absorption of glucose
after recirculation for 30 min in perfused rats. Values are means ⫾ standard
error of the mean, n ⫽ 5. *Significantly different from control, P ⬍ 0.05.
HP, high-methoxyl pectin.
Site
Control
HP
Control
LP
Jejunum
Ileum
8.8 ⫾ 0.5
9.0 ⫾ 0.5
⫺17.6 ⫾ 0.9*
⫺18.7 ⫾ 1.0*
9.2 ⫾ 0.6
9.4 ⫾ 0.7
⫺16.7 ⫾ 0.6*
⫺17.9 ⫾ 0.6*
Values are %, means ⫾ standard errors of the mean, n ⫽ 5.
* Significantly different than the control, P ⬍ 0.05.
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M. Kim / Nutrition 21 (2005) 372–377
serum glucose responses to an oral carbohydrate load. They
also found that long-term supplementation (5 wk) with
cellulose (10%) or pectin (5%) impaired intestinal glucose
absorption and decreased serum glucose responses in rats.
The effect of soluble fibers on intestinal absorption of glucose is controversial. Contradictory data may result from the
differences in the type of soluble fiber (e.g., physicochemical
property), experimental period, experimental technique, levels
of soluble fiber and glucose, and species studied. Its mode of
action remains unclear. Several hypotheses have been proposed to explain its action within the gut. These include a
decreased rate of emptying of the stomach, altered motility in
the stomach and small intestine, poorer mixing of dietary
components in the small intestine, and a decreased rate of
absorption across the epithelial cell membrane. In addition, a
slower rate of diffusion of nutrient from the gut lumen toward
the epithelial surface and an increase (⬃48%) in the thickness
of the mucosal unstirred layer could contribute to the decreased
nutrient absorption by soluble fibers [43]. Guar gum might
result in distention of a perfused intestinal loop, leading to a
wider separation of villi and a larger surface area for inhibitory
effects on absorption to occur [44].
In an attempt to define quantitatively the factors that
control intestinal absorption of nutrients [39,40] and drugs
[45,46], studies are being undertaken in the perfused rat
intestinal preparation as a model of events in humans. It is
believed that the results in rats are likely to have application
to humans [47]. The intestinal perfusion technique is conducted on live animals and avoids the influence of gastric
emptying, and the everted sac technique is conducted in
vitro without blood flow. Therefore, the in situ intestinal
perfusion technique, which is thought to be the ideal methodology for investigation, was used to determine whether
any food component directly affects nutrient absorption in
the small intestine [48].
At the perfusion rate (0.5 mL/min) used in this study,
intestinal glucose absorption was inhibited when high- or lowmethoxyl pectins (10 g/L) were added. However, Schwarz and
Levine [18] reported a lack of inhibition of intestinal glucose
absorption by pectin (10 g/L) at a perfusion rate of 1 mL/min.
In addition, viscosity-related inhibition by guaran depended on
the rate of perfusion and was detectable only at perfusion rates
below 0.4 to 0.5 mL/min [38]. Higher perfusion rates abolished
and even reversed the inhibitory effect. Mucosal unstirred
water layer effects are not readily demonstrated in situ because
of high perfusion rates [49] or segmented flow conditions
[50,51] required to decrease its apparent thickness with aqueous solutions of low viscosity. One of the most important
points in attempts at reproducing physiologic conditions in
animals and humans is the selection of the correct perfusion
rate. The nature of the polysaccharide, its concentration in
solution, and the rate of perfusion may also be important in
such studies.
There was no measurable endogenous secretion of glucose into the loops. Therefore, endogenous glucose was not
corrected. A marked decrease in water absorption from the
perfusate solution containing high- or low-methoxyl pectins
was observed (Table 2). This may be related to the waterholding capacity of these compounds. Blackburn and Johnson [52] showed that preperfusion of rat intestine with a
guaran solution decreased net water absorption during a
subsequent guaran-free perfusion. Further, Stephen and
Cummings [36] observed that guar gum significantly decreased net absorption of water from glucose solution, from
42.7% to 8.3%, and from the maltose solution, from 49.2%
to 5.1%, in humans. After a 24 h-incubation period, they
found that 1 g of guar gum could hold 21.4 g of water. In
this study, water-holding capacities were 43.7 g of water per
gram of high-methoxyl pectin and 23.4 g of water per gram
of low-methoxyl pectin when high- or low-methoxyl pectins
were incubated for 24 h. Using these values, the waterholding capacities of high- or low-methoxyl pectins used in
the present study would be 437 mL/L for high-methoxyl
pectin and 234 mL/L for low-methoxyl pectin when infused
for 24 h. During the perfusion period of 30 min, the high- or
low-methoxyl pectins would hold less. The decrease in net
water absorption as a result of the presence of high- or
low-methoxyl pectins ranged from 260 to 280 mL/L (based
on Table 2). Therefore, the water-holding capacity would be
a major factor responsible for the decreased net water absorption from the solution containing high- or low-methoxyl
pectins. Also, no dehydration due to water secretion during
the period of perfusion was observed.
In conclusion, addition of high- or low-methoxyl pectins
decreased intestinal glucose absorption from perfused intestine. There are distinct influences of structural parameters of
the pectin on the effect of glucose uptake. It is suggested
that products made of high- or low-methoxyl pectins would
be beneficial to healthy people and those who have diabetes.
Especially high-methoxyl pectin may have a greater potential for decreasing postprandial hyperglycemia by decreasing intestinal absorption of glucose. This might result from
an increase in the mucosal unstirred layer thickness due to
the increased viscosity of high-methoxyl pectins.
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