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Faculty of Bioresources, Mie University, Kurima Machiya 1577, Tsu 514-8507, Japan and * Department of Nutrition and Food Science, Gifu Women’s University, 80 Taromaru, Gifu 501-2592, Japan
3To whom correspondence should be addressed. E-mail: takahashitoru71{at}nifty.com.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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30 mL into the small intestine of rats. Cellulose, but not cellobiose, decreased the postinfusion plasma glucose concentration (P < 0.05), although cellulose did not cause adsorption or dilution of glucose. Among the physical properties of the artificial digesta, only viscosity was responsible for the decrease in the plasma glucose concentration, whereas water content, free water, bound water, and osmotic pressure were not. The cellulose-induced increase in digesta viscosity may delay glucose diffusion in the lumen, as found in our previous study. Cellulose also stimulated water absorption from the small intestine (P < 0.05), which may be attributable to increases in the water potential of the digesta moving through the small intestine. The ingestion of cellulose with meals, which increases digesta viscosity, is likely to modulate the postprandial plasma glucose concentration and to reduce the incidence of diarrhea associated with enteral nutrition.
KEY WORDS: • crystalline cellulose • viscosity • intestinal contents • plasma glucose • water potential
Cellulose is an insoluble dietary fiber source that is generally considered to be rather inert in the upper gastrointestinal tract (1), in part because insoluble dietary fibers were thought to have no effect on the viscosity of the digesta (2). Furthermore, the presence of insoluble fibers was thought not to affect absorption (3), with the exception of indirect effects caused by adsorption and dilution (4). This view was based on measurements of the viscosity of the digesta after the complete removal of particles by centrifugation (1). However, using a cone-plate viscometer or a tube-flow viscometer constructed in the laboratory (5), we found recently that insoluble particles and crystalline cellulose considerably increase the viscosity of the whole digesta, including particulate matter, in the stomach, small intestine, and cecum of pigs (5,6), chickens (7), and rats (8).
These and other observations led us to hypothesize that the viscosity of the digesta influences absorption by affecting the diffusion and behavior of nutrients in the intestinal lumen (9,10). In our previous study, we determined that the diffusion and behavior of nutrients in the lumen are directly involved in the flow pattern of the digesta in the lumen (10). Using Reynolds numbers for digesta flow in the lumen, we found that the nutrients should reach the epithelium not by turbulence or vortex but by self-diffusion in the lumen, even when the motility of the intestinal wall was normal (10). We proposed that the diffusion rate, which is negatively correlated with the viscosity of the digesta, becomes the rate-limiting factor for the overall absorption rate of nutrients (10). Accordingly, in rats fed a diet containing crystalline cellulose, the increased viscosity of the digesta might diminish the absorption of nutrients by depressing the diffusion rate (8). Therefore, the purpose of the present study was to determine whether the increased digesta viscosity induced by the addition of crystalline cellulose affects plasma glucose concentrations. We also examined the effects of crystalline cellulose on water absorption using water potential.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Artificial digesta for infusion into the small intestine. We prepared 3 types of artificial digesta containing 20 g/L carboxymethyl cellulose (Nacalai Tesque) and 50 g/L (278 mmol/L) D-glucose in distilled water and supplemented with either 100 g/L cellobiose (Nacalai Tesque) or 100 g/L crystalline cellulose (crystal diameter, 6–10 µm; Avicel; Funakosh), or containing no additives (control). The coefficients of viscosity of the artificial digesta were measured using a digital cone-plate viscometer (RVDV-I with a CPE-52 spindle cone; Brookfield Engineering Laboratories). The viscosities of the control and the cellobiose- and cellulose-containing artificial digesta were 430, 490, and 880 mPa · s, respectively, at a shear rate of 10 s–1 (Fig. 1). We previously reported that the viscosities of the digesta in the stomachs of rats fed diets with and without cellulose were 760 and 440 mPa · s, respectively, at a shear rate of 10 s–1 (8). Thus, the viscosities of the cellulose-containing and control artificial digesta were similar to those of the stomach contents of rats fed diets with and without cellulose, respectively (8). The viscosity of the artificial digesta containing cellobiose was similar to that of the stomach contents of rats fed diets without cellulose.
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We calculated the water flux between the intestinal lumen and the tissues by determining the water content of the artificial digesta before infusion and at the time of killing, 80 min after infusion: Water flux = (Water content before infusion) – (Water content at killing).
Catheterization of the small intestine. Plasma glucose concentrations are affected not only by the absorption of glucose from the small intestine and by glucose metabolism in the tissues but also by gastric emptying (13). In the present study, the effects of gastric emptying were eliminated by infusing the artificial digesta directly into the duodenum via an intestinal catheter, permitting glucose absorption from the small intestine to be examined without confounding factors.
After 24 h of food deprivation, the rats were anesthetized by diethyl ether inhalation through spontaneous respiration and maintained under anesthesia throughout the experiment. Via a midline laparotomy, a 5-mm incision was made on the greater curvature of the stomach after ensuring hemostasis by ligation of the blood vessels on the stomach wall. A 40-mm-long, small-bore silicon tube (i.d., 1.5 mm; o.d., 2.5 mm) was connected to a 1.2-m silicon tube with a slightly larger bore (i.d., 2.0 mm; o.d., 4.0 mm). The free end of the small-bore silicon tube was inserted through the incision in the stomach wall and into the duodenum through the pyloric sphincter so that 10 mm of the tube was placed in the duodenum. To eliminate regurgitation of the artificial digesta, the tube was then fastened in the pylorus using 6–0 nylon monofilament on a curved atraumatic needle (1/2-circle, 14 mm; ELP) and avoiding the larger blood vessels of the stomach wall. The other end of the silicone tube was exteriorized through the laparotomy. The incisions in the stomach and abdominal wall were closed with interrupted 4–0 nylon monofilament sutures.
Before the infusion of the artificial digesta, we confirmed by visual inspection the absence of digesta in the stomach of the experimental rats and in the small intestines of 2 rats not used in the experiment. Immediately after the last plasma glucose determination in all experimental rats, we also confirmed that the artificial digesta that had been infused into the duodenum had not reached the terminal ileum. Thus, the glucose, cellobiose, and cellulose present in the artificial digesta were not fermented in the large intestine during these experiments.
We infused the artificial digesta into the duodenum for 5 min at a rate of 0.6 ± 0.05 mL/min using a peristaltic pump (MP-3; Eyela). The amounts of artificial digesta infused in the control, cellobiose, and cellulose groups (P = 0.1) did not differ among the groups. We also measured the postprandial plasma glucose concentration after the infusion of the artificial digesta.
Density of artificial digesta. Before the infusion, we measured the volumes and weights of the control and the cellobiose- and cellulose-containing artificial digesta using a 50-mL measuring cylinder; we used these values to calculate the densities (kg/L). The density of each artificial digesta (0.972 kg/L for control, 1.009 kg/L for cellobiose, and 1.007 kg/L for cellulose) was then used to calculate the volume of digesta infused [(mass of infused artificial digesta in kg)/(density of artificial digesta in kg/L)] and the weight of glucose and carboxymethyl cellulose infused [(infused volume in L) x 0.05 and 0.002 kg/L, respectively]. Neither the volumes of the infused digesta nor the weights of infused glucose and carboxymethyl cellulose differed among the artificial digesta groups (3.24 ± 0.04, 0.160 ± 0.001, and 0.064 ± 0.001 g, respectively; P > 0.1).
Blood sampling. We collected blood from the caudal vein at 0, 5, 15, 30, 45, 60, and 80 min after the start of the artificial digesta infusion (postinfusion) using a heparin-treated capillary tube, which was centrifuged to obtain plasma for measurement of plasma glucose concentrations (1500 x g, 15 min). Immediately after the blood sampling at 80 min postinfusion, the rats were killed by transection of the jugular vein while still anesthetized. Within 3 min of death, the artificial digesta in the small intestinal lumen were collected for determination of the glucose concentration (mmol/L), coefficient of viscosity (Pa · s), and water content (g H2O/g digesta). The plasma glucose concentrations and glucose concentrations in the artificial digesta collected after killing were measured using a commercial kit (Glu-CII; Wako). The coefficients of viscosity of the artificial digesta were measured using a digital viscometer (RVDV-I with a CPE-52 or CPE-41 cone spindle; Brookfield Engineering). The water contents of the artificial digesta were measured using a vacuum freeze dryer.
Adsorption of glucose to cellulose in vitro. We determined the extent of adsorption of glucose to crystalline cellulose in vitro by adding crystalline cellulose to a solution of 2 g/L (11.1 mmol/L) glucose in water (n = 3) and mixing well. The cellulose suspensions and triplicate samples of the control solution without added cellulose were allowed to stand for 20 min. After centrifugation for 5 min at 10,000 x g, the supernatants were collected, and the glucose concentrations were measured using the Glu-CII kit.
Calculation of water potential and flux of water. The increased absorption of water from the artificial digesta containing cellulose can be explained by examining the water potential in the intestinal lumen (14). Water potential is the tendency of water to move, such that water flows from high to low water potential. It is calculated by subtracting the solute potential from the pressure potential (14). We estimated the pressure drops (the difference in pressure along a tube and the driving force for flow) produced by segmental contractions and peristalsis of the small intestine by using an equation developed from the Hagen-Poiseuill law (15) to calculate the estimated pressure drops for each of the 3 types of artificial digesta passing through in an imaginary circular tube. In this mathematical model, an imaginary 2.0-mm tube was used to represent a rat small intestine of average dimensions (unpublished data). We also assumed that segmental contractions had a width of 2.0 mm in the longitudinal axis and a depth of 2.0 mm, and that each contraction lasted 1 s, which represents a normal physiological condition (16).
First, we calculated the volume flow rate for segmental contractions (sc) from the values of the radius of the imaginary tube (2.0 mm), the duration of the contractions (1 s), and the width of each contraction (2.0 mm) using the equation: Volume flow ratesc (m3/s1) = {[(Radius of imaginary tube (m)]2 · 
· [Constriction width (m)]} · [Duration of contraction (s)]–1 · 2–1.
The digesta in the lumen are also moved by peristalsis. We estimated that, for the 3 types of artificial digesta passing through the same imaginary circular tube, the mean current velocity produced by peristalsis was 30 mm/s, which is consistent with physiological conditions (17).
We then calculated the volume flow rate for peristalsis from the mean current velocity of 30 mm/s using the equation: Volume flow rateperistalsis (m3/s) = 0.030 (m/s) · { · [radius of imaginary circular tube (m)] 2}.
Finally, the pressure drops produced by segmental contractions and peristalsis were calculated for the 3 types of artificial digesta: Pressure drop/100 mm (Pa · 10–1 · m–1) = Volume flow rate (m3/s) + {B/(3 · A) · · [radius of imaginary circular tube (m)3]}/{ · [radius of imaginary circular tube (m)4] · (8 · A), where A and B are constants. A and B were derived from the equation representing the relation between shear stress (dependent factor) and shear rate (independent factor): Shear stress (Pa) = A · Shear rate (s–1) + B, where A and B were 0.22 and 0.38, 0.45 and 3.3, and 7.4 and 10.3 for the control and the cellobiose- and cellulose-containing artificial digesta, respectively, and the shear stress (Pa) was calculated from the viscosity (Pa · s) by multiplying by the corresponding shear rate (s–1). These pressures drops can be considered to be similar to potential pressure differentials from the lumen to the tissues.
The osmolalities of the digesta and the tissues were converted to solute pressures (kPa) using the conversion factor of 2480 kPa/(mmol/kg) (18). The solute potentials of the artificial digesta were calculated from the osmotic pressures of the artificial digesta multiplied by 2480. The solute potential of the tissue was estimated as equivalent to that of normal saline (0.9% NaCl solution) and was calculated using the equation: 9.0 g/L · (58.5 mol/g)–1 · 2480 kPa · 2.0. The difference of the water potential from the lumen to the tissues was estimated as the water flux across the mucosa (14) as follows: Water flux across the mucosa = C · (water potential in the lumen –water potential in the tissue), where C is the hydraulic conductivity across the mucosa and is constant. Given that the water potential is calculated by subtracting the solute potential from the pressure potential (see above), the flux of water across the mucosa can be calculated as follows: Flux of water across the mucosa = C · (
Pressure potentialacross the mucosa – Solute potentialacross the mucosa).
Statistics. The results are expressed as means ± SEM. We analyzed the differences in the plasma glucose concentrations among the artificial digesta groups (control, cellobiose, and cellulose) and over the time course of the experiment (0–80 min) using 2-way ANOVA after log-transformation, with subsequent Tukey multiple comparison tests (19). Similarly, the differences in the viscosities among the artificial digesta collected from the lumen at killing among the groups and measured using various shear rates (1 to 40 s–1) were examined using the same statistical tests. One-way ANOVA with subsequent Tukey multiple comparison tests (19) was used to test the differences in the water contents and the glucose concentrations in the digesta collected from the lumen at killing among the artificial digesta groups and to test the differences in the glucose concentrations in the supernatants of the plain glucose solution and in those of the cellulose suspensions. Differences were considered significant when the P-value was < 0.05. All statistical analyses were performed using JMP version 5 software (SAS Institute Japan).
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Postprandial plasma glucose concentration. There was no significant 2-way interaction between the type of artificial digesta and the time postinfusion on the plasma glucose concentration (P = 0.6; Fig. 2). The plasma glucose concentration was lower in the cellulose group than in the control and cellobiose groups (P < 0.02, Fig. 2).
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Calculated flux of water across the mucosa using water potential. We calculated that flux of water across the mucosa was higher for the artificial digesta containing cellulose than for the control digesta, because the hydraulic conductivity was constant (Table 2).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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We also examined the adsorption of glucose to crystalline cellulose in vitro by determining the glucose concentrations in the supernatants of glucose solutions that had been incubated with and without cellulose. The glucose concentrations did not differ in the 2 conditions, indicating that glucose did not significantly adsorb to cellulose in vitro. Our findings suggest that crystalline cellulose does not exhibit either glucose adsorption or dilution effects. Thus, the effects of adsorption and dilution could be excluded from our evaluation of the effects of the physical properties of the artificial digesta on the plasma glucose concentrations.
The effects of physical properties of the artificial digesta. We summarized the physical properties of the artificial digesta in Table 3. The differences in the water activities of the 3 types of artificial digesta suggest that the free water contents of the artificial digesta containing cellobiose and cellulose were lower than those of the control digesta. The total water contents of the control and the cellobiose-containing artificial digesta did not differ. The bound water, which is the difference between the total water content and the free water content (20), of the artificial digesta containing cellobiose and cellulose was presumed to be higher than that of the control digesta because of the similarity of the water content in the cellulose- and cellobiose-containing digesta. Theoretically, cellobiose and cellulose have a similar number of hydroxyl groups on a weight basis. Accordingly, the amounts of bound and free water were presumed to be similar in the digesta containing cellobiose and cellulose.
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0.278, 0.570, and 0.278 mmol/kg, respectively. Consequently, the osmotic pressure of the digesta containing cellobiose was assumed to be higher than those of the control and cellulose-containing digesta (Table 3). Decreased plasma glucose concentration in rats infused with cellulose-containing digesta. The infusion of artificial digesta containing crystalline cellulose resulted in a smaller plasma glucose concentration than that caused by the infusion of the control digesta or the cellulose-containing digesta (P < 0.02, Fig. 2), which was consistent with only a few previous studies (21). The smaller plasma glucose concentration in the cellulose group and the similarity of the plasma concentrations in the cellobiose and control groups (Control = Cellobiose > Cellulose) were not attributable to differences in the water content, free water, bound water, or osmotic pressure among the 3 types of artificial digesta (Table 3). Among the physical properties, only the viscosity of the artificial digesta was responsible for the plasma glucose concentration (Control = Cellobiose > Cellulose; Table 3).
The concentration of glucose remaining in the artificial digesta collected from the lumen at the time of killing was higher in the cellulose-containing ingesta than in the control digesta (Table 1), suggesting that cellulose retarded glucose absorption. The glucose absorption rate is positively correlated with the plasma glucose concentration (22). Thus, the suppressive effect of cellulose on the plasma glucose concentration was probably attributable to the decreased rate of glucose absorption.
Flow behavior and viscosity of digesta. In examining the process of absorption, it is important to determine the extent of micromixing in the contents of the intestine. Micromixing is the molecular-scale mixing of digesta that directly influences chemical reactions and absorption (23). In our previous study, we estimated that the flow of digesta in the small and large intestines is laminar, even in the presence of segmental contractions and peristalsis. This suggests that rapid micromixing with turbulence does not occur, but micromixing with self-diffusion in the lumen does occur (10). Indeed, complete mixing with turbulence is unlikely to occur in the intestinal lumen (24,25). That mixing with turbulence never occurs allows the nutrients in the lumen to be absorbed only if they reach the epithelium by self-diffusion (10). Although some mixing and folding of digesta is probably induced by segmental contractions of the small intestine (10) and contributes to increased glucose absorption rates (9), the self-diffusion of nutrients is an important determinant of the absorption rate. Therefore, the overall absorption rate is likely to be limited by self-diffusion rather than by transepithelial transport, because the diffusion rate is slower than the transepithelial transport rate (10).
The diffusion of nutrients in the intestinal lumen should decrease at the inverse of the viscosity of digesta (26). Therefore, the viscosity of the artificial digesta may be an important factor influencing glucose absorption (10). Our present findings concerning the effect of crystalline cellulose on glucose absorption are consistent with our previous suggestion that the digesta viscosity affects the overall absorption rate (10).
Changes in water potential caused by cellulose as a possible explanation of increased water absorption. The slightly greater water flux observed in the cellulose group compared with the controls (Table 2) would lead to further increases in the viscosity and in the pressure caused by segmental contractions and peristalsis, and consequently to further stimulation of water absorption. In fact, the estimated differential pressure potential across the mucosa for the cellulose-containing artificial digesta at the time of killing (2.06 kPa) was 7.6 times that of the control digesta (15.7 kPa), as calculated using the equations above and the viscosity of the digesta at the time of killing. Therefore, cellulose apparently stimulated water absorption, which is associated with the increased viscosity and the water potential of digesta.
Viscosity and water flux for the artificial digesta containing cellobiose. The water content of the artificial digesta containing cellobiose was higher than that of the control digesta (Table 1); nonetheless, we observed a negative water flux (i.e., apparent secretion of water into the lumen) in the cellobiose group (Table 1). This may be attributable to the comparatively lower water potential of the cellobiose-containing artificial digesta, which is due to its greater osmotic pressure and solute potential (Table 2).
Another possible explanation, without reference to the water potential, is that the efflux of water into the lumen is positively associated with the osmotic pressure in the small intestinal lumen (27). The higher osmotic pressure of the cellobiose-containing artificial digesta (Table 2) may have "dragged" water into the lumen.
The viscosity of the artificial digesta containing cellobiose was lower than that of the control at the time of killing (P < 0.05, Fig. 3). The viscosity of the digesta is altered by the water flux across the mucosa during passage in the intestinal lumen (27). The decreased viscosity of the artificial digesta containing cellobiose at the time of killing is attributable to the efflux of water into the lumen.
In conclusion, the results of this and previous studies (5–8,10) strongly suggest that the increased viscosity of digesta containing insoluble dietary fibers such as cellulose diminishes glucose absorption by retarding diffusion within the luminal contents and stimulates water absorption by elevating the water potential. Accordingly, insoluble dietary fibers such as cellulose should not be considered to be inert bulk. The activities of insoluble dietary fibers described here should be taken into account in the preparation of meals and experimental diets.
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FOOTNOTES |
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2 Supported by a Grant-in-Aid for Scientific Research (no. 15.8732).
Manuscript received 17 March 2005. Initial review completed 7 May 2005. Revision accepted 27 July 2005.
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