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CSIRO Health Sciences and Nutrition, Adelaide 5000 Australia
4To whom correspondence should be addressed. E-mail: david.topping{at}csiro.au.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSPreparation of acylated starchesDietary studiesRESULTSDISCUSSIONLITERATURE CITED |
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0.18 were prepared on a larger scale. Body weight gain did not differ between rats fed these acylated starches or a control starch for 14 d. Large bowel pH was significantly lower and digesta mass significantly higher throughout the large bowel in rats fed the acylated starches. Cecal + distal colonic starch averaged 12 mg in rats fed the control starch and 103, 134 and 135 (pooled SEM = 6) mg in rats fed acetylated, propionylated or butyrylated starch, respectively. Large bowel SCFA concentrations and pools were significantly higher in rats fed the three acylated starches and were disproportionately greater in the SCFA that had been esterified to the starch. In the cecum, acetate, propionate and butyrate pools were 280, 690 and 1060% higher, respectively, in rats fed the corresponding acylated starch than in those fed the control diet. In the distal colon, the corresponding increases were 320, 940 and 1370%. These data indicate that acylated starches are resistant starch (RS) and raise large bowel SCFA, apparently through bacterial release of the esterified fatty acid and fermentation of the residual starch.
KEY WORDS: • acylated starches • large bowel • rats • short-chain fatty acids • starch
SCFA, principally acetate, propionate and butyrate, are produced in the large bowel of humans and other omnivores by bacterial fermentation. The main fermentative substrates are undigested dietary carbohydrates, including nonstarch polysaccharides (NSP,4 major components of dietary fiber) and resistant starch (RS). Of these, RS seems to be the more important substrate, quantitatively (1,2). Collectively, SCFA act to maintain the normal physiologic function of the large bowel. Their effects include modulation of colonic muscular activity, stimulation of electrolyte and fluid uptake and enhancement of blood flow [for reviews see (2,3)]. Of the major acids, propionate and butyrate are thought to have more specific actions. Butyrate appears to be a preferred oxidative substrate for colonocytes. It also acts to maintain a normal phenotype in these cells through repair of damage to DNA and inhibition of apoptosis in normal cells and its promotion in colon tumor cells in vitro (2,4). Evidence of an effect of butyrate in human colonic cancer is largely indirect, but a study in rats showed that the administration of butyrate to the large bowel in slow-release pellets increased apoptosis in rats treated with the carcinogen, azoxymethane (5). Propionate may also be of metabolic importance in the colon because it exerts some of the antineoplastic and other effects of butyrate, albeit at higher levels (2).
Although the overall supply of SCFA to the colon is important, their availability in specific regions of the large bowel may be critical. Studies in pigs showed that SCFA are high in the proximal large colon and decline toward the distal colon (6–9). This profile is consistent with greater fermentation occurring in the proximal colon (through greater substrate availability) and a decline in concentration due to substrate depletion and absorption of SCFA from the digesta stream. A study in stomal patients indicated that this profile also occurred in humans, with high excretion of SCFA in individuals with transverse colostomy and lower amounts from those with sigmoid colostomy (10). These regional differences in the large bowel may be important in the pathogenesis of colorectal cancer (11), which is highest in the distal colon of populations in Westernized countries (12).
Strategies to raise specific SCFA may be of public health and clinical benefit. One potential route is to increase the consumption of foods high in RS because its fermentation was shown to promote butyrate formation in vitro compared with NSP (13). Foods containing RS (as high amylose starches) were shown to increase fecal butyrate in humans (14,15). However, there is considerable variation among individuals, and it appears that the microflora of some people cannot ferment certain types of RS (16). Further, various RS sources differ in the SCFA profiles that are produced (8,9,16). Finally, transit is a major determinant of the delivery of SCFA (especially butyrate) along the large bowel (17); thus increased fermentation alone may not translate to an increased supply of SCFA to the distal colon. Consumption of SCFA directly in foods or beverages is not effective because they are absorbed rapidly in the fore-gut and fail to reach the large bowel (18). One possibility is to acylate starches with specific SCFA so that they could resist small intestinal amylolysis and be delivered to the hind gut for release by bacterial action. This paper describes experiments in rats to determine whether feeding of starches acylated with acetate, propionate or butyrate could elevate SCFA in the large bowel.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSPreparation of acylated starchesDietary studiesRESULTSDISCUSSIONLITERATURE CITED |
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Adult male rats of the Hooded Wistar strain were used. They were bred in the divisional Small Animal Colony and were housed in groups in standard wire-bottomed cages in a room with controlled temperature (22 ± 1°C) and lighting (lights on at 0800–2000 h). The rats had free access to food and water during the adaptation period and immediately before the sampling procedure (which started at 0900 h). Management of the facility and experimental procedures were under the oversight of the CSIRO Health Sciences and Nutrition Animal Experimentation Ethics Committee and conformed to published guidelines (19).
Chemicals and dietary components.
Maize starch was obtained from Starch Australasia (Lane Cove, NSW). Acetic, propionic and butyric anhydrides and 1-methylimidazole were purchased from Sigma Chemical (St. Louis, MO). Dietary ingredients were purchased as described previously (20).
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Preparation of acylated starches |
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TOPABSTRACTMATERIALS AND METHODSPreparation of acylated starchesDietary studiesRESULTSDISCUSSIONLITERATURE CITED |
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Because starches acylated to the degree required were unavailable commercially, they were produced in the laboratory. Briefly, 110 g of starch was dissolved by stirring overnight in 6 L of dimethyl sulfoxide (DMSO) in a large stainless steel drum; 20 g of 1-methylimidazole (as catalyst) and 50 g of butyric anhydride were then added with stirring and the mixture allowed to stand for 4 h at 20°C. During this time, the mixture was stirred occasionally by hand. Any remaining anhydride was decomposed with water (6 L) and the starch precipitated with 24 L of ethanol. The butyrylated starch was recovered and washed four times with 1 L of ethanol to remove DMSO and butyric acid. The odorless starch was dried at 40°C, milled, weighed and a portion taken for analysis. Starch (110 g) was dissolved in DMSO and taken through the procedure without the acylating reagents to provide the control starch. The amount of butyrate esterified to the starch was determined by GLC (see below) after its liberation by alkaline hydrolysis with NaOH. The DS of this butyrylated starch was 0.25.
Larger-scale production of acetylated, propionylated and butyrylated starch.
For the large-scale preparation of acylated starches (500 g), the procedure was changed slightly. DMSO (30 L) was heated to 80°C in an immersion heater in a metal vessel with continuous stirring and then the heater was removed. For each acylated product, 600 g of maize starch was added to the hot DMSO through a domestic sieve to prevent clumping. The starch was stirred constantly for 1 h to give a clear viscous solution and 110 mL of 1-methylimidazole was added as a catalyst. Anhydride was added to produce a DS of 0.20 with additions of 115, 180 and 230 mL of acetic, propionic and butyric anhydride. After 4 h of incubation, the excess anhydride was decomposed with water and the acylated starches were precipitated with ethanol, washed with ethanol, air dried and milled. An equal weight of starch was taken through the procedure without reagent to serve as a control. The DS was determined by release of the SCFA and was 0.20, 0.16 and 0.19 for acetylated, propionylated and butyrylated starch, respectively. These are referred to as Starches A (acetate), P propionate (P) and B (butyrate) in the text.
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Dietary studies |
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TOPABSTRACTMATERIALS AND METHODSPreparation of acylated starchesDietary studiesRESULTSDISCUSSIONLITERATURE CITED |
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Diets. The purified diets were similar to those used previously in this laboratory and were formulated from casein, sucrose, maize starch and corn oil (20). Vitamins, minerals, methionine and choline were also added. Cellulose (50 g/kg) was added as a source of dietary fiber. The diet contained 401.5 g maize starch/kg together with 150 g/kg of either butyrylated starch or control starch that had been through the procedure without the addition of anhydride. The diets were blended with water, pelleted and air dried as described previously (20). Sufficient food was made to feed the rats for 3–4 d.
Animal feeding and sampling. Two groups of 6 rats (weighing 300–350 g) had free access to the control or butyrylated starch diets and water. After 3 d, the rats were anesthetized and the large bowel removed. The ceca and distal colon were identified, isolated by ligation and the contents extruded and diluted for analysis (21). Samples were obtained from the distal colon of only 5 rats in both groups.
Starch acylated on a large scale.
Diets. The composition of the diets used in this trial was similar to that used in the previous experiment except that Starches A (DS 0.20), P (DS 0.16) and B (DS 0.19) were used. The control group was fed a diet containing starch that had been through the synthetic procedure without the addition of anhydride.
Animal feeding and sampling. Four groups of 8 rats (weighing 200–250 g) were used. They had free access to food and water for 14 d after which they were anesthetized and the large bowel excised and processed as in the first experiment.
Analytical procedures.
Moisture, pH and SCFA in gut contents were determined as described previously (20). Briefly, gut contents were extruded, weighed and homogenized with 3 times their weight with distilled water and then the pH measured with an electrode. Portions were taken for determination of moisture (by air drying to constant weight) and measurement of SCFA by GLC using o-enanthic acid as an internal standard. The starch content of gut digesta was determined after hydrolysis using a commercial kit (22). This method was also used to determine the starch content of acylated starches.
Statistical methods.
Values are presented as means and pooled SEM unless stated otherwise. Statistical differences were performed using the General Linear Model (GLM) of SAS (23). Effects of diet or large bowel site and their interactions were evaluated using ANOVA. When a significant result was observed (P < 0.05), differences between individual means at each sampling site (cecum, proximal or distal colon, as appropriate) were analyzed by the protected differences (PDIFF) procedure. Effects of diet on body weight were analyzed using the same procedure with time and diet as variables. A value of P < 0.05 was taken as the criterion of significance.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSPreparation of acylated starchesDietary studiesRESULTSDISCUSSIONLITERATURE CITED |
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Large bowel SCFA. Acetate, propionate and butyrate were the three major acids measured in the cecum and distal colon of rats fed both diets. In rats fed the control starch, acetate was present in the greatest concentration at both sampling sites (Table 1). Diet and site affected total and individual SCFA (P < 0.001). There was also an interaction between diet and sampling site with higher butyrate concentrations in the cecum and colon of rats fed the butyrylated starch than in controls (P < 0.001). In rats fed the control starch, propionate and butyrate were present in equal concentrations in the cecum and distal colon. However, when rats were fed the butyrylated starch, butyrate was the major acid with concentrations that were substantially (at least 200%) higher than in rats fed the control starch. Concentrations of acetate and propionate were higher in the cecum of rats fed butyrylated starch than in controls but not in the distal colon.
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Food intake and body and liver weights. Neither initial nor final body weights differed among the groups with means of 236 ± 5 g and 308 ± 7, respectively, for all groups, combined.
Large bowel digesta mass, starch and pH. Diet and sampling site affected digesta wet weight and starch independently and interactively (P < 0.001). Thus, in all four groups of rats, digesta was highest in the cecum, lowest in the proximal colon and intermediate in the distal colon (Table 2). Examination of the data at each sampling site showed that digesta wet weight was similar in rats fed Starch A, P or B and significantly higher than in those fed the control starch. The pools of starch in cecal and distal colonic contents were lowest in rats fed the control diet and were significantly higher in rats fed the acylated starches (Table 2). These increases were >10-fold in the cecum and 6-fold in the distal colon. The independent (both P < 0.001) and interactive (P < 0.03) effects of diet and sampling site on colonic digesta pH were significant. The pH was highest in all three viscera of rats fed the control diet (Table 3). Values did not differ in rats fed the acylated starches and were significantly lower than controls at the three sampling sites.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSPreparation of acylated starchesDietary studiesRESULTSDISCUSSIONLITERATURE CITED |
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The initial conclusions that the esterified fatty acid was responsible for the rise in that acid in the large bowel, whereas starch contributed to the general increase in SCFA, were supported by the more extensive second study. In this experiment, acylated starches of DS 0.18 were fed. As anticipated from the first experiment, the greatest increase in each group was in the SCFA that had been esterified to the starch, whereas the other major SCFA were also raised significantly. In this second study experiment, we also measured large bowel starch and found that cecal and colonic starch pools were at least 10- and 6-fold higher, respectively, than in rats fed the control starch. RS is defined as "that fraction of starch and the products of starch digestion which enter the large bowel of healthy humans" (24) and was classified initially into three groups, i.e., RS1, physically inaccessible; RS2, resistant granules; and RS3, retrograded (25). Chemically modified starches used in the food industry for technological (rather than nutritional) reasons also resist amylolysis depending on their degree of substitution (26). On this basis, these have been designated as RS4, chemically resistant (27). It should be noted that the RS4 used in the food industry are of a much lower DS than those used in the present study. Indeed, the lack of commercial availability meant that we had to synthesize them ourselves, and the data show that this was achieved across the desired range. The present data show that classification RS4 is justified and confirm that it was the acylation that conferred resistance to small intestinal amylolysis because the control starch had been through the solvent dispersion and drying processes but not acyl anhydride treatment.
There have been relatively few studies on the nutritional attributes of modified starches. Raben et al. (28) reported lower glycemic indices (GI), indicating slower small intestinal digestion and potential RS status, in humans consuming starches modified by acetylation at a low level or by ß-cyclodextrinisation. Early feeding trials in which rats were fed modified starches, including hydroxypropylated, acetylated and etherized starches, for very long periods showed that cecal digesta mass was higher than in rats fed unmodified starches (29,30). These data are consistent with the proposition that they are RS4 and with the data from the present, much shorter, experiments. In neither the earlier nor current studies was there any evidence of a deleterious effect of the modified starches on the well-being of the rats, with no apparent differences in body weight gain between any of the groups. However, not all RS4 appear to be equal in their effects on large bowel SCFA. Ebihara et al. (31) reported that consumption of hydroxypropylated starches also increased fecal bulk in rats but there was no change in SCFA. This may indicate a greater resistance of these modified starches to bacterial degradation or that SCFA are not the major the end products of metabolism.
SCFA profiles were similar in rats fed the control starch and Starches A, P or B, i.e., highest in the cecum and progressively lower in the proximal and distal colon. These data differ somewhat from an earlier study in which SCFA levels were lower in the proximal colon than in the distal colon (21). The reason for the disparity is unclear but may reflect the difference between the low fiber purified diet used in this experiment and the high fiber unpurified diet used in the previous study. In rats fed the acylated starches, large bowel total SCFA levels were substantially higher at all three sampling sites than in rats fed the control diet. The magnitude of the increase was similar in all groups, and total and all major acids were raised by consumption of the acylated starches. However, the differential effect of each acylated starch was maintained, i.e., the greatest elevation at all sites was in the SCFA that was esterified to the starch and therefore delivered to the large bowel. A direct estimate of the contribution of the esterified SCFA to the total large bowel pool is not possible. However, an approximation may be made by subtracting the mean pool for the individual SCFA in rats fed the other starches from that in those fed the specific acylated starch. The former gives an estimate of nonspecific contribution due to the additional RS due to esterification: e.g., acetate pool (Starch A) - 0.5 x [acetate pool (Starch P) + acetate pool (Starch B)]. Using this approach, calculation of the contribution of the esterified acetate to a total pool of 203.2 µmol was 50.9 µmol. The corresponding values for propionate and butyrate are total pools of 129.0 and 116.0 µmol and specific contributions of 76.0 and 84.4 µmol, respectively. The relatively disproportionate contribution for the latter acids may reflect the fact that they are most sensitive to change in substrate supply (21). Although the exact amylolytic and deacylation processes are unclear, the data also suggest a progressive hydrolysis during passage through the large bowel with effective SCFA delivery even to the distal colon. The higher SCFA values, especially of butyrate and propionate, in the distal region are of some interest given their putative role in diminishing the risk of large bowel disease in that region. pH values also were lowered throughout the hindgut by the feeding of acylated starches. This is not unexpected because greater fermentation (and SCFA production) has been associated with more acidic conditions in the large bowel of rats (21,32) and pigs (9). Lower fecal pH values have been reported in humans after consumption of RS (14). This lowering is ascribed to the production of anions and also the utilization of NH4+ bacterial protein synthesis (2).
As noted previously (21), the mass of digesta was high in the cecum, intermediate in the distal colon and lowest in the proximal colon. This distribution differs from that seen in other omnivores such as pigs, which have a large bowel that closely resembles that of humans (2). In pigs, digesta mass (9) are highest in the proximal large bowel and fall toward the distal colon; although diet can raise or lower these variables, the overall pattern does not change (6–9). The particular profile in rats is thought to reflect the fact that they are cecal fermenters with a complex muscular arrangement in the proximal colon to retain digesta in the fermentation chamber. Notwithstanding this distribution, dietary maneuvers that increase digesta mass and SCFA levels throughout the porcine large bowel, e.g., feeding of wheat bran (9), have generally similar actions in rats [e.g., (33)]. The same was true in the present studies with an overall effect of the acylated starch diets in increasing large bowel digesta. This could occur for two possible reasons, i.e., greater bacterial mass or the presence of unfermented dietary components. In rats fed the control diet, the contribution of starch to total digesta at the two large bowel sites at which it was measured was negligible. In the cecum and distal colon of rats fed the three acylated starches, it accounted for 2.5% of the total weight of digesta. Nyman et al. (34) reported that starch can make a large contribution to fecal mass in rats when its digestion is diminished through feeding of NSP (such as guar gum). However, the current data agree with other studies in which the actual contribution of RS to increased large bowel digesta mass was small (22). It appears more likely that the increase was due to a greater bacterial mass, but this remains to be established.
To our knowledge this is the first demonstration that acylated starches can be used to raise large bowel levels of individual SCFA selectively. The data suggest that acylated starches of DS 0.20–0.25 have potential value as ingredients to improve the nutritional attributes of foods through delivery of SCFA with health-promoting activities directly to the large bowel.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Supported by a special allocation from the Commonwealth Government (May Statement Funds).
3 Present address: Goodman Fielder Limited, Sydney, NSW, Australia
5 Abbreviations used: DMSO, dimethyl sulfoxide; DS, degree of substitution; GI, glycemic index; NSP, nonstarch polysaccharides ("fiber"); RS, resistant starch; Starch A, acetylated starch; Starch B, butyrylated starch; Starch P, propionylated starch.
Manuscript received 22 May 2003. Initial review completed 10 June 2003. Revision accepted 29 August 2003.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSPreparation of acylated starchesDietary studiesRESULTSDISCUSSIONLITERATURE CITED |
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