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Applied Nutrition and Food Chemistry, Center for Chemistry and Chemical Engineering, Lund University, SE-221 00 Lund, Sweden
2To whom correspondence should be addressed. E-mail: asa.henningsson{at}inl.lth.se.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • dietary fiber • resistant starch • fermentation • short-chain fatty acids • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The physiological effects of these carbohydrates depend upon several factors including the extent of colonic fermentation and the fermentation products formed (1
,2
). The end products of colonic fermentation are short-chain fatty acids (SCFA)3
and gases (CO2, CH4 and H2). The SCFA are rapidly absorbed by the colonic mucosa and stimulate salt and water uptake (3
). Butyric acid is the main energy substrate for the colonocytes (4
), but propionic acid is also oxidized (5
). The SCFA are potentially protective against colon diseases such as colon cancer (6
,7
). Butyric acid, and to some extent propionic acid, has been shown to induce differentiation and apoptosis in colon carcinoma cells (8
,9
). Furthermore, subjects with adenomatous polyps or colon cancer frequently have lower fecal levels of butyric acid (10
). Propionic acid also affects metabolism in peripheral tissues and has been proposed to inhibit hepatic cholesterol synthesis (11
). However, experiments conducted to evaluate the possible hypocholesterolemic action of propionic acid using a variety of animal and in vitro models have yielded inconsistent results (12
).
The type of indigestible carbohydrates consumed can influence the distribution of SCFA in the hindgut. For example, guar gum consumption by rats results in a high proportion of propionic acid and pectin in a high proportion of acetic acid upon fermentation (13
,14
). In contrast, high amylose cornstarch appears to be a good source of butyric acid in the rat hindgut (15
,16
). Most studies have been conducted using single substrates, not taking into account the possible synergistic/antagonistic effects of combining carbohydrates. This is important because the human diet contains mixtures, rather than single sources of indigestible carbohydrates.
The physicochemical properties of dietary fibers (DF) may influence their fermentation characteristics. Highly soluble DF such as pectin and guar gum are generally rapidly fermented in the colon, whereas insoluble DF such as cellulose and wheat bran usually are more resistant to fermentation (17
). It is also possible that the fermentability of resistant starch (RS) depends on the source and nature of the starch (18
).
Fermentation takes place essentially in the cecum in rats (19
) and in the upper colon in humans (20
), but the type of substrate may affect the site of fermentation. It was shown previously that the fermentation of easily fermentable RS could be shifted to the distal part of the colon when fed in combination with psyllium, a rather resistant type of DF (16
). This is interesting because most colon cancer appears distally in both humans (21
) and rodents with experimentally induced cancer (22
). Further, the combination of indigestible carbohydrates may affect the SCFA pattern. Topping and co-workers (23
) reported that a mixture of gum arabic and cellulose more efficiently generated butyric acid in the rat hindgut than the individual substrates.
Combinations of indigestible carbohydrate substrates represent an interesting potential with respect to colonic health. In this study, the content and pattern of SCFA from various indigestible carbohydrate sources were studied along the hindgut of rats. Three highly fermentable substrates previously shown to yield different SCFA patterns were chosen; these were guar gum (propionic acid producer), pectin (acetic acid producer) and high amylose cornstarch (butyric acid producer). Wheat bran, a more resistant type of fiber that has been shown to be comparatively slowly fermented in in vitro studies (24
,25
) was also included. In addition, the following combinations of substrates were evaluated to examine their effect on SCFA patterns and/or site of SCFA production: guar gum and pectin, guar gum and wheat bran and high amylose cornstarch and wheat bran. Pectin was combined with guar gum to study whether the SCFA pattern from pectin could be altered, i.e., decrease the proportion of acetic acid. Guar gum and high amylose cornstarch were combined with wheat bran to evaluate the possibility of increasing distal levels of propionic or butyric acids, respectively.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Dietary fiber.
Soluble and insoluble DF in the raw materials was determined gravimetrically according to Asp et al. (26
). The composition of the isolated DF was analyzed by gas-liquid chromatography (GLC) for the neutral sugars as their alditol acetates and with a spectrophotometric method for the uronic acids (27
). Lignin was determined as Klason lignin, i.e., the residue insoluble in H2SO4 (27
). The DF values were corrected for the amount of total starch present in the isolated fiber residues and referred to as nonstarch polysaccharides, NSP, whereas the term DF, also included lignin. The analyses were performed in triplicate.
Resistant starch.
An in vitro model (28
) was used for determination of RS in the high amylose cornstarch and wheat bran. Six human subjects chewed glass beads and rinsed their mouth with 5 mL of water; thereafter the saliva was pooled. Pooled saliva (5 mL) was transferred to a beaker containing the test product and water. The pH was adjusted to 1.5 and pepsin (Merck, Darmstadt, Germany) was added. The samples were incubated at 37°C for 30 min. The pH was adjusted to 5.0 after addition of pancreatin (Sigma Chemical, St. Louis, MO) and amyloglucosidase (Boehringer, Mannheim, Germany). The suspension was incubated for 16 h at 40°C. Undigested starch was precipitated with ethanol and analyzed as liberated glucose after solubilization in KOH and enzymatic treatment with a thermostable 
-amylase (Termamyl 300L DX, Novo Nordisk A/S, Denmark) and amyloglucosidase according to Björck et al. (29
). Pooled saliva was used instead of an initial chewing of the sample because the product was not a realistic food item but a dry flour. The analysis was performed six times per sample. Total starch in feces was analyzed in duplicates as above (29
); after correction for the small amounts of free glucose, it was regarded as RS.
SCFA.
Acetic, propionic, isobutyric, butyric, isovaleric, valeric, caproic, heptanoic and succinic acids were isolated by the method of Richardson et al. (30
). Hindgut and fecal samples were homogenized (Polytron, Kinematica, Switzerland) with 2-ethylbutyric acid (internal standard), after which hydrochloric acid was added to the sample. SCFA were then extracted with diethyl ether and silylated with n-(tert-butyldimethylsilyl)-n-methyltrifluoroacetamide (Sigma Chemical). Samples were analyzed using GLC (HP 6890, Hewlett-Packard, Wilmington, DE) equipped with a HP-5 column and integrated by Chem Station software (Hewlett-Packard).
Composition of the test materials
The chemical composition of the materials used in the test diets is shown in Table 1
. The guar gum was highly soluble and consisted mainly of mannose and galactose. The pectin preparation was almost completely soluble, containing rather pure uronic acid polymers, and small amounts of galactose. The indigestible carbohydrates in the high amylose cornstarch were mainly RS and only small amounts of NSP could be detected. Wheat bran contained mainly insoluble fibers and arabinose, with xylose and glucose the main monomers present. Wheat bran was the only fiber that contained lignin.
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Male Wistar rats (average weight, 79 ± 4 g) were purchased from B&K Universal (Stockholm, Sweden). Rats were housed individually in metabolic cages (13
) and given free access to water. The animal protocol used was reviewed and approved by the Ethics Committee for Animal Studies at Lund University.
Rats were randomly assigned to one of eight dietary treatments (7 rats/treatment). A control diet was prepared according to Table 2
. The test materials were substituted for digestible wheat starch to give a concentration of 100 g indigestible carbohydrates/kg dry diet. Treatments were as follows: 1) control diet (C); 2) guar gum (GG); 3) low methoxylated apple pectin (Pec); 4) high amylose cornstarch (HAS); 5) wheat bran (WB); 6) GG + Pec (1:1, wt/wt indigestible carbohydrate basis); 7) GG + WB (1:1, wt/wt indigestible carbohydrate basis) and 8) HAS + WB (1:1, wt/wt indigestible carbohydrate basis). Intake was restricted to 12 g dry diet/d. After 7 d of adaptation to the diet, a 5-d balance period followed when feces were collected daily. Feces were kept at -20°C and then freeze-dried and milled before analysis of DF and starch. During the following 24 h of the experiment, fresh feces were collected on dry ice for SCFA determination. At the end of the experiments (d 13), the rats were killed using carbon dioxide. The cecum and colon were removed immediately and the colon divided into a proximal and a distal part and then kept frozen (-80°C) until analyzed.
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The term indigestible carbohydrate is the sum of DF (NSP+ lignin) and RS throughout the study. The cecal SCFA pool was calculated by multiplication the concentration of SCFA in the cecum (µmol/g) and the weight of the cecal contents (g). The fecal SCFA excretion was calculated by multiplication of the concentration of SCFA in the feces (µmol/g) with the weight of feces (g) excreted during the last 24 h of the experiment.
All statistical analyses were performed with the Minitab Statistical Software (31
). The means of fermentability, SCFA concentrations and SCFA proportions were analyzed by ANOVA using the General Linear Model procedure according to Minitab. When SCFA values in different parts of the hindgut and in feces were analyzed, comparisons within groups were made. Significance of difference (P < 0.05) between means was determined by Tukey’s test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The feed intake was close to complete (
12 g/d) for all groups except the GG group, which had a somewhat lower intake (Table 3
). However, body weight gains did not differ among groups. Cecal wet weights were greater in rats fed the test diets than in those fed the control diet (P < 0.01). The greatest cecal wet weights were in rats fed GG and GG + Pec and these were greater than in rats fed HAS and the WB diets (P < 0.05). Wet and dry fecal outputs were higher in rats fed the test diets than in those fed the control diet (P < 0.05). The largest outputs were seen in rats fed WB, alone or in combination with GG and HAS (P < 0.05).
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The total fermentability of indigestible carbohydrates was almost complete in rats fed Pec, GG, HAS and GG + Pec (95–96 ± 1%) and was greater than in those fed the other diets (P < 0.0001). WB was most resistant to fermentation, and only 37 ± 1% of the DF was degraded (P < 0.0001). Mixing GG and HAS with WB decreased the total fermentability to 64 ± 2 and 63 ± 1%, respectively (P < 0.0001). The RS in HAS and the galactomannan in GG were somewhat less fermented when mixed with WB (93 ± 2 and 96 ± 0.2%, respectively) than when fed alone [97 ± 1% (P = 0.015) and 98 ± 0.4% (P = 0.0003), respectively].
SCFA
Concentrations and proportions of SCFA in the hindgut of rats.
Acetic acid was the major acid formed in cecum of all rats (36–74 µmol/g) followed by propionic (8–38 µmol/g) and butyric acid (4–17 µmol/g). The cecal concentrations of acetic acid, were higher in rats fed the test diets, other than those containing WB, than in those fed the control diet (P < 0.05) (Table 4
). Rats fed WB had a high cecal proportion of butyric acid (19%), whereas those fed diets containing Pec and HAS generated high proportions of acetic acid (76 and 73%, respectively). The highest proportion of propionic acid was in the cecum of rats fed GG (31%, P < 0.05). These differences occurred also in distal colon and in feces. When Pec and GG were combined, the rats had a higher cecal proportion of butyric acid (15% compared with 10 and 6% for the individual substrates, respectively). Further, mixing GG and HAS with WB resulted in a higher proportion of butyric acid in the cecum of the rats (13 and 16% vs. 6 and 4% with GG and HAS, respectively, P < 0.05). Similar results were obtained in distal colon, and higher proportions of butyric acid were seen in rats when the substrates were mixed with WB (13%) compared with those fed GG or HAS alone (5%, P < 0.05). However, the cecal butyric acid proportion in rats fed GG + WB was still less than that in rats fed WB alone (P = 0.023), and rats fed HAS + WB did not have different butyric acid proportion than rats fed WB. Further, the proportions of propionic and butyric acid in distal colon or in feces were not greater than in the cecum when rats were fed GG or HAS with WB.
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). Thus, the cecal butyric acid pool was twice as high in rats fed the combination of GG and Pec (44 ± 9 µmol) than in those fed the individual components (19 ± 2 and 21 ± 3 µmol, with GG and Pec, respectively) (P < 0.05). This tended to be the case in feces also, and the butyric acid excretion in rats fed the mixture of GG and Pec was higher (16 ± 3 µmol/d) than in those fed GG (8 ± 1 µmol/d; P = 0.083, Fig. 1
). Rats fed the mixture also tended to have a larger cecal pool of butyric acid than those fed the WB diet (30 ± 4 µmol, P = 0.18, data not shown).
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). The size of the fecal excretions of SCFA from the rats fed the different diets did not follow the same order as the cecal SCFA pools. Rats fed the HAS + WB diet had higher fecal SCFA excretions (184 ± 19 µmol/d) than those fed the individual components (77 ± 10 and 116 ± 12 µmol/d in rats fed HAS and WB, respectively, P < 0.05), whereas there was no significant change when rats were fed GG in combination with WB. The ranking of the groups also differed after analysis of fecal and cecal SCFA. The cecal SCFA pool of rats fed GG was twice as high as that of rats fed GG + WB, whereas fecal excretions differed less dramatically (Fig. 2)
.
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), whereas fecal dry or wet weights and the fecal SCFA pool were not correlated.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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,33
). Also the SCFA profile has been reported to be quite stable after this feeding time (34
,35
). Thus, in rats fed guar gum, the SCFA patterns were stable after 7 d (35
), whereas soybean fiber has been reported to require 16 d (34
).
The source of indigestible carbohydrate had an effect on the SCFA pattern. Starches have been suggested to be good sources for butyric acid production in several studies in vitro and in vivo (15
,16
,36
–38
). However, in the present study, rats fed HAS had the lowest proportion of butyric acid among the substrates tested (4%, SCFA basis). Low proportions of butyric acid in the cecum of rats fed high amylose cornstarch have also been obtained by others (39
,40
). Further, in some investigations, easily fermentable RS has been shown to promote the formation of propionic acid (41
,42
), which was not found in the present study. The different SCFA patterns that actually have been reported for RS in the literature may be dependent on the methodology used to study SCFA formation, the nature of the starch (18
,43
) and possibly in the case of RS, the time for adaptation (44
, Henningsson et al., unpublished results).
Low levels of cecal butyric acid were also found in rats fed GG or Pec. Rats fed GG had the highest amount of propionic acid, whereas rats fed Pec the highest proportion of acetic acid, which is in agreement with previous results in vitro (25
) and in rats (13
,14
). Interestingly, when these substrates were combined and fed to rats, a higher butyric acid yield was obtained. As judged from the present work, the 1:1 mixture of Pec and GG seems to favor butyric acid–producing bacteria. A similar additive effect has been seen previously. Topping and co-workers (23
) reported that a mixture of gum arabic and cellulose was more efficient in generating butyric acid in the rat cecum than the individual substrates. It is difficult to explain the increased production of butyric acid in rats fed GG and Pec as a mixture. The microorganisms are metabolizing the colonic substrates and fermentation products in many different pathways (45
). Cross-feeding properties, e.g., metabolism of H2 by acetogenesis or dissimilatory sulfate reduction (46
), the presence of cofactors such as vitamin B-12 in the formation of propionic acid from succinic acid (47
) and the colonic pH are examples of factors that may influence the SCFA pattern. Because of the heterogeneous composition of the microflora, it is better to treat the microflora as a single entity and to study changes in pH and formation of SCFA and gases, than try to investigate separate biochemical events (48
).
One factor that may have an effect on butyric acid production is the transit time through the gastrointestinal tract. Mathers and Dawson (49
) found a relationship between the molar proportion of butyric acid in cecal contents and the cecal transit time in rats fed various diets. In general, higher proportions of butyric acid were related to a shorter cecal transit time. Changes in transit time have been suggested to alter bacterial activity and modify the bacterial pathways; as a consequence, the proportion of individual SCFA is affected (50
). In the present study, cecal transit time was not measured and it remains to be shown whether the transit time was faster in rats fed diets containing WB that generated high amounts of butyric acid, or when GG and Pec were mixed.
GG, Pec and HAS were almost completely fermented in the hindgut of rats, whereas WB was more resistant. These results are in good agreement with previous studies (13
,17
,51
). HAS and GG were less fermented when mixed with WB than when fed alone, suggesting that WB delays degradation of easily fermentable substrates. Similarly, the addition of WB to a diet containing high amylose corn kernels gave higher amounts of starch in the distal regions of the hindgut and feces of pigs (52
). The results from the present study also indicate that WB may be able to deliver HAS more distally, as judged from the higher fecal excretions of SCFA when the individual substrate was mixed with WB. However, the proportions of butyric or propionic acid in the distal parts of the hindgut were not greater than in the cecum in rats fed HAS + WB or GG + WB, respectively. A plausible explanation for HAS could be that this is a poor source for butyric acid production. Further, WB is not an inert substrate but fermented to some extent and combining different fermentable substrates may change the SCFA profile drastically. This was demonstrated by mixing GG and Pec, which gave a much lower cecal proportion of propionic acid (10%) compared with GG alone (31%).
In the present study, the fecal SCFA pool correlated with the amount of stool water. This may be explained by the osmotic effect of SCFA. A more possible explanation, however, is that the WB-mixtures were less fermented, thus maintaining the water-holding capacity of the NSP.
SCFA have been suggested to prevent and treat diseases appearing in the distal part of the colon, i.e., colonic cancer and ulcerative colitis (7
). Indigestible carbohydrates, which release their fermentation products distally, might thus offer advantages in this respect. As judged from the higher fecal SCFA excretion with rats fed HAS + WB, combinations of substrates are to be preferred. However, we must consider the fact that the actual amount of SCFA that will be in contact with the mucosa is unknown because the distribution of SCFA between the core and the surface of the colonic contents may differ, with a lower concentration at the surface than in the core (53
).
We conclude that certain mixtures of indigestible carbohydrates stimulate butyric acid-producing bacteria, with potential benefits for the colonic epithelium. Combining highly fermentable carbohydrate sources such as guar gum and high amylose cornstarch with the more resistant fiber in wheat bran appears to shift the fermentation and in the case of HAS also the release of SCFA to more distal parts of the colon. It remains to be elucidated whether these effects are valid also in humans and have physiologic implications for the human colonic epithelium.
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FOOTNOTES |
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3 Abbreviations: DF, dietary fiber; GG, guar gum; GLC, gas-liquid chromatography; HAS, high-amylose cornstarch; NSP, nonstarch polysaccharides; Pec, pectin; RS, resistant starch; SCFA, short-chain fatty acids; WB, wheat bran.
Manuscript received 22 March 2002. Initial review completed 2 April 2002. Revision accepted 9 July 2002.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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