ARABINOXYLANS AS NUTRITIONALLY FUNCTIONAL FOOD INGREDIENTS

7.8 ARABINOXYLANS AS NUTRITIONALLY FUNCTIONAL FOOD INGREDIENTS

Arabinoxylans as part of dietary fiber have many potential physiological effects along the entire human gastrointestinal tract. These effects are dependent on a

complex mixture of molecular and physical properties of arabinoxylan preparations as well as on the site, rate, and extent of their digestion and fermentation in the gut. In the large bowel, the undigested arabinoxylans may change the composition of the microbial flora, affect the activity of bacterial enzymes, and influence the end products of bacterial fermentation, thus having an effect on colonic health. It is generally agreed that arabinoxylans and various xylooligosaccharides enhance the growth of potentially health-promoting bacteria, the so-called probiotics, although

Functional Food Carbohydrates

some inconsistencies as to the effectiveness of arabinoxylans in proliferation of specific bacterial species are reported in the literature. Jaskari et al. 37 and Crittenden

et al. 38 reported that xylooligosaccharide preparations (containing mainly β-(1→4)- Xyl oligosaccharides ranging in size from DP 2 to DP 5) support the growth of many Bifidobacterium and Bacteroides species, as well as Lactobacillus brevis, but are not fermented by Escherichia coli, enterococci, Clostridium sp., and the majority of Lactobacillus sp. Katrien et al. 134 generated various arabino-xylooligosaccharides with DPs of 5 to 10 containing mainly doubly branched xylose residues and observed that they were fermented completely by Bifidobacterium adolescentis, Bifidobacte- rium longum , and Bacteroides vulgatus. The same authors reported that intact ara- binoxylans from wheat were fermented by Bifidobacterium longum and Bacteroides ovatus , but glucuronoarabinoxylans from sorghum could only be fermented by Bacteroides ovatus . It appears that the presence of arabinosyl substitution affects the ability of microorganisms to ferment the intact polysaccharide since arabinox- ylans can support the growth of some bacterial species, whereas unsubstituted xylans are not fermented by any of the probiotic bacteria. Unsubstituted xylans are known to form insoluble aggregates and hindered accessibility of the bacterial enzymes to the substrates. Water-insoluble arabinoxylans are also not likely to be digested in the large intestine. The degree of branching and the distribution of arabinose along the xylan backbone may additionally influence the degradation of arabinoxylans. When four types of rye bread containing either whole rye, pericarp/testa, aleurone, or endosperm-enriched fractions (Table 7.2), and therefore differing in arabinoxylans structure, were fed to pigs, substantial differences in the rate and extent of digest- ibility were found. 135 The pericarp/testa diet exhibited the lowest digestibility and the endosperm the highest (Figure 7.14). The endosperm fractions contained the highest amount of water-soluble arabinoxylans and a large content of unsubstituted xylose residues. Mono- and disubstituted xylose, terminal xylose, and nonterminal

−0.20 Iieum Caecum Proximal Middle

Distal Faeces

FIGURE 7.14 Digestibility of arabinoxylans in intestinal segments of pigs receiving diets based on ( 䡩 ) whole rye, (•) pericarp/testa, (▫) aleurone, and ( 䉱 ) endosperm. Values are means and standard deviations of five pigs. (Adapted from Glitsø, L.V. et al., J. Sci. Food Agric.,

79, 961, 1999. With permmission.)

Arabinoxylans

arabinose residues were concentrated in the unfermented fractions, collected from the intestinal material of pigs, showing that these structural features were difficult

to digest. Harris et al. 36 also reported that the wheat-lignified pericarp-rich fractions were not degraded, whereas the wheat aleurone-rich fractions were partially degraded when fed as a dietary supplement to rats. Hopkins and coworkers 137

compared the degradation of cross-linked (via H 2 O 2 –peroxidase) and non-cross- linked arabinoxylans by the intestinal microbiota of children and reported that the non-cross-linked arabinoxylans were utilized more quickly than their more viscous cross-linked counterparts. The degradation of non-cross-linked arabinoxylans resulted in initial production of arabinose, followed by release of xylose residues, whereas the xylan backbone of the cross-linked arabinoxylans appeared to be more resistant to degradation, probably because the dense structure of the covalently bound chains restricted access of xylanolytic enzymes to their target sites. Treatment of the pericarp-enriched fractions with alkali considerably increased the fermentability and suggested that in the lignified tissues, the solubilization of arabinoxylans by breaking the alkali-labile cross-links was a more important determinant of arabinox- ylans’ degradability than arabinoxylans’ structure itself (Table 7.4). 136 Aura et al. 138 investigated the effects of various preprocessing of rye bran on the fermentation rate of arabinoxylans. Rye bran was extruded and treated with xylanase, and a part of xylanase-treated rye bran was separated into a soluble rye bran extract and an insoluble residue. Xylanase treatment of extruded rye bran made the cell wall polysaccharides more accessible to colonic bacteria, and thus increased the initial

TABLE 7.4 Amount of Total Fermented Carbohydrates (CHO) and Amount of Short- Chain Fatty Acids (SCFAs) Produced during In Vitro Fermentation of Ileal Effluent, Collected from Cannulated Pigs Fed Rye Bread Diets, with Fecal Inocula from Pigs

g CHO

Molar Distribution of SCFAs

fermented mmol SCFA

Diet a Treatment

(in 48 h)

(in 48 h)

Acetate Propionate Butyrate b-SCFAs

Whole rye 1.64 18 40 41 17 0.9 Pericarp/testa

1.37 20 40 37 20 2.9 Pericarp/testa

NaOH 2.64 20 52 33 15 nd Pericarp/testa

NaClO 2 0.68 7 54 36 11 nd Endosperm

NaOH 1.54 18 49 31 20 nd Endosperm

NaClO 2 1.27 12 47 21 32 0.4 Note : nd = no data; b-SCFAs = branched short-chain fatty acids (isobutyrate, isovalerate). a Rye bread diets based on either whole rye or on one of three rye-milling fractions enriched in pericarp/testa,

aleurone, or endosperm, as shown in Table 7.2, fed to pigs. Source : Adapted from Glitsø, L.V. et al., J. Sci. Food Agric., 80, 1211, 2000. With permission.

Functional Food Carbohydrates

rate of fermentation. The soluble rye bran extract showed the fastest fermentation rate and the highest extent of fermentation as determined from the consumption of neutral sugars (arabinose, xylose, and glucose) and the production of short-chain fatty acids (SCFAs). The molar proportions of acetic, propionic, and butyric acids after 24 h of fermentation were 60:21:19 for the extruded rye bran, 66:18:16 for the xylanase-treated rye bran, and 67:16:17 for the soluble rye extract. The authors reported that the Ara/Xyl ratio increased during fermentation, indicating that the nonsubstituted xylan was preferentially consumed during fermentation.

In addition to prebiotic properties, the effects of arabinoxylans on lipid metab- olism and mineral absorption have recently been investigated. The study involving diets containing water-soluble arabinoxylans extracted from maize bran with alkaline solutions revealed striking effects of these polymers on cecal fermentation as well as on lipid metabolism and mineral balance in rats. 139 It was reported that the arabinoxylan diets caused an enlargement of the cecum and cecal walls and induced significant proliferation of the beneficial microflora. The authors postulated that fermentation of arabinoxylans and the resulting production of SCFAs were involved in a decrease of serum cholesterol and improve the calcium and magnesium adsorp- tion. One of the interesting effects of corncob arabinoxylan was production of a relatively high percentage of propionate rather than butyrate. Propionate is believed to affect various metabolic pathways; for example, it inhibits cholesterogenesis and lipogenesis. 140 Hopkins et al. 137 investigated in vitro breakdown of arabinoxylans by the intestinal microbiota, collected from fecal samples from children, and also found

a relatively high production of propionate together with acetate. These results support the contention that the species of genus Bacteroides are particularly active in arabi- noxylan metabolism. This contrasts with starch digestion, which is considerably more bifidogenic and associated with significant production of acetate, butyrate, and lactate (Table 7.5). However, the studies by Grasten et al. 141 reported that in healthy humans, the pentosan bread diet increased the concentration of butyrate, whereas the inulin bread diet increased the concentration of acetate and propionate in feces. Fecal levels of SCFAs only partly reflect the SCFA production in the colon, because they can be absorbed rapidly in the proximal colon. However, the authors claimed that a high fecal concentration of SCFAs also indicates their high level in the distal colon, the site of most colon tumors. In addition, the authors postulated that butyrate has beneficial effects on maintaining normal functions in colonocytes, and implied that pentosans are beneficial in the human large intestine. 141

The unfermented arabinoxylans can absorb water and increase fecal bulk, thus resulting in the dilution of intestinal contents and lowering the concentration of putative carcinogens, such as the secondary bile acids. Also, fecal bulking results in decreased time of transit through the colon, and therefore in reduction of the exposure time to irritants or carcinogens. As indicated above, the fine molecular structure of arabinoxylans may have an important effect on their fermentability, and thus on their specific physiological functions and efficacy to treat various bowel disorders. Edwards et al. 142 postulated that terminal arabinose residues are particu- larly susceptible to fermentation, and since the distribution of these residues along the xylan backbone is nonrandom, there will be regions of these polymers that are more or less fermentable in the gut. The portion of arabinoxylans that is resistant

Arabino

TABLE 7.5 Production of Fermentation Acids in pH-Controlled Fecal Incubation Mixtures Containing Arabinoxylans,

xylans

Ferulic Acid Cross-Linked Arabinoxylans, and Starch a

Incubation

Production (mM) of:

Substrate

Period (h)

Total SCFAs

Cross-linked arabinoxylans

72.9 (13.6) c ND

5.8 (1.5) Note : ND = not detected. a The values are means (n = 10). The values in parentheses are standard errors of the means. The concentrations of minor SCFAs were <0.1 mM. Source : Adapted from Hopkins, M.J. et al., Appl. Environ. Microbiol., 69, 6354, 2003. With permmission.

Functional Food Carbohydrates

to fermentation will retain its native structure and will likely increase the fecal bulk, thereby alleviating the symptoms of certain bowel disorders, such as constipation. For example, the psyllium arabinoxylans, because of their more complex and more substituted structure, are thought to be more efficient in inducing the laxative effects than their wheat bran counterparts. Marlett and Fischer 143 conducted a series of human clinical studies to test the hypothesis that a gel-forming, nonfermented fraction of psyllium seed husk is the active component responsible for the laxative and cholesterol-lowering properties of psyllium. The nonfermented fraction of psyl- lium appeared to be a highly branched arabinoxylan with both arabinose and xylose side chains. However, as pointed out by the authors, psyllium arabinoxylans must possess unique structural features that hinder their fermentation by typical colonic microflora and distinguish them from the extensively fermented arabinoxylans from

wheat or oats. In 2004, Fischer et al. 8 identified the physiologically active gel- forming fraction of psyllium seed husk to be a neutral arabinoxylan branched with single xylose units as well as with trisaccharides consisting of Araf- α-(1→3)-Xylp- β-(1→3)-Araf.

Another physiological role of arabinoxylans may be associated with the man- agement of diabetes in humans. Lu et al. 32 showed that addition of arabinoxylan- rich fiber to bread eaten at breakfast lowered postprandial glucose and insulin responses in healthy humans. Similar results were obtained by Zunft and cowork-

ers. 33 The mechanism by which arabinoxylans affect the postprandial glucose response is not clear, but it has been postulated that due to their viscosity-generating properties, arabinoxylans impair mixing of the food mass and can markedly affect the degree of contact of food substrates with the enzymes that digest them in the small intestine. In this way, arabinoxylans may slow down the rate of gastric emp- tying and reduce small intestine motility, which results in delayed glucose absorption.

The beneficial role of arabinoxylans in the human diet may also be associated with the presence of ferulic acid covalently bound to these polymers. Recent studies

have shown that ferulic acid has strong anti-inflamatory properties, inhibits chemi- cally induced carcinogenesis in rats, and plays a role as an antioxidant, inhibiting

lipid peroxidation and low-density lipoprotein (LDL) oxidation and scavenging oxygen radicals. 144 Katapodis et al. 3 evaluated the antioxidant activity of feruloylated oligosaccharides derived from partial hydrolysis of wheat flour arabinoxylans. The researchers found that feruloyl arabinoxylotrisaccharide (FAX 3 ) had profound anti- oxidant activity in 2,2-diphenyl-1-picrylhydrazyl reduction assay and inhibited the copper-mediated oxidation of human LDL. This antioxidant activity of FAX 3 may

be important in preventing or reducing the progression of arteriosclerosis by inhib- iting the peroxidation of lipids. The rate of release of ferulic acid from arabinoxylans may affect its bioavailability and bioactivity, and human studies are needed to evaluate which form of ferulic acid, free or bound, would be more effective. Adam et al. 144 showed that the cereal matrix severely limits ferulic acid bioavailability in rats. Rondini et al., 145 on the other hand, demonstrated that plasmas of rats fed with wheat bran, where ferulic acid is mainly bound to arabinoxylans, showed a better antioxidant activity than those of the pure ferulic acid-supplemented group. The contribution of other bran components acting as antioxidants cannot, however, be disregarded in the latter study. Recent studies on the cell culture and animal models

Arabinoxylans

have indicated that arabinoxylans show positive oxidative burst activity in murine macrophage cells in vitro and tend to increase the body weight and reduce the attachment of pathogen Salmonella to ileal tissue in broiler chicks undergoing mild

heat stress in vivo. 6 Deters et al. 146 showed that water-soluble and gel-forming polysaccharides from psyllium seed husk promote proliferation of human epithelial cells (skin keratinocytes and fibroblasts) via enhanced growth factor receptors and energy production.