Arabinoxylan-oligosaccharides (AXOS) affect the protein/carbohydrate fermentation balance and microbial population dynamics of the Simulator of Human Intestinal Microbial Ecosystem

The influence of diet on the gut microbiota

Diet is a major factor driving the composition and metabolism of the colonic microbiota. The amount, type and balance of the main dietary macronutrients (carbohydrates, proteins and fats) have a great impact on the large intestinal microbiota. The human colon contains a dense population of bacterial cells that outnumber host cells 10-fold. Bacteroidetes, Firmicutes and Actinobacteria are the three major phyla that inhabit the human large intestine and these bacteria possess a fascinating array of enzymes that can degrade complex dietary substrates. Certain colonic bacteria are able to metabolise a remarkable variety of substrates whilst other species carry out more specialised activities, including primary degradation of plant cell walls. Microbial metabolism of dietary carbohydrates results mainly in the formation of short chain fatty acids and gases. The major bacterial fermentation products are acetate, propionate and butyrate; and the production of these tends to lower the colonic pH. These weak acids influence the microbial composition and directly affect host health, with butyrate the preferred energy source for the colonocytes. Certain bacterial species in the colon survive by cross-feeding, using either the breakdown products of complex carbohydrate degradation or fermentation products such as lactic acid for growth. Microbial protein metabolism results in additional fermentation products, some of which are potentially harmful to host health. The current 'omic era promises rapid progress towards understanding how diet can be used to modulate the composition and metabolism of the gut microbiota, allowing researchers to provide informed advice, that should improve long-term health status.

Effects of arabinoxylan-oligosaccharides (AXOS) on juvenile Siberian sturgeon (Acipenser baerii) performance, immune responses and gastrointestinal microbial community

Arabinoxylan-oligosaccharides (AXOS) are a newly discovered class of candidate prebiotics that exert different properties depending on their structure. In this study the effects of two different structures of AXOS, namely AXOS-32-0.30 (average degree of polymerization: 32, average degree of substitution: 0.30) and AXOS-3-0.25, were investigated on growth performance, immune responses, gut microbial fermentation and gut bacterial composition of juvenile Siberian sturgeon (Acipenser baerii). After a two weeks acclimation, fish (25.9 ± 0.9 g) were distributed over 24 aquariums (8 replicates per treatment) and fed a control diet or a diet containing 2% AXOS-32-0.30 or AXOS-3-0.25 for 12 weeks. Growth performance and feed utilization tend to improve in sturgeon fed on diets supplemented with AXOS-32-0.30, however not significant. Survival was high in all groups. Both AXOS preparations significantly enhanced the phagocytic activity of fish macrophages compared to the control group, while the alternative haemolytic complement activity and total serum peroxidase content improved only in the group fed AXOS-32-0.30 (P < 0.05). The lysozyme activity was not affected by AXOS addition. Simultaneously, the amount of short-chain fatty acids (SCFAs) was highest in the hind gut of sturgeon fed AXOS-32-0.30. The concentrations of acetate, butyrate and total SCFAs in fish fed AXOS-32-0.30 was significantly higher than in the groups fed the control diet or AXOS-3-0.25. Study of the bacterial community in the sturgeon hindgut using PCR-denaturing gradient gel electrophoresis (PCR-DGGE) revealed that both preparations of AXOS induced changes in the bacterial composition. According to redundancy analysis (RDA), hindgut microbiota of each treatment group clustered apart from one another (P = 0.001). DNA sequencing of the dominant DGGE bands recovered from the different treatments showed that AXOS mainly stimulated the growth of lactic acid bacteria and Clostridium sp., with more pronounced effects of AXOS-32-0.30. It is concluded that AXOS improves sturgeon health through prebiotic action, but the induced effects depend on the specific structure of AXOS. A higher degree of polymerization of AXOS had a stronger beneficial impact in this sturgeon species.

A randomised, double-blind, placebo controlled cross-over study to determine the gastrointestinal effects of consumption of arabinoxylan-oligosaccharides enriched bread in healthy volunteers

Background

Prebiotics are food ingredients, usually non-digestible oligosaccharides, that are selectively fermented by populations of beneficial gut bacteria. Endoxylanases, altering the naturally present cereal arabinoxylans, are commonly used in the bread industry to improve dough and bread characteristics. Recently, an in situ method has been developed to produce arabinoxylan-oligosaccharides (AXOS) at high levels in breads through the use of a thermophilic endoxylanase. AXOS have demonstrated potentially prebiotic properties in that they have been observed to lead to beneficial shifts in the microbiota in vitro and in murine, poultry and human studies.

Methods

A double-blind, placebo controlled human intervention study was undertaken with 40 healthy adult volunteers to assess the impact of consumption of breads with in situ produced AXOS (containing 2.2 g AXOS) compared to non-endoxylanase treated breads. Volatile fatty acid concentrations in faeces were assessed and fluorescence in situ hybridisation was used to assess changes in gut microbial groups. Secretory immunoglobulin A (sIgA) levels in saliva were also measured.

Results

Consumption of AXOS-enriched breads led to increased faecal butyrate and a trend for reduced iso-valerate and fatty acids associated with protein fermentation. Faecal levels of bifidobacteria increased following initial control breads and remained elevated throughout the study. Lactobacilli levels were elevated following both placebo and AXOS-breads. No changes in salivary secretory IgA levels were observed during the study. Furthermore, no adverse effects on gastrointestinal symptoms were reported during AXOS-bread intake.

Conclusions

AXOS-breads led to a potentially beneficial shift in fermentation end products and are well tolerated.

Effects of a wheat bran extract containing arabinoxylan oligosaccharides on gastrointestinal health parameters in healthy adult human volunteers: a double-blind, randomised, placebo-controlled, cross-over trial

Wheat bran extract (WBE) is a food-grade soluble fibre preparation that is highly enriched in arabinoxylan oligosaccharides. In this placebo-controlled cross-over human intervention trial, tolerance and effects on colonic protein and carbohydrate fermentation were studied. After a 1-week run-in period, sixty-three healthy adult volunteers consumed 3, 10 and 0 g WBE/d for 3 weeks in a random order, with 2 weeks' washout between each treatment period. Fasting blood samples were collected at the end of the run-in period and at the end of each treatment period for analysis of haematological and clinical chemistry parameters. Additionally, subjects collected a stool sample for analysis of microbiota, SCFA and pH. A urine sample, collected over 48 h, was used for analysis of p-cresol and phenol content. Finally, the subjects completed questionnaires scoring occurrence frequency and distress severity of eighteen gastrointestinal symptoms. Urinary p-cresol excretion was significantly decreased after WBE consumption at 10 g/d. Faecal bifidobacteria levels were significantly increased after daily intake of 10 g WBE. Additionally, WBE intake at 10 g/d increased faecal SCFA concentrations and lowered faecal pH, indicating increased colonic fermentation of WBE into desired metabolites. At 10 g/d, WBE caused a mild increase in flatulence occurrence frequency and distress severity and a tendency for a mild decrease in constipation occurrence frequency. In conclusion, WBE is well tolerated at doses up to 10 g/d in healthy adults volunteers. Intake of 10 g WBE/d exerts beneficial effects on gut health parameters.

In vitro fermentation of arabinoxylan oligosaccharides and low molecular mass arabinoxylans with different structural properties from wheat (Triticum aestivum L.) bran and psyllium (Plantago ovata Forsk) seed husk

Ball milling was used for producing complex arabinoxylan oligosaccharides (AXOS) and low molecular mass arabinoxylans (AX) from wheat bran, pericarp-enriched wheat bran, and psyllium seed husk. The arabinose to xylose ratio of the samples produced varied between 0.14 and 0.92, and their average degree of polymerization (avDP) ranged between 42 and 300. Their fermentation for 48 h in an in vitro system using human colon suspensions was compared to enzymatically produced wheat bran AXOS with an arabinose to xylose ratio of 0.22 and 0.34 and an avDP of 4 and 40, respectively. Degrees of AXOS fermentation ranged from 28% to 50% and were lower for the higher arabinose to xylose ratio and/or higher avDP materials. Arabinose to xylose ratios of the unfermented fractions exceeded those of their fermented counterparts, indicating that molecules less substituted with arabinose were preferably fermented. Xylanase, arabinofuranosidase, and xylosidase activities increased with incubation time. Enzyme activities in the samples containing psyllium seed husk AX or psyllium seed husk AXOS were generally higher than those in the wheat bran AXOS preparations. Fermentation gave rise to unbranched short-chain fatty acids. Concentrations of acetic, propionic, and butyric acids increased to 1.9-2.6, 1.9-2.8, and 1.3-2.0 times their initial values, respectively, after 24 h incubation. Results show that the human intestinal microbiota can at least partially use complex AXOS and low molecular mass AX. The tested materials are thus interesting physiologically active carbohydrates.

Prebiotic and other health-related effects of cereal-derived arabinoxylans, arabinoxylan-oligosaccharides, and xylooligosaccharides

Arabinoxylans (AX) from cereals are cell wall components that constitute an important part of the dietary fiber intake in humans. Enzymatic hydrolysis of AX yields arabinoxylan-oligosaccharides (AXOS), consisting of arabinoxylooligosaccharides and xylooligosaccharides (XOS). This reaction takes place in the production of AXOS and of cereal-derived food products such as bread and beer, as well as in the colon upon ingestion of AX. This review mainly focuses on the available evidence that AXOS and XOS exert prebiotic effects in the colon of humans and animals through selective stimulation of beneficial intestinal microbiota. In addition, in vitro experiments and in vivo intervention studies on animals or humans are discussed that have investigated potential health-related effects resulting from the dietary intake of AX, AXOS, or XOS.

Safety Assessment of a Wheat Bran Extract Containing Arabinoxylan-Oligosaccharides: Mutagenicity, Clastogenicity, and 90-Day Rat-Feeding Studies

Wheat bran extract (WBE) is a food-grade preparation that is highly enriched in arabinoxylan-oligosaccharides. As part of the safety evaluation of WBE, its genotoxic potential was assessed in a bacterial reverse mutagenicity assay (Ames test) and a chromosome aberration assay on Chinese hamster lung fibroblast cells. These in vitro genotoxicity assays showed no evidence of mutagenic or clastogenic activity with WBE. The safety of WBE was furthermore evaluated in a subchronic toxicity study on rats that were fed a semisynthetic diet (AIN 93G) containing 0.3%, 1.5%, or 7.5% WBE for 13 weeks, corresponding to an average intake of 0.2, 0.9, and 4.4 g/kg body weight (bw) per day, with control groups receiving the unsupplemented AIN 93G, AIN 93G with 7.5% inulin, or AIN 93G with 7.5% wheat bran. Based on this rat-feeding study, the no-observed-adverse-effect level (NOAEL) for WBE was determined as 4.4 g/kg (bw)/d, the highest dose tested.

Tolerance of arabinoxylan-oligosaccharides and their prebiotic activity in healthy subjects: a randomised, placebo-controlled cross-over study

The tolerance and prebiotic effect following oral intake by healthy human subjects of arabinoxylan-oligosaccharides (AXOS), produced by partial enzymic hydrolysis of the wheat fibre arabinoxlyan, were studied. A total of twenty healthy subjects participated in the present randomised, placebo-controlled cross-over study. They consumed 10 g AXOS or placebo per d each for 3 weeks with a 4-week wash-out period in between. Before and immediately after each intake period, blood samples were taken to measure haematological and clinical chemistry parameters and the subjects completed a questionnaire about gastrointestinal symptoms. Additionally, urine was collected over 48 h for analysis of p-cresol and phenol content by GC-MS, and faeces were collected over 72 h for analysis of microbiota using real-time PCR. Of the subjects, ten also performed a urine and faeces collection 2 weeks after the start of intake (during intervention). A limited number of tested blood parameters were influenced in a statistically significantly way by either AXOS or placebo intake, but these changes remained within the normal range. Blood lipids remained unchanged. AXOS had no statistically significant effect on the range of gastrointestinal symptoms, except for a mild increase in flatulence. Urinary p-cresol excretion, an indicator of protein fermentation, was significantly decreased after 2 weeks of AXOS intake. The levels of bifidobacteria were significantly increased after 2 and 3 weeks of AXOS intake as well as after 3 weeks of placebo. However, the effect of AXOS on bifidobacteria was more pronounced than that of placebo. In conclusion, AXOS are a well-tolerated prebiotic at the dose of 10 g/d. AXOS intake increases faecal bifidobacteria and reduces urinary p-cresol excretion.