Introduction
Xylooligosaccharides (XOS) are gaining recognition as valuable prebiotics with diverse health benefits. These sugar polymers, composed of xylose, are derived from the xylan fraction of plant fiber and are produced from the xylan fraction in plant fiber. Unlike other prebiotics based on C6 sugars, XOS feature a unique C5 structure, where C is a quantity of carbon atoms in each monomer. They selectively feed beneficial bacteria in the digestive tract, promoting overall well-being. Their C5 structure is fundamentally different from other prebiotics, which are based upon C6 sugars.
What are Xylooligosaccharides?
Xylooligosaccharides (XOS) are non-digestible oligosaccharides composed mainly of xylose units with high prebiotic potential. XOS are mixtures of oligosaccharides containing β-1,4-linked xylose residues which naturally occur in bamboo shoots, fruits, vegetables, milk, and honey. These prebiotics are indigestible food supplements that play an important role in stimulating the growth of beneficial bacteria for health. These oligosaccharides, with a chain of 2-10 xylose units linked by β-1,4-xylosidic bonds, resist digestion in the upper gastrointestinal tract, allowing them to reach the lower intestine where they are metabolized by probiotic bacteria. This type of link allows XOs to resist the attack of gastric enzymes, so they pass through the upper gastrointestinal tract without being digested, until they reach the lower intestine and are metabolized by the probiotic microbiota. Smaller chains with a polymerization degree of 2-4 are particularly effective in promoting the growth of specific probiotics, especially those belonging to the Bifidobacterium genus.
XOS vs. AXOS
Arabinoxylans (AX) are xylans with arabinose substitution at the C-2 and/or C-3 positions of the xylan backbone, and are most present in cereals like wheat, barley, maize, and rice. Hydrolysis of AX leads to a mixture of unsubstituted and arabinose-substituted XOs, the so-called arabinoxylan-oligosaccharides (AXOs). Each species of probiotic has specific preferences between XOs and AXOs assimilation. When in the colon, AXO and XO fragments are further degraded to xylose and arabinose by extracellular and/or intracellular arabinofuranosidases and xylosidases produced by specialized bacteria, including Bifidobacterium species.
History and Availability
Xylooligosaccharides have been commercially available since the 1980s, originally produced by Suntory in Japan. They have more recently become more widely available commercially, as technologies have advanced and production costs have fallen.
Health Benefits of Xylooligosaccharides
Xylooligosaccharides (XOS) are categorized as prebiotic fibers that play a significant role in enhancing gut health. XOS enhances gut health by selectively stimulating the proliferation of beneficial bacteria. Xylooligosaccharides act as a prebiotic, selectively feeding beneficial bacteria such as bifidobacteria and lactobacilli within the digestive tract. In general, XOs increase digestion and absorption of nutrients, but also present health benefits by preventing the growth of pathogenic bacteria.
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Gut Health and Microbiota Modulation
The dynamic and complex populations of gastrointestinal microorganisms play a pivotal role in human health. The intestinal microbiota not only exert metabolic activities but also participate in the defense against invading pathogens. It has been suggested that disruptions to intestinal microbial balance may lead to diseases including chronic intestinal diseases, colorectal cancer, type 2 diabetes, and obesity.
Probiotics are bacteria that provide health-promoting properties for the host lining of the colon. The most commonly used and/or studied probiotics are largely species of the genera Bifidobacterium and Lactobacillus, all of which can be found in the host's own microbiota and fermented foods. Evidence suggests that these probiotic bacteria can alleviate lactose intolerance, inhibit the growth of harmful bacteria, prevent colon cancer, decrease cholesterol levels, improve digestion, reduce inflammation, and stimulate the immune system. Prebiotics can nourish probiotics and encourage them to function more efficiently, allowing the bacteria to stay within a healthy balance.
XOS has been found to be predominantly utilized by members of the Bifidobacterium genus. Furthermore, the consumption of XOS results in increased indigenous Bifidobacterium spp. levels in the gastrointestinal tract and fecal short-chain fatty acids in rats.
The inhibition of pathogenic bacteria due to the action of XOs is mediated by two mechanisms: (i) increasing the proliferation and colonization of probiotic bacteria in the intestinal epithelium, and (ii) decreasing the pH by inducing the production of organic acids, such as lactic acid and acetic acid. Thanks to these mechanisms, XOs indirectly prevent gastrointestinal infections, maintaining fecal water levels and preventing diarrhea. In addition to suppressing the activity of enteric bacteria, they decrease the production of toxic compounds such as amines.
Impact on the Gut-Liver Axis
The gut-liver axis is a crucial connection between the gastrointestinal tract and liver function. The gut-liver axis refers to the bidirectional communication between the gut and the liver. This connection impacts various processes, including detoxification and glucose homeostasis. A healthy intestinal barrier prevents the translocation of harmful bacteria and endotoxins. Gut microbiota plays a significant role in liver health and regulation of the immune system. An imbalance in intestinal microbial diversity can result in gastrointestinal disorders. This imbalance can lead to chronic inflammation and negatively impact liver function.
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Liver Health Benefits
Xylooligosaccharides (XOS) offer several noteworthy benefits for liver health. Xylooligosaccharides have been shown to reduce inflammatory responses in the liver. They modulate the gut microbiota, producing short-chain fatty acids (SCFAs). Research indicates that XOS can improve gut health by promoting beneficial bacteria that produce SCFAs. This, in turn, mitigates the systemic inflammation linked to obesity and fatty liver disease. Xylooligosaccharides may also support lipid metabolism, specifically in reducing liver fat and improving lipid profiles. The mechanism involves enhancing fat oxidation and decreasing the accumulation of triglycerides in the liver.
The role of xylooligosaccharides in improving insulin sensitivity is significant. Increased insulin sensitivity can prevent the onset of metabolic conditions like insulin resistance, often associated with liver damage and fatty liver disease. By lowering blood glucose levels, XOS helps reduce the risk of hyperglycemia and associated liver complications. Xylooligosaccharides contribute to antioxidant activity in the liver through SCFAs. These fatty acids are known to have protective effects against oxidative stress, a contributor to liver damage. By neutralizing free radicals and reducing oxidative damage, these SCFAs support liver health.
Additional Health Benefits
Xylooligosaccharides beneficial characteristics as prebiotic in humans.On the other hand, XOs present specific indirect actions within the intestinal tract. These prebiotics are used and metabolized by probiotic bacteria, generating an increase in the production levels of small-chain fatty acids (SCFAs), with butyrate and propionate SCFAs being the most produced. High levels of SCFAs maintain the integrity of the gastrointestinal barrier by regulating cecal cell proliferation and apoptosis, and by encouraging goblet cell differentiation via Notch and Wnt/β-catenin signaling pathways. It has also been reported that dietary supplementation with XOs can increase the expression of molecular chaperones and improve the ubiquitination of proteases, which would also demonstrate that XOs have a healthy protective activity in the mammalian intestine.
These characteristics give XOs greater potential compared to other established prebiotics such as fructooligosaccharides, galactooligosaccharides, and inulin, hence making them interesting supplementary food ingredients. Further, probiotics of the Bifidobacterium genus present a greater predisposition for the consumption of pentoses compared to prebiotics composed of hexoses (fructooligosaccharides and galactooligosaccharides). In addition, it has been reported that due to the high induction of responses beneficial to health, smaller daily doses of XOs are necessary in food supplementation compared to other prebiotics, with XO dietary supplementation requiring only 1.4-2.8 g per day. This makes XOs more economically competitive than other prebiotics, which has high potential for consumer preference as a healthy food product.
Immunomodulatory and Anti-inflammatory Effects
XOS is suggested to be an immunomodulator to prevent adverse immune-related conditions. Indeed, XOS was shown to have immunomodulatory effects by regulating expression of several proinflammatory mediators in vitro. XOS not only suppressed TNF-α, IL-1β, IL-6 and NO expression, but also triggered IL-10 production in lipopolysaccharide (LPS)-stimulated RAW264.7 cells. XOS feeding significantly decreased expression of IL-1β and IFN-γ and attenuated systemic inflammation.
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Anticancer Properties
It has been shown that XOS exposure showed effect in preventing cancer. Indeed, β-1,3-Xylooligosaccharides with an average DP of 5 extracted from green alga Caulerpa lentillifera inhibited the number of viable human breast cancer MCF-7 cells in a dose-dependent manner, and induced apoptosis. Moreover, XOS supplementation reduced the level of lipid peroxidation and increased the activities of glutathione-S-transferase and catalase in colonic mucosa and liver, which may have contributed to the inhibition of colon carcinogenesis.
Antioxidant Activity
Several notable studies demonstrated that XOS had exhibited strong antioxidant and free radical scavenging activity, thus suggesting a potential use in biomedical applications. The scavenging ability of XOS was shown to be dose-dependent, and this potential is likely attributable to efficient release of phenolic compounds and transfer of hydrogen atoms from the phenolic compounds to free radicals.
Antimicrobial Effects
It has been reported that XOS have significant antimicrobial effects against several pathogenic bacterial. A host of clinically important both Gram-negative and Gram-positive bacteria have been documented to be sensitive to XOS exposure.
Xylooligosaccharides in Animal Husbandry
XOS have been used for animal nutrition and health improvement due to their potential biological functions, such as antioxidant, anti-inflammatory and antimicrobial effects. Previous studies have demonstrated the benefits of XOS on the growth performance of animals. The growth promoting effect of XOS has been shown to be related to improvement in nutrient digestibility.
Production of Xylooligosaccharides
The cost of the production of XOs is a limiting factor, so their obtention from agro-industrial waste, including many grain by-products, has been considered a potential cost-reduction strategy as this is a low-cost, renewable, and abundant raw material. Xylan, the second most abundant biopolymer in nature, constitutes most of the hemicellulose that can be degraded into XOs, using combinations of pretreatments and enzymatic hydrolysis. Despite the benefits of using agro-industrial residues, their varying composition is a challenge, because depending on the chemical composition, the residues may be more or less suitable for XOs production. Residues with higher amounts of xylan and low amounts of lignin are better options. Corn cob, wheat straw, rice straw, corn stover, switchgrass, and sugarcane bagasse are some of the relevant residues for XOs production, due to their high levels of hemicellulose.
Agro-Industrial By-Products as a Source
In recent years, lignocellulose biomasses have emerged as a source of raw material in the production of high-value-added biomolecules, including XOs prebiotics. The great distribution worldwide, its great abundance, and its renewable nature make lignocellulosic biomass a natural source that is easily accessible and cheap compared to other raw materials. Within the huge set of lignocellulosic biomasses, agro-industrial residues or by-products obtain greater interest due to their high production volumes.
Agro-industrial by-products are mostly composed of cellulose, hemicellulose, and lignin structures. Cellulose and hemicellulose can be enzymatically hydrolyzed into glucose and xylose; however, cellulose is strongly associated with hemicelluloses and lignin, preventing the access of hydrolytic agents, and its crystalline structure is also an extra obstacle to hydrolysis. To increase the efficiency of the use of lignocellulosic residues, it is necessary to perform a step before the enzymatic hydrolysis, called pretreatment, to extract xylan from the plant cell wall. Several pretreatment techniques have been reported in the literature, such as steam explosion, solvent extraction, and thermal pretreatment using acids or bases, organosolv, deep eutectic solvents, and hydrothermal pretreatment.
Pretreatment Methods
During pretreatment, hemicellulose can be depolymerized to produce various xylosugars, among them short-chain polysaccharides with different degrees of polymerization, which are considered attractive because of their potential use to promote the growth of intestinal bacteria and improve immunity. Alkaline pretreatment has the disadvantage of partially degrading carbohydrates and causing equipment corrosion and environmental contamination. Acid pretreatment generates undesirable sugar monomers and many toxic by-products, such as furfural, hydroxymethylfurfural, and formic acid, and excessive degradation of xylan to xylose results in lower purity of the XOs. Hydrolysis with acetic acid has been used to prepare XOs because of its fast reac…
Degradation Methods
XOS are the degraded products prepared by chemical, physical or enzymatic degradation of xylan derived from biomass materials such as sugarcane residues, corn cobs, rice straw, etc. They are composed of xylose units linked by β-1, 4-xylosidic bonds, which have a branched structure by the addition of different side groups (Moreira et al.).
The chemical degradation process, especially the acid and the alkaline hydrolysis methods, has been widely used for the mass production of XOS in industry due to its advantages such as simple operation and low production cost. Several studies have been conducted on producing XOS with various inorganic acids. XOS production can also be obtained from corn cob xylan using weak sulphuric acid at 90°C during 30 min.
Production XOS products by physical degradation is relatively simple and environmentally friendly compared to chemical degradation. For example, XOS can be obtained from milled aspen wood using a microwave oven, processing at 180°C for 10 min were and nextly subjected to fractionation to oligo- and polysaccharides by size-exclusion chromatography.
The industrial process of XOS production from natural xylan-rich agricultural residues involve enzymatic hydrolysis. As compared to the acid and alkaline hydrolysis method, production by the enzymatic degradation is relatively more economical, quick, and eco-friendly. Furthermore, enzymatic hydrolysis neither requires any special equipment nor produces undesirable byproducts. Thus, the production of XOS by enzymatic means was done from plant sources rich in xylan including corn cobs, sugarcane bagasse, wheat bran, birch wood, oat spelt, beech wood, natural grass, oil palm frond etc.
Applications of Xylooligosaccharides
XOs prebiotics can be consumed in many forms in food and feed. For animals, it is normally incorporated at a given percentage in the daily diet. In the case of human consumption, it can be ingested directly as a pure supplement, as in the case of capsule intake, or it can be incorporated in beverages (like juices), biscuits, and breakfast cereals, among other things. The addition of XOs in those products has great acceptance according to the literature since they contribute to sensorial properties by increasing sweetness and taste intensity.
Incorporation into Food Products
In this study, we investigated the effects of xylooligosaccharides- (XOS-) enriched rice porridge consumption on the ecosystem in the intestinal tract of human subjects. The results showed that 6-week daily ingestion of the XOS-enriched rice porridge induced significant increases in fecal bacterial counts of Lactobacillus spp. and Bifidobacterium spp., as well as decreases in Clostridium perfringens without changing the total anaerobic bacterial counts, compared to that of placebo rice porridge. In conclusion, the intestinal microbiota balance was improved after daily consumption of 150 g of rice porridge containing XOS for 6 weeks, demonstrating the prebiotic potential of XOS incorporated into foods.
Dosage and Safety
Current research on XOS suggests a safe daily dosage range of 2 to 10 grams. Exceeding this recommended dose may not enhance its effects and could lead to gastrointestinal discomfort, such as gas or bloating. XOS can be found in various forms, including dietary supplements and certain food sources. Dietary supplements are available in powdered and capsule forms. Integrating XOS-rich foods alongside supplementation may offer synergistic benefits for those seeking to improve liver health. Individuals with specific health conditions must exercise caution when using XOS. Individuals taking certain medications that affect liver function should also consider potential interactions.
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