Inclusion of Dietary-Fibers in Nutrition Provides Prebiotic Substrates to Probiotics for the Synthesis of Beneficial Metabolites SCFA to Sustain Gut Health Minimizing Risk of IBS, IBD, CRC

Usually, everyday meals constitute materials and ingredients for food preparation derived from different agricultural sources. Although most customers are aware of the benefits of a balanced diet, they mainly focus on a diet based on the daily requirements of protein, fat, and carbohydrates in their meals. However, the vital aspect of the including dietary fibers in diets is overlooked, which is equally important as is the daily requirement of calories and protein intake for maintaining the muscle mass. Some societies consume a diet heavily based on animal-sourced materials, which is deficient in components of plant-derived beneficial fibers. In such consumers, the smooth functioning of the digestive system and the overall metabolism could be affected in due course of time.


Introduction
The dietary intakes are typically comprised of food items and nutrients sourced from plants, animals, and sometimes supplements.Generally, the food elements collectively influence the activities of the metabolic system of a person.The biochemistry between the components in nutrition and physiology establishes a balanced state of microbiota in the gastrointestinal tract (GIT).Consequently, the condition of consumers' general well-being is affected.The constituents in a balanced dietary serving of food should include those essential materials that can be effectively assimilated in the digestive system.In response, they can execute a physiological effect on consumers' health.The GIT microbiota, if adversely affected by malnutrition, will impact on the initiation of certain disease/s [1].A functional diet can include ingredients in the form of dietary compounds and dietary fibers sourced from various agricultural resources.Since the components of regular nutrition might affect the ecology of gut microbiota and its functioning, they consequently affect, positively or negatively, the health condition of consumers [2].The balanced gut microbiota with the intake of nutraceutical food prepared with prebiotic substrates and specific strains of probiotics can relieve hosts of gastric discomforts and allergic reactions to certain foods and their additives [3].
The objectives of research studies are relevant to this subject, which included understanding the dietary fibers and their influence on the activity of probiotics.Gut microbiota is a collective term for all microorganisms that survive in all vertebrates' GIT.The gut is the main site for the existence of microbiota.Hence, an individual's gut microbiota plays a dynamic role in maintaining health, or initiating and developing disease/s under balanced or disturbed conditions of microbiota, respectively [4,5].Various factors may prompt the changes in the organization and functioning of gut-microbiota, for example, more importantly, an imbalanced diet, then environmental conditions, low immunity-related health conditions, or treatment with prescribed antibiotics.Consequently, frequently imbalanced gut-microbiota may result in several gut disorders and chronic diseases [6].
In the last few decades, there has been a rise in research studies on the gut microbiota and the focus of investigations has commenced to include clinical trial studies.That would help to understand the mechanism by which the gut microbiota influence the general health of a person.There are research reports on the role and effectiveness of gut microbiota in the mitigation of several diseases, for instance, gut inflammation, inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and colorectal cancer (CRC) [1][2][3][4][5][6][7].Therefore, the routine intake of dietary components including prebiotics and probiotics, has been reported to be influential in sustaining health, as well as with their proven potential as psychobiotics for cognitive health issues treatment through the mechanism of gut-brain signaling [8].In this article, we aim to discuss the important topic of dietary fibers derived from plants, their types, sources, and their role as prebiotic beneficial for the efficient functioning of probiotic strains resident or supplemented in GIT.

Description of Dietary Fibers
Dietary fibers (DF) are those parts of plant material ingested as the constituent of food based on plant sources.DF include oligosaccharides, cellulose, non-cellulosic polysaccharides such as hemicellulose, pectic substances, gums, mucilages and a non-carbohydrate component lignin.Biologically, DF can be defined as the edible parts of vegetation and their products that are not absorbed in the human small intestine, their digestion being resistant to gastric enzymes.However, DFs are used by gut probiotic microbiota in complete or partial fermentation, depending on their composition [9].Natural Fibers synthesized by plants are mainly available in most agricultural provisions, such as vegetables, tuber crops, fruits, pulses, beans, legumes, grains and cereals.Consumption of such products provides necessary dietary fibers in the GIT system [10,11].Although these fibers contain various types of lignocellulosic structures and polymeric carbohydrates that cannot be digested by enzymes available in the human gut, they cannot be assimilated in the gut.However, these are components of the diet essential for the sustainability of microbiota residing in the gut [10].The dietary fibers can be classified into insoluble and soluble forms (Table 1) based on their sources.*Several agricultural materials are good sources of both types of fibers, which consumers can incorporate into their diets, according to their preferences and availability of items seasonally/geographically (A list of DF-food sources is presented in Table 2).
Most insoluble types of fibers, when consumed as an integral part of a meal, are beneficial for the digestive system as they contribute to faecal-bulking.Their main function is to provide a gutcleaning effect for a regular bowel movement.Contributing to another vital aspect, most soluble dietary fibers give rise to the formation of viscous gels in the GIT system [10].The presence of both types of fibers in meals has key impacts on the diversity and population of beneficial gut microbiota.Due to the fact that the physicochemical properties of various dietary fibers differ greatly depending on their source, it is essential to screen novel plant-derived fibers, aiming at the sustainability of gut microbiome and understanding their potential mechanism on gut microbiota-associated human diseases [11].
Microbiota in the host's gut forms a composite community of microbial species that regulate (in the presence of prebiotics) many key biological activities essential for good health [12].In recent decades, epidemiological evidence demonstrated the increased consumption of industriallyprocessed ready-meal diets, which are low in dietary fibers but rich in fat and sugar.This dietary change may cause a partial diminution of some beneficial bacterial species, disturbing the balance in the ecology of normal gut microbiota.Consequently, dysbiosis may result in dysfunctions of microbiota, leading to an increase in the development of recurring inflammatory illnesses, for instance, conditions related to cardiovascular, obesity, type 2 diabetes, IBD, CRC, allergies, and autoimmune diseases [13].

Description of Prebiotics
Technically, prebiotic materials have been identified as non-digestible components in food, conferring benefits linked with the adjustment of the host's probiotic microbiota in the gut [12].All those materials that provide nutrition to gut microbiota and promote growth for long-term survival in the GIT, are considered prebiotics.The accepted definition given by the International Scientific Association for Probiotics and Prebiotics (ISAPP) for a material to be classified as a prebiotic is "A substrate that is selectively utilized by microorganisms in the gut, conferring a health benefit" [14].Prebiotics are naturally present in most foods prepared with materials sourced from agricultural products, and hence, they foster the growth of beneficial bacterial species in the gut.Fibers in prebiotic materials provide valuable assistance in the smooth functioning of the digestive system, and in this way, overall health conditions are improved.
Consequently, the standard approach for sustaining the beneficial microbiota in the gut would be the addition of diverse materials sourced from plants in the diet.For instance, fresh seasonal or preserved fruits, legumes, root vegetables, tubers, unrefined flour from whole and multi-grains, nuts and seeds, which are stocked with naturally occurring dietary fibers as sources of prebiotics.
The dietetic quality of a regular meal with a mixture of functional ingredients is as important as the daily requirement of calories.This approach is even more critical for providing nutrition for elderly and sick people undergoing repeated antibiotic therapy, who have weaker gut microbiota [15].Balanced meals enriched in dietary fiber-prebiotics are also significant for those individuals with a reduced physically active routine and with a lower requirement of calories [16].

Difference between Dietary Fibers and Prebiotics
The relationship between fibers in food and prebiotics often leads to diverse opinions on differentiating the two terminologies.ISAPP has proposed a practical similarity of prebiotics with fibers [14].This comparison allows us to identify the measures of both, which influence the propagation of gut microbiota, needed to contribute to the host's health.Fiber is a considerable, nonetheless inadvertently overlooked, component in the routine meals of many people.Even though not all types of fibers can be assimilated by humans, yet, these can be used by the microbial strains inhabiting the gut.As a result, microorganisms growing in the gut collectively constitute a composite community of several species.DF acting as prebiotics, provides nutrition to probiotic strains, which are normally resident in the gut, or ingested by some people through the intake of supplements formulated with probiotics [14].
Soluble dietary fibers (Table 1) are identified as a form of abundant prebiotics available in most items of fleshy fruits, vegetables, and cereals like oats.Most fibers are non-digestible carbohydrates derived from plants, although their inclusion in the diet from assorted food sources is useful in supporting the regularized digestive system.Prebiotics and fibers both stimulate gut health by regulating bowel movements.Published reports have indicated that dietary fibers, through the regulation of gut functioning, could be beneficial in reducing various gut problems such as GIThealth syndromes [7], irritable bowel disease, CRC, allergies, and autoimmune diseases [13] and Crohn's Disease [17].

Sources of Prebiotics
Most fibers in a diet containing components from different plant sources act as prebiotics and are non-digestible materials Nonetheless, these provide useful fermentable oligo and polysaccharides (Table 2).Prebiotics may enhance the vigour and metabolic function of beneficial gut microflora.Prebiotic materials are resistant to low pH conditions and remain unchanged by digestive enzymes in the stomach.They securely move in the passage of GIT and are fermented in the large intestine by the inhabitant microbial species.Such prebiotic materials efficiently modify the balance and activity of beneficial microbiota.The reason is that the gastric enzymes required for the hydrolysis of bonds in the polymer molecules of prebiotics are not present in the digestive system.Consequently, the prebiotic materials make their safe way via the small intestine and reach the colon, where these can be fermented by commonly present probiotic strains of Lactobacillus, and Bifidobacteria [21].
Prebiotics are classified according to the number of monomer saccharides linked in the composition of dietary fibers, for example, di-saccharides (2 units), oligo-saccharides (>2), and polysaccharides (n units) [22,23].Compared to unfermentable fibers, fermentable fibers have a much more compelling effect, in terms of effective prebiotics, on the metabolism of gut microbiota and the maintenance of consumers' gut health [24].The widely used standard for the categorization of prebiotic materials is based on oligosaccharides (Table 3).

Prebiotic Materials in Commercial Use
Several prebiotic products are marketed for dietary consumption, such as oligosaccharides.These are selected for their ability to surpass the activity of gastric enzymes, nevertheless, they can be selectively used by the microbial population resident in the gut.Various functional prebiotic materials are often used as mixer ingredients in commercial synbiotic formulation and are also available as individual prebiotic products.The most researched materials are FOS, GOS, and Inulin.Prebiotic materials are sold in health shops for use with or without probiotics.Frequently used prebiotic items contain inulin, FOSs, GOSs, IMOSs, XOSs, lactulose, polydextrose, and lactitol etc. [25].Supplementary poly-saccharides pectin, cellulose, hemicellulose, or starch, are also used in a few commercial prebiotic compounds and in the formulation of synbiotic products [26].
Commercial supplement products are available in designed combinations of prebiotic materials with specific strains of probiotic microbial cultures.These synbiotic preparations, if integrated into the diet of adults in reasonable measures (about 5-20 gm per day depending on the type of constituent-prebiotic material), encourage the growth of essential gut bacteria Bifidobacteria and Lactobacilli in the GIT [27].Oligosaccharide XOSs are composed of 2-10 xylose units linked with β-(1,4) bonds, and these are produced from xylan-containing plant material like banana-pseudostem via enzymatic hydrolysis [28].XOSs have the ability to boost the growth of beneficial gut microflora, hence they have been recognized as one of the critical dietary fibers.Consequently, XOSs have established their use as an effective prebiotic ingredient in synbiotic products on a commercial scale [29].

Prebiotics Regulate the Functioning of Probiotics in the Gut
Food preparations with an active population of probiotic cultures combined with prebiotics are characterized as nutraceuticals or functional foods, gaining widespread acceptance in the health food market [3,16,30].Globally, consumers have become interested in the intake of functional diets to sustain their health and improve the quality of their lives [31].Specific strains of lactic acid bacteria have established their beneficial probiotic health effects like maintenance of intestinal microbiota by the prevention of pathogenic species in the intestine.Reports have suggested live cells of probiotic strains are suitable starter cultures for the production of fermented foods and functional beverages.Besides, their metabolites also have applications as food additives and they can be used by adding directly to foods [32].
Prebiotic materials individually or in mixtures combined with probiotic strains in the formulation of synbiotic products, can balance gut microbiota and sustain gastrointestinal health [34].Therefore, prebiotics have become part of nutraceuticals because of their role in moderating the gut microbiota via their metabolic activities.However, different prebiotic materials can affect the growth and functions of different strains of probiotics.In a study performed in-vitro, the growth medium was prepared with a variety of prebiotic materials, inulin, fructo-oligosaccharides and galacto-oligosaccharides, incorporating them individually to test the growth of two typical probiotic strains Lacticaseibacillus rhamnosus and Bifidobacterium animalis subsp.lactis.The results revealed that all three sources of prebiotics accelerated the growth of both probiotic strains in the fermentation medium grown under both conditions, mono-culture, or co-culture.However, results demonstrated that each prebiotic substrate affected the growth of bacteria at a different rate.It is probably due to the mechanism of Lactobacillus strains in the fermentation process producing a specific enzyme galactosidase to utilize the prebiotics added as a carbohydrate substrate [35].
Another example is the use of FOS, which has proven a very effective carbohydrate source acting as a prebiotic for the growth of probiotics.FOS has been reported to increase the growth rate of two strains, Bf-1 and Bf-6, of Bifidobacterium [36].The bifidobacteria were able to hydrolyze FOS for their use, for the reason that they produce a viable enzyme fructo-furanosidase, which is required to break down FOS.Studies have proven two enzymes galactosidase and fructofuranosidase being involved in the hydrolysis of prebiotic materials GOS and FOS, respectively.Enzymes from microbial sources have been reported as useful biocatalysts for the processing of raw natural substrates derived from edible parts of plants and other agricultural residual materials [37][38][39].
Substrates used as prebiotics are fermented selectively in the gut, allowing actual modifications, both in the gut microbial community and their activity providing benefits to the host [40].The purpose for the use of prebiotics lies in the understanding that it should increase the proliferation of essential probiotics in the gut and assist in the synthesis of their metabolites, which affect the alleviation of gut health issues and CRC [13].Prebiotics can employ health-beneficial effects on the colon through their use by specific strains of bacteria synthesizing useful metabolite compounds [41].For the sustainability of resident microflora, prebiotic materials serve nutrients that effectively steer the growth of probiotic bacterial species [42], which could depend on the molecular structure of prebiotic materials.
Studies have revealed that foods containing inulin (prebiotic) can improve the growth of propionate producers in Bacteroidetes inhabitants.In a study, the traditionally processed meat products were re-formulated for an inulin-rich product, towards the production of functional foods used to reduce polyps in two animal models of colorectal cancer [43].The inulin component in diet also benefits in reducing the Firmicutes population.It should be considered that higher percentages of Bacteroidetes and Firmicutes are predominantly related to those diseases, which are associated with gut inflammation [44,45].Some of the essential prebiotics that can be easily taken by the GIT microbiota are referred to as non-digestible oligosaccharides.Such prebiotics have unique glycosidic bonds of the anomeric carbon from monosaccharide units, and these bonds cannot be hydrolyzed by the gastric enzymes of humans [46].The commonly used non-digestible oligosaccharides-prebiotics are fructo-oligosaccharides, galacto-oligosaccharides, and xylooligosaccharides, which can influence the composition of microbiota to ease the severity of colorectal cancer [47].The impacts exerted by galacto-oligosaccharides have been studied on the ecology of gut microbiota of adult hosts taking antibiotic medication, where prebiotic was found supporting an increased number of Bifidobacterium spp.and Lactobacillus spp resident in the intestine [48].
A team of researchers studied the genetic expression of enzymes implicated in the intake of xylooligosaccharides by Lactobacillus spp [49].A randomised controlled study in Kenyan infants reported the prebiotic galacto-oligosaccharides relieving the antagonistic effects of fortification with iron on the gut microbiome [50].The influence of galacto-oligosaccharide mixture has been studied on immunity parameters and metabolomics in the gut microbiota of elderly persons [51].Prebiotic impacts caused by xylo-oligosaccharides have also been reported to improve the microbiota balance in human studies by Lin et al [52].

Metabolites Produced from Prebiotics' Fermentation in GIT
Prebiotics being indigestible by human gastric enzymes usually move unchanged in the gastrointestinal tract and are available to gut bacteria as suitable substrates for fermentation.As a result, the main production of short-chain fatty acids (SCFAs), like acetic, propionic, and butyric acid, occurs [53].These SCFAs perform important functions in maintaining gut microbial ecology and influence their metabolic system.Moreover, lactic acid produced by LAB can aid in neutralizing the alkaline pH in the colon and thus establishes a more neutral environment for beneficial bacteria to grow.Bifidobacterium and Lactobacillus inhibit pathogens under favorable intestinal pH due to the synthesis of their metabolites [54].This has been reported that SCFAs influence the drop in alkaline pH of the gut, inhibiting the growth of species of microbial pathogens [54,55].The SCFAs are absorbed by epithelial cells in GIT for use as a source of energy and as metabolic regulators.This activity improves the growth of villi, crypt development, tight junctions, and mucin production [55].The action of butyrate influences the formation of the intestinal epithelium [56].SCFAs are also important for promoting salt and water absorption in the colon by the process of nutrient control and ion transporters, which can also help prevent diarrhoea triggered by a condition of short-bowel syndrome (SBS) [57].These factors result in effective remedial actions for conditions of irritable bowel syndrome and inflammatory bowel disease.These findings indicate that the release of SCFAs by probiotics with the use of prebiotics may be a significant mechanism for sustaining gut health.
The definite contribution of prebiotics to human health is coupled with their capacity to adjust the viability of probiotics, and subsequently with the regulation of secretion of metabolites, extracellular polysaccharides, and SCFA in the gut.With the regular intake of a diet comprising functional food elements (synbiotic preparations), prebiotic materials and probiotic cultures are supplemented in the gut.Their presence in the gut encourages the state of a microbial balance in the host's GIT-microbiota.Studies have recognized the beneficial properties of probiotics actively sustaining in the gut, supporting the deterrence of intestinal disorders, defense against cancer, stimulation of immune function, and reducing symptoms of irritable bowel syndrome and cholesterol level, and contributing to several therapeutic benefits [16].Some of these impacts, as discussed above, are facilitated by the activities of SCFAs, which are produced by probiotics with the fermentation of prebiotic substrates in the gut.
Probiotic strains actively residing in the gut implement respective health effects by various mechanisms.Principally for their endurance in GIT, probiotics strive for nutrients available in the form of prebiotic materials, and in this approach, they obstruct the growth of harmful microorganisms by hindering their adherence to gut epithelial cells.The lactic acid bacteria produce antagonistic complexes like bacteriocins and organic acids that impose an inhibition stimulus on the pathogen's development and deter the colonization of any opportunistic organisms [58].Other activities applied by probiotics are regulating the immune system by affecting immune-globulin production, increasing the cytotoxic property of natural killer cells, and the adjustment of cytokine secretion.The beneficial metabolite exopolysaccharide (EPS) has been studied for effectively alleviating gastritis in Helicobacter pylori-infected mice by a mechanism of down-regulating mRNA expression levels of pro-inflammatory cytokines IL-6, IL-8, IL-1β and TNF-α, and supporting the upregulation of mRNA expression of inflammatory cytokine IL-10 in gut cells.EPS was also found to be effective in positively regulating the ecology of GIT microflora [59][60][61].Consequently, in consideration of several benefits as discussed above, the use of probiotics along with suitable prebiotic materials in functional foods has created a wide-reaching enterprise in the food industry.

Dietary Intervention Studies
Studies have incorporated the interventions of prebiotics with probiotics to achieve complex advantages in diverse systemic illnesses related to inflammatory, gastrointestinal, cardiovascular, and neurological [60].Helpful probiotics resident in the host's GIT system would selectively use certain prebiotics materials for developing and sustaining their growth in the gut [31].Thus, the inclusion of selected prebiotics in the intervention study is supposed to enhance the number of favorable gut bacteria and exclude other infective bacteria.Furthermore, the integrity of the gut barrier and properties related to immunomodulation develop in the presence of SCFAs, which are released through the fermentation of oligosaccharides [30].The intake of diets containing DF and grains has been correlated with lowered risks of CRC, proposing a shielding impact of these prebiotic molecules [61].
The topic of the quality of consumed carbohydrates and human health has been studied in some animal models and also in-vitro studies, and has been reviewed in systematic and meta-analyses [62,63].Prebiotics, as nutrients for probiotics, can help retain intestinal microbial homeostasis and mitigate the condition of dysbiosis, which could be beneficial in preventing gut inflammation and CRC.These nutrients can deter the onset of dysbiosis by reassuring the growth of beneficial bacteria needed for the production of short-chain fatty acids, the maintenance of the barrier of the intestinal epithelium, the development of anti-inflammatory immunity, and pro-apoptotic mechanisms [64].The beneficial bacteria sustaining in gut microbiota with the support of prebiotics, produce metabolites effective as antimicrobials for pathogens.
Probiotics degrade polymer prebiotics into oligo and monosaccharides which connect with the lectin receptor on the surface of epithelial cells, this binding mechanism blocks the settlement of opportunistic disease-causing strains at the receptor site [55].Furthermore, some species of gut residents stimulate the immune system by signaling the dendritic cells [54,55].The consumption of prebiotics-rich diets influences gut microflora composition and their metabolic activity.The chemical structure of prebiotics controls their physiological effect on gut microbiota that can use these prebiotics as an energy source in the intestine [54].Hence, the properties of prebiotics based on their structural configuration are associated with the fluctuations in the gut-microbiota and can cause improvements in hosts' metabolism linked to several health disorders like obesity, inflammations, glucose homeostasis syndrome, and abnormal plasma lipid levels [65].
Furthermore, the use of prebiotics indirectly causes a decline in triglyceride levels in the serum and thus they might affect the absorption of minerals in the large intestine, protecting against inflammatory bowel syndrome by the production of butyrate [66].Therefore, prebiotics can be considered as an important functional food for the colon, which can help in the improvement of general health through the microbiota residing in the colon [67].The sustainability of well-known probiotic strains depends on selecting the most profitable combination of prebiotic substrates.Therefore, ingesting correctly chosen probiotics and DF as natural sources of prebiotics may be useful in developing positive effects, both for individual probiotics and several strains working synergistically [68].Although, the consumption of most prebiotic materials is without serious risk to health; however, the consumption of their excess quantities might induce unwanted effects -the commonly experienced effects are bloating and gas production causing abdominal pain with flatulence [56].
Although the prebiotic materials are available in the form of a variety of plant-DF (Table 1, Table 2 and Table 3), however, for the intervention studies, the most frequently used DF are inulin, fructooligosaccharides, and galacto-oligosaccharides [54,69].Their widely accepted use as prebiotics has also been supported by results from in vitro studies [70,71].FOS and GOS were tested in vitro for the requirements of the current criteria for effective prebiotics [72].In addition, reports of studies have revealed that a minimum dose of 4 g a day of FOS or higher up to 8 g would be required to improve significantly the population of Bifidobacteria in the human gut [73].The mixing of a few prebiotic materials in optimal doses may increase the probiotic population and stimulate their action [67].Besides, the use of prebiotic sources as an unaccompanied material or mixed with probiotic strains in synbiotic preparations can improve gut health by preventing the onset of IBD, IBS, and CRC by selectively stimulating the metabolism of health-promoting bacteria (Figure 1).

Prebiotics Studied for Their Use in Functional Food
The awareness of functional food for well-being, which is essentially correlated to the diet consumed, has been rising [74].Thus, consumers are more interested in buying food products containing required nutritional components with health promotion properties [75].This has regulated an approximately 10% increase in the functional food business, opening the market for new nutraceutical products, such as food with prebiotic-DF and probiotic components [76].This has been again stressed in studies that prebiotics are dietary ingredients needed to improve the establishment of gut microbiota, which in turn adjusts the state of health by hindering the development of diet-related disorders [77].
Furthermore, prebiotics sourced from different plant materials (Tables 1-3) may amend and enhance the physicochemical and sensorial quality of food.Hence, these natural substrates, placed in four different groups in Table 2 are potential ingredients for developing new food products [78].In addition, to supply cost-effective and consumer-friendly options, the food industry can incorporate prebiotics sourced from various seasonal and regional agricultural materials in the formulation of economically attractive products [79,80].The standard strategy has been the direct combination of the prebiotics into food products by simple fortification [75].FOS are oligomers of fructose and are synthesized enzymatically through the transfructosylation of sucrose by the activity of the enzyme fructosyltransferase [81].The capability of common probiotics Lactobacilli and Bifidobacteria to ferment particular prebiotic substrates containing oligosaccharides and polysaccharides can also be significant and explored for developing synbiotic functional foods [82].
Therefore, it is worth evaluating the prebiotic potential of different foods and the ingredients to prepare foods.The prebiotic index (PI) value is used as an evaluation to decide on the selection of prebiotic-rich foods.The value of PI is considered by comparing the increase in the probiotic strains' growth (an increase in the cell populations of bacteria is a constructive effect), with the growth in the presence of a less desirable ingredient (an increase in bacterial population is a negative effect) [67,83,84].Ghoddusi et al. [70] testified that inulin, FOS, polydextrose, and IMO, individually or in a blend, influenced on the value of PI.Since the materials used as prebiotics have different functionalities due to their chemical structures affecting the rate of their fermentation by GIT bacteria, this factor determines an influence on PI.Figueroa-Gonzalez et al. [67] studied the proficiency of several strains of probiotics (including L. casei Shirota, L. casei 1, L. casei 2, L. rhamnosus GG, and L. rhamnosus) to ferment different prebiotics like inulin, GOS, and lactulose.According to the study outcomes, all assessed probiotics could grow in a medium added with the selected prebiotic.Nevertheless, their growth occurred differently at each incubation time.Remarkably, all probiotics, except L. casei Shirota, had higher growth in inulin and GOS than the control (lactulose).
Buddington et al. [85] reported that inulin and FOS provided adequate protection from pathogens Salmonella typhimurium and Listeria monocytogenes in mice with unusual crypt foci in their colon compared with in vitro samples of the cell line.Additionally, probiotic Bifidobacterium, and prebiotic TGO, could be applied in a murine model for the anti-infective activity against Salmonella [86].In a study conducted observing the efficacy of prebiotics in the mixture, GOS proved to have an advanced performance over other prebiotic materials [82].In a different study, the measure of SCFA synthesis was taken as a reference point for the effectiveness of prebiotics, where the level of SCFAs was calculated as a sum of all acids including acetic, butyric, and propionic.
The response surface analysis indicated that the synthesis of SCFAs in the fermentation of FOS was directly related to the use of the prebiotic substrate [87].

Conclusion
As discussed in previous sections, different prebiotic-DF materials can accelerate the development of strains of gut probiotics, hence they should be selected based on their composition which could favour their enhanced functionality (Figure 1).Among the four most effective prebiotics (FOS, inulin, GOS, and mannan oligosaccharide) inulin has been routinely used as a prebiotic component in the food industry.Nevertheless, to get a pure preparation of this long-chain polysaccharide, the process of its extraction from plant sources like chicory root, artichoke, and asparagus is costly.Alternatively, FOS and GOS are short-chain OS that can be obtained from plant sources like sugar cane.Yet, different sources may have marginally different properties for prebiotic specifications.Subsequently, the screening of a mixed prebiotic ratio might result in a potentially useful ingredient for the production of cost-effective functional food.Suitable ratios of mixed prebiotics should be used in synbiotic food supplements to improve probiotic stimulation and establish a balance in the gut microbiota, considering the results of intervention studies conducted with human volunteers.

Future Prospectives
For the research and development of prebiotic sources for their formulation in functional foods, the selection of materials should be based on their fermentability by gut bacteria.For example, prebiotics FOS, GOS, and XOS are rapidly fermented by beneficial strains of probiotics, succeeding in a higher score of the prebiotic index.Moreover, the number of sugar units in OS -prebiotic molecules i.e. the degree of polymerization (DP) is an important factor in their fermentation and PI score.Preferred prebiotics are FOS, GOS and XOS with a lower DP of 2-10, for the reason these are easily fermented and have a higher prebiotic index score, compared to inulin with a higher DP [88][89][90].It should also be considered that starch and cellulose are types of agriculturally sourced substrates that need longer fermentation time, hence not preferred as efficient prebiotics, however, with the innovative process-technology they could have a different utility to synthesize other valueadded products [90][91][92][93][94][95][96].Although the research on a variety of bioactive materials derived from plant-bioresources, and probiotic strains is ongoing for their quality, function, and use in the biosynthesis of therapeutic compounds [97][98][99][100], and in the fortification of bioactive molecules in food [101,102]; nevertheless, the prebiotic effect of OS also needs to be examined on the activity of those probiotic strains, which have been isolated from regional fermented food products.The emerging trend is that novel functional food to cater to consumers with different diet preferences can be prepared with combinations of prebiotic-DF substrates in the food-fermentation process, employing characterized strains of probiotics.Such products can be specifically used as nutraceuticals for supplementing gut microbiota in hosts who suffer from gut dysbiosis, food discomforts, and are allergic to certain foods or their additive ingredients.

Table 1
Description of the two types of plant-derived dietary fibers.

Table 2
Sources of Prebiotics Derived from Plants*.