Dietary Polysaccharides as Modulators of the Gut Microbiota Ecosystem: An Update on Their Impact on Health

A polysaccharide is a macromolecule composed of more than ten monosaccharides with a wide distribution and high structural diversity and complexity in nature. Certain polysaccharides are immunomodulators and play key roles in the regulation of immune responses during the progression of some diseases. In addition to stimulating the growth of certain intestinal bacteria, polysaccharides may also promote health benefits by modulating the gut microbiota. In the last years, studies about the triad gut microbiota–polysaccharides–health have increased exponentially. In consequence, in the present review, we aim to summarize recent knowledge about the function of dietary polysaccharides on gut microbiota composition and how these effects affect host health.


Food Polysaccharides, an Overview
Carbohydrates are divided in several categories based on their number of sugar units: (a) monosaccharides have one sugar molecule; (b) disaccharides have two sugar molecules; (c) oligosaccharides have three to ten sugar units and may be produced by the breaking down polysaccharides; and (d) polysaccharides are macromolecules of monosaccharides consisting of more than ten units [1]. Polysaccharides are the major components of dietary fiber [2]. They bind to bile acids in the small intestine, thereby lowering serum cholesterol and normalizing blood lipid levels [3]. Most of the structures of polysaccharides are associated with numerous biological benefits for gut health and are frequently found in more complex structures that also contain digestible carbohydrates and proteins [4].
Food products contain polysaccharides derived from many sources, including farms, forests, oceans, fermentation vats, and chemical modification of natural polysaccharides, such as cellulose and starch [5]. Of the source and polysaccharide types, examples include algal (seaweed extracts) derived from agar, algins, carrageenans, and furcellaran, higher insoluble plants derived from cellulose, fruit extracts derived from pectin, corn starches, rice starches, wheat starches, beta-glucans, guar gum, locust bean gum, tara gum, psyllium seed gum, and tamarind seed polysaccharides [6].
For instance, hydrocolloids (plant-derived ingredients such as pectin, guar gum, locust bean gum, and konjac mannan) are a class of food ingredients mainly composed of polysaccharides and some proteins that are widely used in several food products [7]. Other polysaccharides are also commonly found in dietary products including starch,

Food Polysaccharides and Gut Microbiota
The intestinal or gut microbiota is "the set of microbes that colonize our digestive tract and interact with each other and with the host" [20,21]. Indeed, the microbes that reside in our gut have a remarkable potential to influence physiology, both in disease and the health of the host. The gut microbiota modulates, directly or indirectly, most of our physiologic functions, including metabolic and pathogenic functions, as well as the immune system maturation [22]. The microbiome also encompasses all of the genetic information contained in the microbiota [23], creating a dynamic, interactive microecosystem capable of changing in time and scale, along with being integrated into macro-ecosystems including eukaryotic hosts, and being crucial to their health and functioning [24]. A gut ecosystem with a wide variety of species may be more resilient to environmental influences than one that lacks diversity, since functionally linked microbes within an intact ecosystem may be able to balance the function of other species that have become extinct. In conse-quence, a higher diversity is commonly regarded as an indicator of a healthy digestive system [25,26]. Thus, an equilibrated microbiota community frequently exhibits high taxonomic diversity, stable core microbiota, and high microbial gene richness [27,28]. In healthy conditions, the intestinal microbiota is stable, resilient, and interacts symbiotically with the host [27,28]. By contrast, an imbalance in gut microbiota composition and function (dysbiosis) has been linked to cardiovascular disease [29], cancer [30,31], respiratory diseases [32,33], diabetes [34], inflammatory bowel disease [35], brain disorders [36], chronic kidney disease [37], and liver disease [38], among others.
Physiological properties of the gastrointestinal tract are revealed by the composition of the microbiota in a given region, which is stratiform both transversely and longitudinally. Microbiota density and composition are influenced by nutritional, chemical, and immunological gradients along the gut [39]. The large intestine has high levels of oxygen, acids, and antimicrobials, as well as a longer transit time than the small intestine [40]. However, facultative anaerobes with the ability to adhere to epithelial or mucus surfaces are thought to survive in the large intestine, as are rapidly growing bacteria [40]. Besides, according to animal studies, the microbial community of the small intestine is essentially dominated by Lactobacillaceae (traditionally classified as oxygen-tolerant anaerobes) [41]. A diverse and dense bacteria community occurs in the colon, primarily anaerobes with the ability to utilize complex carbohydrates, which are undigested in the small intestine. It has been reported that Prevotelaceae, Lachnospiraceae, and Rikenellaceae constitute the majority of species in the colon [39,42].
There is a spatial preservation of microbiota diversity and composition in the colorectal mucosa region [43,44]. On the contrary, the compositions of the mucosal and fecal/luminal regions are drastically different [45]. Bacteroidetes are more abundant in fecal/luminal samples than in the mucosa samples. Firmicutes, specifically Clostridium cluster XIVa, are enriched in the mucus compared with the luminal/fecal regions [46,47].
Concerning the interaction of gut microbiota-food polysaccharides, several dietary polysaccharides are fermented by the gut microbiota [55]. In this regard, the results of recent interventional studies suggest that dietary fiber increments may reduce diversity. This is because the microbes that digest fiber become exclusively enriched, resulting in a change in intestinal composition and, through competitive interactions, decreased diversity [56]. Gut bacterial degradation by dietary polysaccharides happens in two phases: (1) internal anaerobic glycolysis and (2) polysaccharides are hydrolyzed extracellularly to produce mono-and disaccharides [57].
Bearing in mind the above mentioned, by increasing the growth of certain intestinal bacteria during intestinal fermentation (among others), polysaccharides can alter the microbiota profile of the intestinal microbiota and change the physiology of the host, both locally and remotely [58].
On the other hand, Bifidobacterium longum, an example of bacteria with the ability of microbial fermentation, has the advantage of using the fucosylated oligosaccharides present in human milk to inhibit the growth of specific bacteria such as Escherichia coli and Clostridium perfringens [59]. In addition, Bacteroides species may consume those fucosylated oligosaccharides as a carbon source [60]. Infants born to mothers with nonfunctional fucosyltransferase 2 (FUT2), which is required for the fucosylation of milk oligosaccharides, have low levels of Bacteroides and Bifidobacterium in their feces [61]. In humans, patients with insulin resistance show elevated levels of Dorea and Coprococcus. Polysaccharide-containing bacteria possess degradation properties, and their associations with fecal sugar derivatives were generally positive, while Alistipes showed a negative correlation [62]. Dorea strain administration on a high-fat diet mice intensified insulin resistance and obesity compared with Alistipes administration. The authors of this work reported that the gut microbes' effects on metabolic diseases are mediated through polysaccharides' microbial fermentation and their derivatives [62]. Several studies in mice involving species of Bacteroides have shown that controlling the intake of polysaccharides in the mouse diet allows species selection that are capable of metabolizing the complex glycans present, such as human milk oligosaccharides [60], fructans [63], fucosylated mucin glycans [64] and mannan [65], among others.
For a comprehensive understanding of the effects of polysaccharides on gut health and the host, more detailed information is required. Therefore, the present review aims to elucidate the knowledge of the function played by dietary polysaccharides on gut microbiota composition and how these effects affect host health. We addressed the impact of several polysaccharides in health-promoting effects through the modulation of gut microbiota. Finally, we summarize recently reported studies in the field conducted on humans.

Dietary Polysaccharides and Short-Chain Fatty Acids (SCFAs)
Short-chain fatty acids (SCFAs) are metabolites produced by bacteria that can pass through the intestinal barrier and interact with host cells, thereby affecting the immune response [66]. When fiber is anaerobically fermented by gut microbiota, polysaccharides and proteins are metabolized into SCFAs [1]. In Figure 1 we summarize the bacterial degradation of polysaccharides in the intestine by fermentation.
A growing body of evidence suggests that SCFAs are capable of modulating the inflammatory response of immune cells, including neutrophils, dendritic cells, macrophages, monocytes, and T cells [67][68][69].
Obligate anaerobes hydrolyze nondigestible carbohydrates into oligosaccharides, which are fermented in an anaerobic environment. Anaerobes convert hexoses to pyruvate by a process similar to glycolysis before oxidizing pyruvate to acetyl-CoA in conjunction with reduction of an electron carrier or, in many cases, hydrogen gas [70,71]. From there, acetyl CoA is converted into various SCFAs.
As soon as SCFAs are produced, they are absorbed by colonocytes, primarily through sodium-dependent monocarboxylate transporters or H+-dependent monocarboxylate transporters. SCFAs affect intestinal mucosal immunity and influence barrier integrity and function by binding to G protein-coupled receptors, including free fatty acid receptors 2 and 3, as well as GPR109a/HCAR2 and GPR164 [72,73]. In a mouse model of colitis induced by dextran sulfate sodium, SCFAs binding to GPR43 and GPR109A stimulated K+ efflux and hyperpolarization, resulting in NLRP3 inflammasome activation and increased levels of IL-18 in serum [74]. Hence, SCFAs and their receptors contribute to health benefits associated with dietary fiber, as well as the way in which metabolite signals feed through to a major path for gut homeostasis.
The normal gut microbiome makes 50-100 mmol·L −1 SCFAs per day and works as a source of energy for the host's gut epithelium [78]. These SCFAs can be rapidly absorbed in the colon and serve many diverse roles in regulating gut inflammation, motility, energy harvesting, and glucose homeostasis [79,80].
The most common SCFAs are acetates, butyrates, or propionates, and a large proportion of these acetates undergo lipogenesis in adipose tissue and undergo oxidization in muscle, while some are converted into butyrates by bacteria [28]. Both butyrate and propionate protect the host from hypertensive cardiovascular damage [81] and butyrates are also associated with intestinal barrier integrity and may have beneficial effects on the epithelium of the gut [82].

Dietary Polysaccharides Influence Immunity by Acting as Prebiotics by Changing Gut Microbiota Composition
Biologically, polysaccharides perform a wide variety of functions and are capable of producing prebiotics that stimulate the microbiota in the intestines. The intestinal microbiota also exerts beneficial effects by selectively degrading polysaccharides, which can be used by the intestinal microbiota as a source of energy to maintain the physiologic effects of the intestinal bacteria and regulate their composition [69]. Some polysaccharides, such as dietary fibers, resist hydrolysis in the stomach and the small intestine of humans. According to Dolan et al., prolonged deficiency of dietary fiber can permanently alter gut microbiota and result in gut dysbiosis [83].
Non-fermentable polysaccharides are excreted in the large intestine while fermentable polysaccharides are digested by the microbiota that inhabits in the large intestine and are fermented to produce diverse metabolites that provide the host with energy [84,85].
Certain polysaccharides act as immunomodulators and influence the regulation of immune responses during the progression of some diseases [86]. Moreover, natural polysaccharides are capable of enhancing immunity by promoting beneficial microorganisms and increasing immune cell function [87].
Sheng et al. have reported that Hericium erinaceus-derived polysaccharides can help to restore humoral and cellular immunity in a murine model by improving the phagocytic function of natural killer cells, phagocytes, secretory IgA, and increasing the activity of AKT and MAPK signaling pathways [88]. Several studies have demonstrated that polysaccharides from ginseng can enhance immunity in sows by increasing the levels of interleukin (IL)-2, IL-6, immunoglobulin (Ig)-G, tumor necrosis factor-alpha (TNF-α), and interferon-gamma (IFN-γ) in both milk and serum [89]. Some other polysaccharides isolated from Robinia pseudoacacia and young barley leaves have also been shown to enhance IgA-related cytokines, leukocytes, transforming growth factor-beta (TGF-β), and IL-10 levels [87,90,91].
Bacteroides possess the ability to degrade dietary polysaccharides, as well as the polysaccharides on the surface of other gut microbes, and this is the major factor that enables them to thrive within the gut environment [77]. These species could metabolize dietary polysaccharides to SCFAs [92].
Polysaccharides isolated from Artemisia sphaerocephala might prevent the diversity decrease associated with bacteria belonging to Proteobacteria and Helicobacter in an animal model of high-fat diet-induced obesity [93]. Also, Chlorella pyrenoidosa and Spirulina platensis can restructure the gut microbiota in an animal model of obesity using a high-fat diet, increasing beneficial bacteria from Bacteroidia, Clostridia, and Mollicutes, and decreasing some bacteria from Verrucomicrobia and Actinobacteria [94].
Some reports have shown that alginate in brown seaweed modulates the obesity-related with a high-fat diet by regulating SCFA production and changing the Bacteroidales and Clostridiales [95]. Laminaria japonica soluble polysaccharides diminish non-alcoholic fatty liver diseases in a high-fat diet animal model through decreasing the Firmicutes/Bacteroidetes ratio and stimulating Verrucomicrobia and propionate-producing bacteria Akkermansia (a bacterium of the phylum Verrucomicrobia) and Bacteroides [94].
Accordingly, Akkermansia muciniphila is involved in the metabolism of mucin and the maintenance of intestinal integrity [96]. The increment of Akkermansia muciniphila after polysaccharide interventions has been related to benefits to the host (e.g., [97][98][99]). By contrast, other studies define Verrucomicrobia phylum as "unfavorable" for the prevention of obesity, and higher levels of this bacteria have been associated with this disease [96]. These discrepancies may be due to the fact that not all subspecies of Verrucomicrobia (e.g., Akkermansia muciniphila) may display the same specific properties, the model used in the study (animal model, or humans), as well as the basal state of the microbiota (eubiosis or dysbiosis, healthy or not subject, etc.). Overall, the fact is that the bacteria belonging to the phylum Verrucomicrobia are widespread contributors to the cycling of carbon and have the capacity for starch degradation is a crucial component of plant biomass [100].

Human Studies Examining Polysaccharide Modulation of Gut Microbiota and Its Association with Improved Health
The intestinal microbiota plays a vital role in human physiology through the production of metabolites that regulate essential activities that facilitate a symbiotic relationship between the microbes and the host. Polysaccharides are key regulators of colon physiology and the changing intestinal environment [101], and they are selectively used by gut microbiota to enhance the selection, colonization, and survival of probiotic bacteria acting as prebiotics [102].
The consumption of prebiotics is currently increasing, as well as the interest in them as functional foods. Therefore, research aimed at deciphering the mechanisms involved and their precise health effects has augmented exponentially.
In this regard, numerous clinical trials have already been conducted addressing a wide range of diseases (from obesity to chronic kidney disease) through dietary intervention with different polysaccharides. These studies are mainly focused on evaluating the potential of these polysaccharides as modulators of the intestinal microbiota to counteract the detrimental effects of the pathology (Table 1).
A clear example of the latter is inulin, a functional food found naturally in various plants and vegetables, which is a widely used ingredient in diverse efficacy studies thanks to its prebiotic properties [103]. Inulin is being investigated as a potential modulator of the gut microbiota with benefits for human health. The most notable recently reported changes induced by inulin are an increase in Bifidobacterium, an improvement in function, as well as benefits in host metabolism for a variety of metabolic diseases, including obesity, type 2 diabetes, kidney disease, intestinal disease, and non-alcoholic fatty liver disease [97,[104][105][106][107][108][109][110][111][112][113][114].
In addition to being a dietary fiber beneficial to health, RS is also defined as the portion of starch that cannot be digested or absorbed by humans in their small intestine. By fermenting RS, the gut microbiota can produce SCFAs [115]. In recent years, clinical investigations addressing the use of RS as a microbiome-modifying strategy have proliferated. In this particular case, supplementation with RS in patients with renal disease has led to an elevation in Faecalibacterium and a decrease in systemic inflammation [116], as well as elevated SCFA producers' microbes [107]. Additionally, a SCFA increment after RS intervention has been positively correlated with the relative abundance of Faecalibacterium, Ruminococcus, Roseburia, and Barnesiellaceae [117] and is effective in reducing body fat in healthy individuals [98].
The consumption of β-glucans has been shown to reduce calorie intake, lower cholesterol levels, and improve immunity [118]. Moreover, several clinical trials have also shown changes in gut microbiota composition and metabolic parameters. After dietary interventions with this prebiotic, changes in gut microbiota composition related to the increase of healthy bacteria (Bifidobacterium and Akkermansia) were observed in patients at high risk of developing metabolic syndrome [119,120]. Furthermore, in patients suffering from chronic kidney disease, β-glucan intake significantly altered the levels of the uremic toxin of intestinal origin and improved the state of the intestine [99].
On the other hand, non-invasive therapies such as prebiotic intake are becoming increasingly popular as a means to improve the quality of life of older adults [121]. In this regard, we found studies that examined the impact of polysaccharides on elderly people, but the results were conflicting. For instance, while Kiewiet et al. have reported changes in microbiota that were associated with improvements in health [111], Ganda et al. observed no significant effects following the intervention [122].

Future Perspectives in the Nutrition Field
Cell plant walls are composed of diverse types of polysaccharides and proteins which play vital roles in biology, including the regulation of cell expansion and tissue attachment, exchange of ions, as well as defense against pathogenic microorganisms. Further, there is evidence that fermentable dietary fiber from polysaccharides has biological activities that are low in toxicity, and have anti-oxidant, anti-inflammatory, anti-tumor, and antiviral effects. Moreover, evidence also suggests that polysaccharides play an active role in the symbiotic relationship between the gut microbiota and the host. Indeed, microbes convert complex polysaccharides into monosaccharides through a variety of biochemical pathways mediated by enzymatic activities. Together with polysaccharides, colonic bacteria also produce lactic acid, which reduces colonic pH and alters gut microbial composition. As immunomodulators, bacterial polysaccharides protect host cells from pathogenic microbial neighbors, and host-derived polysaccharides interact with gut microbes to influence gut health.
However, it is necessary to point out that fibers derived from polysaccharides obtained from different types of plants have different chemical compositions and physicochemical properties. Consequently, plant-based diets will provide a variety of dietary fibers as well as a variety of microbiota compositions. The genetics and the pre-diet microbiome of the host will also add variability to the effects of plant-based diets on microbiota composition.
On the other hand, although many of the physiological and nutritional effects of dietary polysaccharides are widely known, the different mechanisms of action have yet to be fully elucidated, as occurs, for example, with NSP. Along the same line, polysaccharides from marine algae are increasingly being used as prebiotics. These compounds are a rich source of dietary fiber, which are not decomposed by the enzymes of the upper gastrointestinal tract. Polysaccharides from marine seaweeds also have a detoxifying effect. Conversely, other factors, such as the complex chemical structure of some of these products must still be completely understood [138], as well as the high presence of sulfate residues in some of them, which may limit their fermentation by the gut microbiota and increase toxicity.
Several factors may affect the consistency of the results regarding polysaccharides' effects on the gut microbiota, including methodological sampling and bioinformatic pipelines. To make conclusions regarding this issue, it is necessary to take these factors into account.
To conclude, despite the recent work in this field, we are only beginning to understand how dietary polysaccharides affect health by modulating gut microbiota. Progress is challenged by the wide variety of dietary polysaccharides, their interactions with other molecules such as proteins, and by the vast variations in gut microbiota profiles. In this sense, it is essential to emphasize the importance of carefully selecting a sampling method when analyzing the composition of the microbiota in order to avoid contradictory results and to be able to obtain a solid understanding of all the processes and the precise role of all