Dietary Polyphenol, Gut Microbiota, and Health Benefits

Polyphenols, which are probably the most important secondary metabolites produced by plants, have attracted tremendous attention due to their health-promoting effects, including their antioxidant, anti-inflammatory, antibacterial, anti-adipogenic, and neuro-protective activities, as well as health properties. However, due to their complicated structures and high molecular weights, a large proportion of dietary polyphenols remain unabsorbed along the gastrointestinal tract, while in the large intestine they are biotransformed into bioactive, low-molecular-weight phenolic metabolites through the residing gut microbiota. Dietary polyphenols can modulate the composition of intestinal microbes, and in turn, gut microbes catabolize polyphenols to release bioactive metabolites. To better investigate the health benefits of dietary polyphenols, this review provides a summary of their modulation through in vitro and in vivo evidence (animal models and humans), as well as their possible actions through intestinal barrier function and gut microbes. This review aims to provide a basis for better understanding the relationship between dietary polyphenols, gut microbiota, and host health.


Introduction
Polyphenols, described as plants' secondary metabolites, are probably the most abundant antioxidants in our daily life. The main dietary sources of these compounds include fruits, vegetables, grains, green tea, coffee, etc. [1]. Total dietary polyphenol intake is as high as 1 g per day for each adult, which is about 10-times higher than the intake of Vitamin C, and even 100-times higher than that of Vitamin E and carotenoids [2]. During the last few decades, there has been tremendous research output related to the health-promoting effects of polyphenols, including their antioxidant, anti-inflammatory, antibacterial, antiadipogenic, and neuro-protective activities [3,4].
It has been reported that most dietary polyphenol intake remains unabsorbed in the small intestine, while the unabsorbed parts may accumulate in the large intestine and are extensively metabolized by the gut microbiota [5]. Therefore, intestinal microbiota play an important role in the biotransformation and metabolism of the original polyphenolic structures into low-molecular-weight metabolites, which can be readily absorbed and contribute to host healthy benefits. However, little is currently known regarding the possible mechanism among dietary polyphenols, gut microbes, and host health.
Dietary polyphenols influence gut microbiota compositions in the host, which further affect the host's metabolism. In turn, intestinal microbiota can metabolize polyphenols into bioactive, low-molecular-weight phenolic metabolites to modulate the regulatory metabolism network. In this regard, this review aims to provide an assessment of dietary polyphenols' biological significances on host health, a summary of their modulation through in vitro and in vivo evidence (animal models and humans), as well as their possible action through intestinal barrier function and gut microbes.

Dietary Polyphenols and Their Sources
Dietary polyphenols are one of the most abundant and widely distributed na products in plants. At present, according to the structure, dietary polyphenols are div into four categories: phenolic acids, flavonoids (the largest subclass of polyphenols) yphenolic amide, and other non-flavonoids ( Figure 1). Phenolic acids can be furthe vided into two main types, benzoic acid and cinnamic acid derivatives based on C and C3-C6 backbones [6]. Flavonoids include flavonoids, flavanones, isoflavones, cones, flavanols, flavonols, flavanonols, anthocyanins, and so on [7]. Polyphenolic am have N-containing functional substituents, two such groups are capsaicinoids and enanthramides. The non-flavonoids include mainly stilbenes and lignans. In additi phenolic acids, flavonoids, and phenolic amides, there are several non-flavonoid poly nols found in foods that are considered important to human health, such as resver ellagic acid and its derivatives, curcumin, etc. The remarkable feature of the chem structure is that it has a different amount of phenolic hydroxyl groups, which can b vided into phenolic monomers and polymerized polyphenols. Phenolic monomer clude flavonoids and non-flavonoids. The former generally involves a common ca skeleton of diphenyl propane in which two benzene rings are connected by a linear t carbon chain, while the latter is two benzene rings connected by the vinyl group [8] ymeric polyphenols are oligomers or polymers polymerized by monomers known as nins. Polyphenols are widely distributed in nature, including in fruits, vegetables, ce beans, tea, coffee, honey, and red wine, which are the main sources of dietary poly nols. Specifically, caffeic acid and ferulic acid are the most common phenolic acids in Caffeic acid is abundant in vegetables, fruits, and coffee, while ferulic acid is mainly tributed in rice bran, wheat bran, and other cereals. Among the flavonols, quercetin most common, which is commonly found in onions. Flavanols or flavan-3-ols are commonly called catechins, which are abundant in red wine, chocolate, and lotus Isoflavones are mainly found in the leguminous family of plants. Anthocyanidi plants mainly exist in glycosidic forms, which are commonly referred to as anthocya [9] and are largely distributed in strawberries, blueberries, and cherries. Some poly nols have N-containing functional substituents, such as capsaicinoids in chili pepper avenanthramides in oats, which belong to polyphenolic amides. The second major Polyphenols are widely distributed in nature, including in fruits, vegetables, cereals, beans, tea, coffee, honey, and red wine, which are the main sources of dietary polyphenols. Specifically, caffeic acid and ferulic acid are the most common phenolic acids in food. Caffeic acid is abundant in vegetables, fruits, and coffee, while ferulic acid is mainly distributed in rice bran, wheat bran, and other cereals. Among the flavonols, quercetin is the most common, which is commonly found in onions. Flavanols or flavan-3-ols are often commonly called catechins, which are abundant in red wine, chocolate, and lotus root. Isoflavones are mainly found in the leguminous family of plants. Anthocyanidins in plants mainly exist in glycosidic forms, which are commonly referred to as anthocyanins [9] and are largely distributed in strawberries, blueberries, and cherries. Some polyphenols have N-containing functional substituents, such as capsaicinoids in chili peppers and avenanthramides in oats, which belong to polyphenolic amides. The second major non-flavonoid group mainly consists of stilbenes, with resveratrol being the main representative, which is found in red and purple grape skins and grape wine. Another important nonflavonoid group is the lignans, which exist in bound forms in flax, sesame, and many grains.

Dietary Polyphenols and Their Biological Significance
As the most general plant-derived bioactive components in our diet, dietary polyphenols have received tremendous attention among nutritionists, food researchers, and consumers. Phenolic compounds are generally involved in defenses against plant pathogens and atmospheric agents, including bacteria, fungi, and viruses, and many abiotic stresses like drought, salinity, and UV. Polyphenols exhibit antimicrobial and antioxidant properties that can help plants to evade pathogenic infections and, at the same time, protect the major tissues from the toxic effects of reactive oxygen species [10]. Currently, they represent a topic of great scientific attention due to interest in their biological significance for humans. Both in vitro and in vivo studies have shown their health-promoting effects, including their antioxidant, anti-inflammatory, antibacterial, anti-adipogenic, and neuro-protective activities.

Antioxidant Properties
The effectiveness of phenolic compounds in the inhibition of oxidative processes is potentially related to their reactive species scavenging activity. Due to the structure of the hydroxyl group on the benzene ring, polyphenols scavenge free radicals by H-atom transfer from the active OH group of the polyphenol to the free radical [6]. This allows polyphenols to indirectly activate antioxidant responses and generate non-toxic levels of intermediates, specifically the electrophilic forms of hydroquinone and quinone [11]. On the other hand, polyphenols inhibit the formation of or deactivation of the active species and precursors of free radicals, thus reducing the rate of oxidation and ultimately suppressing the generation of free radicals. They donate an electron to the free radical, neutralizing the radicals, and causing themselves to become stable (less reactive) radicals, thus stopping the reactions [12]. Treatment of HepG2 cells with (−)-epigallocatechin-3-gallate from green tea stimulates the nuclear translocation of nuclear factor erythroid 2-related factor 2 (Nrf2), which modulates the expression of antioxidant genes [13]. Resveratrol improves antioxidant defenses in pancreatic tissue because it enhances the activity of antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione-S-transferase (GST) [14].

Anti-Inflammatory Properties
Oxidative-stress-induced inflammation is mediated by the activation of cellular signaling processes of nuclear factor-kappa B (NF-kB) activation and activator protein-1 (AP-1) DNA binding [15]. It affects the expression of pro-inflammatory genes such as interleukin-1beta (IL-1β), IL-6, tumor necrotic factor alpha (TNF-a), and inducible nitric oxide synthase (iNOS) [16]. Preclinical and clinical studies suggest that polyphenols are able to express anti-inflammatory properties [17]. Although the precise mechanisms deserve further clarification, dietary polyphenols have shown benefits in distinct disorders [18]. Dihydroxylated phenolic acids produced from dietary proanthocyanidins potentially lowered the secretion of cytokines, including TNF-α, IL-1β, and IL-6, from healthy individuals [19]. Supplementation with 0.8% quercetin decreased interferon-γ, IL-1α, and IL-4 in male C57Bl/6j mice [20]. The administration of 10 mg/kg of quercetin also reduced the plasma nitrate plus nitrite (NOx) concentration and TNF-α production in adipose tissue of obese Zucker rats, resulting in an important anti-inflammatory effect [21].

Antibacterial Properties
Dietary polyphenols and plants rich in polyphenols have been demonstrated to be natural antimicrobials against both Gram-positive and Gram-negative bacteria. Epigallocatechin gallate (EGCG) was able to bind directly to the peptidoglycan from Staphylococcus aureus, affecting its cell integrity and destroying the osmotic protection of the cell wall [22]. Other than bacterial cell walls, tea polyphenols also damaged the inner cyto- plasmic membrane of Serratia marcescens, increasing its permeability and releasing small cellular molecules [23]. Moreover, polyphenols can exhibit antibacterial activity via antibiofilm agents. Cranberry proanthocyanidins limited the motility-particularly swarming motility-and reduced the biofilm formation of Pseudomonas aeruginosa [24]. However, due to the structural diversity of polyphenol classes, the mechanisms of their antimicrobial activities have not yet been fully resolved.

Anti-Adipogenic Properties
Stimulating the development of beige adipocytes (so called 'browning') can reduce adverse obesity effects and help to improve metabolic health [25,26]. Dietary polyphenols have been demonstrated to effectively activate adipose tissue browning and relieve obesity and lipid accumulation through the induction of beige adipocytes. Daily ingestion of a catechin-rich beverage increases brown adipose tissue density in healthy young women, supporting the brown adipogenesis of polyphenols [27]. Also, in mice fed with a high energy diet, vanillic acid could accelerate thermogenesis and mitochondrial synthesis in both classical brown adipose tissue (BAT) and inguinal white adipose tissue (WAT) [28]. Resveratrol decreased triglycerides (TG) accumulation in the liver by suppressing the expression of adipogenesis-related genes, such as acetyl-CoA carboxylase (ACC), peroxisome proliferator-activated receptor (PPAR-γ), and sterol regulatory element binding protein (SREBP-1) [26,29]. Piceatannol treatment suppressed protein levels of the adipogenic transcription factors PPAR-γ, while it increased ACC protein expression [30]. Therefore, a positive relationship may exist between dietary polyphenol and anti-adipogenesis, and the underlying mechanisms are worthy of exploration.

Neuro-Protective Properties
The neuro-protective effects of dietary polyphenols have received considerable attention in recent years, suggesting that polyphenols may be effective in reversing neurodegenerative pathology and age-related declines in neurocognitive performance. Animal evidence demonstrates that blueberries are effective at reversing age-related deficits in rat spatial working memory, and (−)-epicatechin enhances the retention of mice spatial memory and may relate to their potential to influence the synthesis of neurotrophic factors [31,32]. In addition, curcumin could disrupt existing plaques and partially restore distorted neurites in an Alzheimer mouse model [33]. Resveratrol can activate the phosphorylation of protein kinase C and secretes transthyretin to prevent Aβ aggregation in cultured rat hippocampal cells [34]. However, a direct association between dietary polyphenol and an improvement in neurological health has not been made at present.

Impact of Dietary Polyphenols on Gut Microbiota
Emerging evidence demonstrates that gut microbiota plays an important role in maintaining the physiological function of host health and the pathogenesis of various diseases, including obesity, diabetes, inflammatory bowel disease, and even neurodegenerative disorders. Diet can alter the composition of gut microbiota, which in turn affects host metabolism. The alteration of gut microbiota by the administration of probiotics, prebiotics, or fecal microbiota transplantation is already well established. However, the gut microbiotamodulating effects of polyphenol are less clear. Nevertheless, there is growing evidence showing that dietary polyphenol may directly modulate the gut microbiome, i.e., increasing beneficial microbial or decreasing harmful microbial species in the gut microbiota. In this part, we summarize the in vitro and in vivo studies that studied the effects of polyphenol supplementation on the gut microbiota.

In Vitro Modulation of Dietary Polyphenols on Gut Microbiota
In vitro experiments on polyphenols and polyphenol-rich food sources have been studied through extraction, digestion, and fermentation to demonstrate that they could modulate the resident bacteria. A series of in vitro studies with polyphenol from different Polyphenols can selectively inhibit the growth of pathogenic bacteria. Flavonoids in red wine showed a slight inhibition of the Clostridium [35]. Ellagic acid and anthocyanins in raspberry juice may inhibit the growth of Ruminococcus [37]. Grape polyphenols can inhibit the growth of Clostridium histolyticum [36]. On the other hand, polyphenols can promote the growth of beneficial bacteria in the gut, such as Bifidobacterium. Tannin in pomegranate, gingerol in ginger, grape polyphenols, and sorghum polyphenols can promote the growth of Bifidobacterium [41,45,46]. Tannin can also promote the growth of Lactobacillus [45]. Gingerol and grape polyphenols can promote the growth of Enterococci [36,45]. Sorghum polyphenols can cooperate with fructooligosaccharides to enhance the abundance of lactic acid bacteria, Roseburia, and Prevotella [46]. However, Kemperman's research shows that polyphenols in red wine and black tea can reduce the abundance of Bifidobacterium [39]. They conducted in vitro experiments using fluids from the colon and found that catechins and flavonoids in black tea could stimulate Klebsiella, Enterococci, and Akkermansia and reduce Bifidobacteria, B. coccoids, Anaeroglobus, and Victivallis. Anthocyanins and catechins in red wine can promote the growth of Klebsiella, Alistipes, Cloacibacillus, Victivallis, and Akkermansia, and reduce the growth of Bifidobacteria, B. coccoides, Anaeroglobus, Subdoligranulum, and Bacteroides [39]. Mango peel is another high-polyphenol food, with gallates, flavonoids, gallotannins, gallic acid, and so on, and in vitro fermentation of mango peel could increase the growth of Bifidobacterium and Lactobacillus.

In Vivo Modulation of Dietary Polyphenols on Gut Microbiota of Animal Models
Similarly, in vivo studies have shown that polyphenol supplementation can modulate gut microbiota in animal models, including the increase of beneficial microbes and the decrease of harmful microbes. Detailed information on the published in vivo studies, from invertebrate Drosophila and zebrafish to vertebrate rat, mouse, chick, pig, etc., have been listed in Table 2. Both vertebrate and invertebrate model organisms confirmed that polyphenol supplementation can increase the number of beneficial bacteria in the gut, such as Bifidobacterium and Lactobacillus. Mango supplementation in mice fed with a high-fat diet can prevent the loss of beneficial intestinal bacteria, especially Bifidobacteria, Akkermansia, and Aldercrutzia [47]. Orso applied a diet of chestnut shell extract rich in tannin to a zebrafish intestinal inflammation model and found that it promoted the growth of healthy and beneficial bacteria (Enterobacteriaceae and Pseudomonas) [48]. Supplementation with polyphenols can also change the ratio of Firmicutes to Bacteroides. Cranberry extract is rich in phenolic acids, flavonoids, anthocyanins, and other polyphenols, which can reduce the ratio of Firmicutes to Bacteroides in mice induced by a high-fat/high-sugar diet [49]. Moreover, a polyphenol diet intervention can selectively inhibit pathogenic bacteria. Polyphenols from Smilax china L. rhizome can reduce the relative abundance of Desulfovibrionaceae, Lachnospiraceae, and Streptococcaceae [50], and grape pomace reduces potentially pathogenic bacteria to humans, such as Salmonella, E. coli, Shigella, Yersinia, and Proteus [51]. The combination of quercetin and resveratrol can significantly inhibit the relative abundance of Desulfovibrionaceae, Acidaminococcaceae, Coriobacteriaceae, Bilophila, and Lachnospiraceae, which may be related to diet-induced obesity [52]. Blueberry polyphenols were used to interfere with ovariectomized rats, with an upregulation of Bacteroides dorei and Lachnoclostridium and a decrease of Rickenellaceae and Eubacterium [53].

In Vivo Modulation of Dietary Polyphenols on Gut Microbiota of Humans
Clinical studies further confirmed the regulatory effect of polyphenols on human intestinal micro-organisms (Table 3). Consistent with in vitro and in vivo animal studies, supplementation with polyphenols such as anthocyanins and flavonoids increase the abundance of Bifidobacterium and Lactobacillus, which are two intestinal protective agents in the human gut [90,91]. Blueberries are rich in anthocyanins, which can increase the number of Bifidobacteria and lactic acid bacteria in healthy volunteers [92]. Almonds and almond skins are heavily rich in a range of flavonoids, including catechin, flavonol, and flavanone glycosides, and adding almonds or almond skins to the diet can increase the number of Bifidobacteria and Lactobacillus in feces [93]; Moreno-Indias found that polyphenols in red wine can increase the number of Bifidobacteria and Lactobacillus [94]. Besides, a diet rich in polyphenols can regulate the ratio of Firmicutes to Bacteroides in the human body. Daily consumption of cranberries rich in proanthocyanidins can reduce the number of Firmicutes in the body and increase the number of Bacteroides [95]; however, Yuan used tea polyphenols in tea to intervene in healthy volunteers and found different results. The diet that intervened with tea polyphenols resulted in an increase in the number of Firmicutes in feces, a decrease in the number of Bacteroides, and an increase in the ratio of Firmicutes to Bacteroides [96]. Queipo-Ortu found that the combined action of alcohol and polyphenols could increase the number of Enterococcus, Prevotella, Bacteroides, Bifidobacterium, Bacteroides uniformis, Eggerthella lenta, and Blautia coccoides-Eubacterium, but had no significant effect on the changes of Lactobacillus [97]. The effect of polyphenols on gut microbiota is related to the number of initial microbiota in the intestinal tract. Mayta-Apaza classified them according to the initial number of Bacteroides in the body, and different microbial compositions led to different performances after receiving a dietary intervention. After receiving sour cherry juice, the volunteers with high initial Bacteroides reduced Bacteroides and Bifidobacterium and increased the Lachnospiraceae, Ruminococcus, and potential polyphenol metabolite Collinsella. The volunteers with low Bacteroides responded to the increase of Bacteroides and Bifidobacterium and the decrease in the relative abundance of Lachnospiraceae, Ruminococcus, and Collinsella [38]. The effect of polyphenols on gut microbiota is related to the intake of polyphenols. Tzounis found that high-dose cocoa flavanone beverages increase the number of Bifidobacterium, lactic acid bacteria, and Enterococci; increase the number of E.rectale-C.coccoides; and reduce the number of Histolytic Chlamydia. A low dose of cocoa flavanone beverage will not cause a significant change in the number of Bifidobacteria, but will increase Clostridia [91].

Mechanism of Dietary Polyphenol and Gut Microbiota Affecting Host Health
The gut microbiota and the host maintain normal physiological function and morphology of the intestine by forming a mutually beneficial relationship. Gut microbiota not only play a bridge role between the diet and host in digesting dietary food complexes, but also yields short-chain fatty acids and other metabolites to regulate human health. Studies have shown that only a small portion of polyphenols (5-10% of the total polyphenol intake) are absorbed in the small intestine, while most (90-95% of the total polyphenol intake) are transported to the human large intestine [104]. Diet polyphenol can modulate the gut microbial composition, and, at the same time, gut microbiota also improve the bioavailability of polyphenols by converting them to bioavailable metabolites (Figure 2). the decrease in the relative abundance of Lachnospiraceae, Ruminococcus, and Collinsella [38]. The effect of polyphenols on gut microbiota is related to the intake of polyphenols. Tzounis found that high-dose cocoa flavanone beverages increase the number of Bifidobacterium, lactic acid bacteria, and Enterococci; increase the number of E.rectale-C.coccoides; and reduce the number of Histolytic Chlamydia. A low dose of cocoa flavanone beverage will not cause a significant change in the number of Bifidobacteria, but will increase Clostridia [91].

Mechanism of Dietary Polyphenol and Gut Microbiota Affecting Host Health
The gut microbiota and the host maintain normal physiological function and morphology of the intestine by forming a mutually beneficial relationship. Gut microbiota not only play a bridge role between the diet and host in digesting dietary food complexes, but also yields short-chain fatty acids and other metabolites to regulate human health. Studies have shown that only a small portion of polyphenols (5-10% of the total polyphenol intake) are absorbed in the small intestine, while most (90-95% of the total polyphenol intake) are transported to the human large intestine [104]. Diet polyphenol can modulate the gut microbial composition, and, at the same time, gut microbiota also improve the bioavailability of polyphenols by converting them to bioavailable metabolites ( Figure 2).

Dietary Polyphenols Affect the Composition of Gut Microbiota
Dietary polyphenol has a definite role in the composition and functional profile of the gut microbiota. Polyphenols promote the growth of beneficial microbes, such as Lactobacillus and Bifidobacterium, which are two major health beneficial probiotics and bring benefits to human health, such as improving gastrointestinal disorders, suppressing diarrhea and constipation [105], alleviating lactose intolerance [106], relieving irritable bowel symptoms [107], and preventing inflammatory bowel disease [108]. A systematic review by Ma et al. with a meta-analysis revealed that polyphenol supplementation profoundly increased the abundance of Lactobacillus by 220% and Bifidobacterium by 56%. On the other hand, polyphenols can inhibit the growth of harmful microbiota, and Clostridium histolyticum and Clostridium perfringens in Clostridium are common pathogenic bacteria. Clostridium histolyticum causes inflammatory bowel disease [5] and Clostridium perfringens produces many toxins and hydrolytic enzymes, which are related to gastrointestinal disease

Dietary Polyphenols Affect the Composition of Gut Microbiota
Dietary polyphenol has a definite role in the composition and functional profile of the gut microbiota. Polyphenols promote the growth of beneficial microbes, such as Lactobacillus and Bifidobacterium, which are two major health beneficial probiotics and bring benefits to human health, such as improving gastrointestinal disorders, suppressing diarrhea and constipation [105], alleviating lactose intolerance [106], relieving irritable bowel symptoms [107], and preventing inflammatory bowel disease [108]. A systematic review by Ma et al. with a meta-analysis revealed that polyphenol supplementation profoundly increased the abundance of Lactobacillus by 220% and Bifidobacterium by 56%. On the other hand, polyphenols can inhibit the growth of harmful microbiota, and Clostridium histolyticum and Clostridium perfringens in Clostridium are common pathogenic bacteria. Clostridium histolyticum causes inflammatory bowel disease [5] and Clostridium perfringens produces many toxins and hydrolytic enzymes, which are related to gastrointestinal disease and necrotizing enteritis [109]. Ma's review system by meta-analysis showed that polyphenols derived from different foods all suppress the abundance of Clostridium pathogen species in the human gut microbiota, with tea being the most effective polyphenol food source for reducing Clostridium [110]. Dietary polyphenols can also regulate the ratio of Firmicutes to Bacteroides, which is related to body weight, and the ratio of Firmicutes to Bacteroides in obese patients is higher [111]. Xue's studies have shown that four dietary polyphenols, rutin, quercetin, chlorogenic acid, and caffeic acid, can reduce the ratio of Firmicutes to Bacteroides in in vitro gut microbiota experiments [112]. However, due to the different types of polyphenols, polyphenol dosage, and research methods, the results of different studies are different to some extent, resulting in the changes between microbes not being completely consistent.

Dietary Polyphenols Affect the Metabolites of Gut Microbiota
Short-chain fatty acids (SCFAs) are the most well-studied microbial metabolites so far. SCFAs are a saturated aliphatic organic acid [113] that are produced by the incomplete metabolism of plant-derived carbohydrates by intestinal flora present in an anaerobic environment [114]. Acetate, propionate, and butyrate are the main SCFAs in the gut (accounting for 90% of the total SCFAs) [115]. Wu's studies have shown that EGCG can significantly increase the number of SCFAs-producing bacteria, especially Akkermansia, and then promote the production of SCFAs, thereby enhancing anti-inflammatory effects and colon barrier integrity, which reduces enteritis [116]. Previous studies have shown that Akkermansia muciniphila can promote the production of acetate and propionate, and the nutritional interaction between Akkermansia muciniphila and butyrate-producing bacteria promotes butyrate production [117]. Liu's experiment showed that after a week-long intervention with an Aronia-berry-rich diet, the polyphenol diet extracted by Aronia berry was 57% higher than that in the control group [3]. In the human model intestinal system, the in vitro fermentation of wild cherry juice increased the microbial production of propionate and butyrate [118]. McDougall found that after ingesting anthocyanin-rich raspberry, the concentration of bile acid in an ideal fluid of ileostomy subjects changed significantly, wherein the glycine and taurine derivatives of cholate and deoxycholate increased [119]. Fotschki further described the beneficial effects of raspberry dregs on the bile acid profile of the cecum in a hyperlipidemic mouse model [120]. Studies by Huang have shown that EGCG can significantly reduce the content of intestinal bile acid; increase the excretion of bile acid, cholesterol, and total lipids in feces; and alleviate metabolic abnormalities and fatty liver induced by a high-fat diet in mice [121]. Therefore, after dietary polyphenols reach the gut, microbiota can then further produce metabolites, and, once absorbed and transported to target tissues and organs, contribute to metabolite health.

Dietary Polyphenols Affect the Bacterial Cell Membrane
Dietary polyphenol can interfere with the bacterial cell function of the cell membrane. For example, flavonols and flavones in the Staphylococcus genus can increase membrane cytoplasm permeability. Studies have shown that the antibacterial effect of polyphenols is more effective against Gram-positive bacteria. Inouye pointed out that because of the hydrophilic outer membrane outside the cell wall of Gram-negative bacteria, the passage of chemicals is prevented. Gram-negative bacteria are more resistant to plant secondary metabolites, including phenols [122]. When polyphenols were ingested, the growth of Gram-negative Salmonella and Escherichia strains was inhibited, but the growth of Grampositive lactic acid bacteria was not affected [123]. The effect of polyphenols on bacteria depends on the interaction between compounds and the bacterial cell surface, which can inhibit bacterial growth by disturbing the function of the cell membrane [124]. Tea polyphenols, such as tea catechins, have a strong affinity to the lipid bilayers of the cell membrane through hydrogen bonds with the bilayer surface, thus penetrating underneath the surface and giving play to antibacterial, anticancer, and other beneficial effects [125]. EGCG has antibacterial activity against Staphylococcus; possible mechanisms include damaging the lipid bilayer of the cell membrane, reducing mucus production and affecting the formation of biofilm, and binding and neutralizing with enterotoxin B [126]. Therefore, the effect of polyphenols on the bacterial cell membranes is considered to be one of the mechanisms for regulating metabolic health.

Biotransformation of Polyphenols by Gut Microbiota
With respect to the complicated structures and high molecular weights, dietary polyphenols have low bioavailability and are difficult to be absorbed in the small intestine. About 90% of dietary polyphenols arrive at the colon in an intact form where they are biotransformed and metabolized into bioactive, low-molecular-weight phenolic metabolites through the residing microbiota [127]. Chen discovered that gut bacteria can deconjugate mulberry anthocyanin (cyanidin-3-glucoside, cyanidin-3-rutin, and delphinidin-3-rutinoside) to lower molecular-weight metabolites, and metabonomic data showed that the first two compounds were decomposed into protocatechuic, vanillic acid, and p-coumaric acids, while the latter was converted to syringic acid and gallic acid [128,129]. The core bacteria that can metabolize anthocyanins are Bifidobacterium spp. and Lactobacillus spp. [130,131] with probiotic effects to produce antibacterial substances, to compete with pathogens for adhering to the epithelium and for nutrients, to regulate the host immune system, and to inhibit the production of bacterial toxins [132]. The flavonoids (flavonols, flavones, and flavanones) can be biotransformed into p-hydroxyphenylacetic acid, protocatechuic acid, p-hydroxybenzoic acid, vanillic acid, hydrocaffeic acid, coumaric acid, 3-(4hydroxyphenyl) propionic acid, and other aromatic metabolites [133]. Soybean isoflavones can be converted to dihydrodaidzein, dihydrogenistein,6 -OH-O-desmethylangolensin, and cis-4-OH-equol by anaerobic bacteria in the distal region of the small intestine and colon [134][135][136]. The bioavailability of ellagic tannin, which was found in pomegranate and grape, is low, but they can be metabolized by intestinal micro-organisms into urolithins with antioxidant activity and preventive effects for chronic diseases such as cancer, diabetes, and cardiovascular and neurodegenerative diseases [137,138]. Therefore, polyphenol metabolites produced by gut microbiota have potentially beneficial effects on the host.

Conclusions
There is increasing evidence in the literature to emphasize that dietary polyphenols have potentially beneficial effects on host health through interactions with gut microbiota. Numerous studies listed in this review, both in vitro and in vivo, demonstrated the relationship between dietary polyphenols and gut microbiota, while the possible mechanism may be through the alteration of gut microbiota composition, the production of gut microbiota metabolites, the modulation of intestinal barrier function, and the biotransformation and metabolism of dietary polyphenols. However, a clear and deep understanding of these mechanisms between polyphenols and gut microbiota is necessitated, especially considering the metabolic pathways, which will allow for new therapeutic targets in the future.