In vivo, flavonoid metabolism is closely associated with GM. Microorganisms interact and cooperate with each other for the completion of complex metabolic functions. Under the action of GM, flavonoid glycosides are first hydrolyzed to aglycones in the intestinal tract, then reduced, with the hydrogenation of the C-ring, after which O–C2 bonds in the C-ring are cleaved to form phenolic ketones and phenolic acids (Figure 1). Therefore, hydrolysis and reduction are the main reactions of flavonoids in intestinal metabolism. In addition, GM can perform degradation of glycoside flavonoid macromolecules O-glycoside and C-glycoside, ester hydrolysis, hydrolysis (Jin et al., 2007), and deglucuronylation; partial dehydrogenation (Hor-Gil and Fatemeh, 2000), dimethoxylation (Shi et al., 2014), and demethylation of aglycones; and hydrogenation, α-oxidation, β-oxidation, and cracking of aromatic rings of substituted aliphatic groups. After the aromatic ring of the flavonoid aglycone is cracked, the hydroxylated forms of phloroglucinol and phenylacetate or phenylpropionate are produced.

For example, the main components of Scutellaria baicalensis Georgi include baicalin, wogonoside, and xyloside. These glycosides are converted by GM into the corresponding aglycones, such as baicalein, wogonin, and xyloside A, respectively, with a higher anti-complement activity than that of the prototypes. Antibacterial activity evaluation showed that the activities of all aglycones were stronger than those of the corresponding glycosides (Xing et al., 2014). With respect to anti-platelet aggregation, baicalein activity was significantly stronger than that of baicalin, which has a positive effect on improving atherosclerosis. It has been found that baicalin is first transformed into its aglycone baicalein in the intestinal tract of rats, absorbed, then converted into baicalin in the small intestine and liver (Akao et al., 2000). These findings suggest that the insoluble glycosides may be hydrolyzed into easily absorbed aglycones to make them better absorbed by the small intestine, and then converting them back to glycosides using host enzymes, thereby promoting glycoside absorption. Hesperidin, a flavanone glycoside, is a natural phenolic compound with a wide range of biological effects (Hajialyani et al., 2019). To clarify hesperidin metabolism by GM, hesperidin was incubated with human and rat fecal suspensions. By comparison, when hesperidin was orally administered to rats, hesperetin was detected in the urine, but hesperidin was not, indicating that hesperidin was metabolized to hesperetin, which was the main metabolite during incubation (Lee et al., 2004). Hesperidin can dilate blood vessels, and reduce blood lipid levels, capillary permeability, and vascular wall fragility. Protocatechuic acid is the main metabolite of anthocyanin fermented by human GM. It exhibits obvious antioxidant activity, scavenging of free radicals, inhibition of platelet aggregation, and anti-hypoxia activity, which are significant for improving CVDs. The appearance of low molecular weight metabolites indicates that anthocyanin has been transformed by GM (He et al., 2005). Kaempferitrin is a flavonoid glycoside isolated from Siraitia grosvenorii (Swingle) C. Jeffrey ex Lu and Zhang, which is incubated and transformed by the bacteria in human intestines to produce afzelin, kaempferol, p-hydroxybenzoic acid, and kaempferol-7-O-α-L-rhamnoside (Yang et al., 2005). It has various pharmacological activities, such as lowering of blood pressure, reduction of blood lipid levels, and inhibition of platelet aggregation.

Flavonoids can promote the growth of beneficial intestinal microorganisms. Flavonoids, such as hesperidin and quercetin have been reported to modulate the composition of GM by promoting the growth of probiotic bacteria and effecting a reduction in the growth of pathogens. A study conducted by Simmering (Rainer et al., 2002) examined the influence of flavonoids on the population of Eubacterium ramulus in the human intestinal tract. A controlled study was performed with human subjects who received a flavonoid-free diet and a single high dose of quercetin, rutin, or buckwheat. Results showed that oral quercetin and rutin intake significantly increased the bacterial load of E. ramulus in humans. Metabolomic analyses have also shown that grape seed extract, rich in flavanols, had a strong value-added effect on the proliferation of Lactobacillus plantarum, Lactobacillus casei, and Lactobacillus bulgaricus (Tabasco et al., 2011).

Moreover, flavonoids have anti-microbial activity, and a large number of studies have shown that they can inhibit the proliferation of symbiotic bacteria and pathogenic microorganisms, and thus have a certain impact on CVD. The quantity of Lactobacillus and Escherichia coli per gram of fecal contents in rats fed soybean isoflavones was calculated using the bacterial culture method. The results showed that the erobic E. coli numbers in the intestinal tract of rats fed soybean isoflavone decreased significantly, while the count of the facultative anaerobic Lactobacillus spp. increased significantly (Zhu et al., 2013). This is similar to the results of a study by Duda-Chodak (Duda-Chodak, 2012) in which the liquid medium was supplemented with various polyphenol monomers, whose effects on GM were evaluated by measuring the turbidity of the medium after 24 h. It was found that quercetin significantly inhibited the growth of Bacteroides galactosus and Lactobacillus. Furthermore, Gwiazdowska (Daniela et al., 2015) studied the effects of different polyphenol monomers (2, 20, and 100 μg/ml) on the growth of Bifidobacterium Bifidum using the 96-well microtitration plate method. The results showed that rutin and quercetin promoted the growth of B. Bifidum, but high-dose quercetin inhibited the growth of B. Bifidum. These observations indicate that the effect of flavonoids on GM depends on the dosage of flavonoids, and different concentrations of flavonoids have different inhibitory effects on different bacterial strains. Currently, flavonoids, as antibacterial drugs, have been widely used in the medical and food preservative industries.

Atherosclerosis, one of the main diseases that harm human cardiovascular and cerebrovascular health is mainly caused by lipid metabolism disorder and vascular endothelial inflammation (Chistiakov et al., 2015). It is a chronic inflammatory reaction of the arterial wall (Wu M. Y. et al., 2017), mainly manifested by the formation of atherosclerotic plaques on the arterial wall, resulting in loss of arterial elasticity and eventually leading to adverse cardiovascular events (Greaves and Gordon, 2013; Mailer et al., 2017; Peter, 2017). Recent studies show that the changes in GM composition and diversity are closely related to the occurrence of chronic low-grade inflammation and atherosclerosis (Jones et al., 2013; Witjes et al., 2015); it was observed that the Yersinia and Rhodobacter population decreased, while that of Collinsella erofaciens and monocytes increased (Svetel et al., 2013). This was illustrated in a study of the feces of 12 patients with obvious symptoms of carotid atherosclerotic plaques and 12 healthy individuals matched according to gender and age (Karlsson et al., 2012). The data showed that the number of coliforms in the intestinal tract of the patients increased, while the number of Rhodobacter and Eubacterium increased in the healthy controls. Further, in a metagenomic association study on feces from 218 individuals with atherosclerotic CVD and 187 healthy controls (Jie et al., 2017), significant differences in GM were found between individuals with atherosclerotic CVD and the control group, including a relative reduction in Bacteroides and Prevotella populations, and the enrichment of Streptococcus and E. coli.

GM can affect the development of atherosclerosis through a variety of ways. It can not only promote an inflammatory reaction, aggravate the development of atherosclerotic plaque, or lead to plaque rupture but also affect the occurrence and development of atherosclerotic plaque by regulating host cholesterol and lipid metabolism. It is generally believed that GM is an activator of inflammation, especially that related to lipopolysaccharide (LPS) synthesized by Gram-negative bacteria and a strong inflammatory trigger (Du et al., 2017). LPS produced by GM can promote the progression of chronic low-grade inflammation, which is recognized as a risk factor for atherosclerosis. GM dysbiosis is accompanied by increased intestinal permeability, which leads to increase in endotoxin levels in the blood. Additionally, high-dose LPS can significantly increase the expression of nuclear factor kappaB (NF-κB) and other inflammatory factors, whereas low-dose LPS can induce macrophage differentiation into a mild proinflammatory state through Toll-like receptor 4, interleukin-1 receptor-related kinase-1, and Toll-interacting protein on the cell surface and promote the occurrence of atherosclerosis (Maitra et al., 2012). Moreover, GM inhibits the expression of fasting-induced adipocyte factor, increases triglycerides in blood circulation, and promotes deposition of adipocytes, resulting in increased lipid levels and leading to dyslipidemia and induction of atherosclerosis (Backhed et al., 2004). Furthermore, the signaling molecules produced by GM significantly influence cholesterol reverse transport. For example, low-dose endotoxin LPS can significantly reduce the expression of ATP-binding cassette (ABC) subfamily A member 1 and ABC subfamily G member 1 by cholesterol-reversion factor and inhibit the reverse process of cholesterol accumulation in macrophages, which promotes foam-cell accumulation in the vascular endothelium to form plaques and the occurrence of arteriosclerosis (Maitra and Li, 2013).

As a potential promoter of atherosclerosis and cardiac metabolic diseases, TMAO significantly affects the development of atherosclerotic lesions, resulting in increased attention being given to its molecular mechanisms in the human body (Tang et al., 2013). TMAO can not only significantly accelerate the development of atherosclerosis in apolipoprotein E-deficient mice but also activate the mitogen-activated protein kinase and NF-κB signaling pathways in endothelial cells and smooth muscle cells, thereby triggering inflammatory reactions in vessels and causing increased accumulation of foam cells in the vascular wall and atherosclerosis (Zeisel and Warrier, 2017). These findings suggest that reducing in vivo TMAO levels by inhibiting the proliferation of TMA-producing bacteria might improve the symptoms of atherosclerosis. One study reported that TMAO can inhibit the reverse transport of cholesterol and regulate the activity of cholesterol transporters in macrophages, thus leading to atherosclerosis (Koeth et al., 2015). Conversely, another study aimed at the key TMA enzyme (cutC/D) produced by bacteria and developed two strong inhibitors that can permanently inactivate cutC/D without affecting the survival ability of the symbiotic bacteria. After oral administration, the selective accumulation of the inhibitor in the intestinal symbiotic bacteria can almost completely inhibit the production of TMA and TMAO caused by hypercholinergic food, reduce platelet aggregation and thrombosis, and thus reduce the risk of atherosclerosis. Simultaneously, it reverses the changes in the intestinal bacterial composition induced by high choline foods, and increases the levels of Akkermansia muciniphila (Roberts et al., 2018). The TMA/TMAO pathway likely represents only one of many microbe-dependent pathways that will ultimately be linked to CVD pathogenesis. Early intervention of GM and its metabolites can help to better control the occurrence and progression of atherosclerosis, stabilize atherosclerotic plaques, and reduce the risk of MI.

MI, caused by the necrosis of myocardium due to prolonged ischemia, was one of the most critical stress to the patients (Wu Z. X. et al., 2017). Nevertheless, different from other chronic CVDs, MI is a kind of severe stress injury. Under this kind of trauma, the structure of GM and its metabolites undergo stress changes. Patients with MI will have a typical GM dysbiosis, with a significant reduction of probiotics, a large number of pathogens, impaired intestinal barrier function, increased intestinal permeability and a series of changes. Based on the analysis of the blood microflora of 49 healthy controls, 50 patients with coronary heart disease, and 100 patients with MI, it was found that the diversity and richness of the blood microflora in patients with MI increased, and more than 12% of the bacteria in the blood was derived from GM (Zhou et al., 2018). Studies based on MI rat models have shown that the GM abundance was significantly higher in rats with MI than in the sham operation group, upon comparing the intestinal barrier injury of both rat groups (Wu Z. X. et al., 2017). Further, MI led to a decrease in Lactobacillus, and SCFAs were found to alleviate MI, indicating that GM and its metabolites have a significant repair effect on the cardiac tissue after MI. Similar studies have confirmed that the dysbacteriosis destroys the intestinal mucosal barrier, causing GM to enter systemic circulation, thus increasing the incidence of adverse cardiovascular events after MI. The incidence of adverse cardiovascular events increased significantly in patients with MI who had significantly increased GM (Lactobacillus, Bacteroides, and Streptococcus) populations (Org et al., 2015). Another study also found that the use of antibiotics to reduce the bacterial translocation in mice after MI can reduce the systemic inflammatory response and myocardial damage (Zhou et al., 2018).

Additionally, GM and its metabolites can, not only decrease the risk of MI by affecting intestinal barrier function, but also influence MI occurrence and development in other ways. Studies have shown that bacterial DNA found in arterial plaques is similar to that found in GM, and these bacteria may be related to plaque stability, implying that unstable atherosclerotic plaques increase the risk of MI (Koren et al., 2011). Elevated TMAO levels can affect the repair of mitochondria and myocardial metabolism and increase the risk and severity of MI (Makrecka-Kuka et al., 2017). TMAO can also promote the release of calcium ions into the cells through adenosine diphosphate, thrombin, and collagen, improve the sensitivity of platelets, and then promote thrombosis, which may directly stimulate the occurrence of MI (Zhu et al., 2016). Further experiments showed that antibiotics can reduce the MI area of acute myocardial infarction in mice, with the mechanism possibly related to the regulation of intestinal microorganisms (Lam et al., 2016). Furthermore, a recent study found that before myocardial infarction, probiotics supplementation in mice treated with antibiotics was beneficial at restoring cardiac function and prolonging survival time. That study found that GM-derived SCFAs played an important role in maintaining host immune components and repair ability after myocardial infarction, suggesting that regulation of intestinal flora might provide opportunities for rehabilitation after myocardial infarction (Tang et al., 2019); however, their ability to prevent coronary atherosclerosis and reduce the incidence of acute MI remains unclear. Thus, the mechanism by which GM and its metabolites influence the occurrence and development of MI is extremely complex, and needs to be studied further.

HF is the ultimate stage after MI. The structure and function of the heart are seriously damaged. Currently, surgery and drug treatment can delay the progress of HF to some extent, but the mortality rate is still very high. In recent years, great advances have taken place in understanding the pathophysiological mechanism of HF, from hemodynamics to neurohumoral immune regulation. The role of GM in HF has attracted widespread attention. In patients with HF, due to volume overload, intestinal congestion and ischemia could occur, leading to the dysbiosis of GM and an increase in intestinal permeability (Pasini et al., 2015). This will allow too many toxins to enter into blood circulation, activate cytokines, generate a systemic inflammatory response, and aggravate the progress of HF (Krack et al., 2004; Cannon and Mcmurray, 2014; Cui et al., 2018). Similar results were observed in a more recent study of patients with HF who were more likely to be infected with pathogenic bacteria than those in the control group (Pasini et al., 2015). Candida, Campylobacter, and Shigella were positively correlated with disease severity. Compared with the control group, 78.3% of the patients with HF had increased intestinal permeability, which was higher in patients with moderate and severe HF than in patients with mild HF. These results show that HF destroys GM balance, and the destruction of the intestinal mucosal barrier leads to bacterial translocation, an increase in systemic endotoxins, potential inflammation, and aggravation of HF.

Additionally, GM metabolites are closely related to HF. Recent studies have shown that TMAO is directly involved in the development of HF. TMAO levels were significantly higher in the blood of patients with HF than in that of healthy controls, and the prognosis of patients with high TMAO levels was poor (Tang et al., 2014). For instance, results from a study of a large number of chronic HF patients undergoing treatment show that increased TMAO content is positively correlated with the probability of death, worsening of patients’ condition, and probability of relapse in recovered patients within a short period of time (Chun and Rui, 2017). Further, Organ et al. (Organ et al., 2016) established a mouse HF model by surgery and found that mice fed TMAO showed more severe symptoms and signs of HF than mice fed a normal diet. Because 3,3-dimethyl-1-butanol can reduce the level of plasma TMAO, another animal study found that feeding 3,3-dimethyl-1-butanol to mice with HF induced by aortic coarctation significantly improved cardiac structural remodeling and electrical remodeling, suggesting a relationship between low levels of plasma TMAO and these improvements (Wang et al., 2020). Moreover, TMAO directly leads to progressive tubulointerstitial fibrosis and dysfunction, which might represent a potential mechanism associated with HF progression (Tang et al., 2015). Although there are several studies on the mechanisms underlying the effects of metabolites on HF, the GM composition and the specific mechanism by which the metabolites affect HF still need to be further studied.