TLRs are the key components of the pattern recognition receptor family, which significantly influence the innate immune system by recognizing pathogen-associated molecular patterns and injury-associated molecular patterns. So far, 10 different Toll-like receptors (TLRs, including TLR1-10) have been identified in humans, each exhibiting specificity for various microbial components. For instance, TLR2, as a dimeric cell membrane protein, is responsible for the recognition of bacterial lipoproteins and peptidoglycans. TLR4 is known for its recognition of LPS, while TLR5 is sensitive to bacterial flagella. Intracellular TLR3 detects double-stranded RNA, and TLR9 identifies bacterial DNA containing unmethylated CpG sequences (Bezhaeva et al., 2022).

In the context of gut microbiota dysregulation, LPS is secreted by Gram-negative bacteria, engages with a range of immune receptors and proteins, including TLR4, LPS-binding protein, and CD14 (Ryu et al., 2017). This interaction triggers TLR4 activation on the intestinal epithelium, leading to disruption of tight junctions between intestinal epithelial cells (Park and Lee, 2013; Mohr et al., 2022), increased intestinal permeability (Hietbrink et al., 2009), and facilitation of bacteria and endotoxins into the bloodstream. Once in the systemic circulation, LPS and peptidoglycans provoke an immune response by binding to TLR4 and nucleotide-binding oligomerization domain-like receptors (NLRs) (Rodrigues-e-Lacerda et al., 2023). Consequently, LPS is identified as a pivotal factor in induction of systemic inflammation (Cao et al., 2018).

The microbial constituents originating from the gut microbiota that can enter the bloodstream and reach the heart, inducing inflammation through activation of TLRs. This inflammatory cascade can ultimately lead to myocardial dysfunction and fibrosis (Zhang et al., 2023). Studies have demonstrated that TLRs, which orchestrate innate immune responses, are primarily responsible for inflammatory responses in the hearts of HF patients (Sharma et al., 2016), including TLR2, TLR3, TLR4, TLR5, and TLR9 (Yu and Feng, 2018). Specifically, TLR2 has been shown to mediate cardiac hypertrophy and inflammation in mice induced by Ang II via the TLR2/MyD88/NF-κB signaling pathway (Ye et al., 2021). Antagonizing TLR2 can block activation of NF-κB, suppress expression of inflammatory cytokines such as IL-1β, IL-6, IL-18, and attenuate cardiomyocyte apoptosis and cardiac fibrosis (Wang et al., 2019). TLR4 is implicated in cardiac inflammation and development of hypertrophy through multiple signaling pathways, including TLR4/MyD88/Calcium/calmodulin-dependent protein kinase II (CaMK II), TLR4/MyD88/NF-κB, TLR4/MyD88/PI3K/protein kinase B (AKT) and TLR4/MyD88/MAPK (Zhang et al., 2023). Animal experiments have found that Lactobacillus reuteri GMNL-263 can reduce inflammation, hypertrophy, and fibrosis in myocardial tissue by inhibiting theTLR4 pathway (Chiang et al., 2021). Both TLR3 and TLR4 have been identified to facilitate Ang II-induced cardiac hypertrophy through the TRIF pathway (Han et al., 2018). Furthermore, TLR5 has been linked to cardiac fibrosis and dysfunction by stimulating inflammation and endothelial-mesenchymal transition (Liu et al., 2015). Myocardial cell hypertrophy and fibrosis have been identified as the major pathophysiological manifestations of HFpEF (Fayyaz et al., 2024; Krittanawong et al., 2024).

SCFAs, primarily acetate, propionate, and butyrate, are critical mediators in the “gut microbiota-inflammation-HFpEF axis” and are terminal products of gut microbial catabolism (Yuan et al., 2024). Within the digestive tract, SCFAs can increase expression of intestinal tight junction proteins (Wang et al., 2012), regulate Treg/Th17 balance via peroxisome proliferator-activated receptor-gamma (PPAR-γ) pathway, and maintain gut epithelial cell physiology (Wen et al., 2021), thereby reducing gut permeability and alleviating low-grade systemic inflammation (Ma et al., 2022).

Butyrate reduction is prevalent in cardiovascular disease and associated with disease severity (Chakaroun et al., 2023). This is mainly because SCFAs are known to influence the pathology of HF by activating multiple G protein-coupled receptors, inhibiting histone deacetylase (HDAC) activity, and improving the cardiac inflammatory response (Kaye et al., 2020; Yukino-Iwashita et al., 2022; Taslim et al., 2023). Specifically, propionate diminishes the activity of NF-κB, IL-6, STAT1, and STAT3 signal transduction through down-regulation of HDAC activity, leading to a decrease in Th1 and Th17 immunoreactivity (Han et al., 2024). Additionally, propionate suppresses downstream inflammatory responses mediated by STAT1 and STAT3 (Jeong and Choe, 2023). Butyrate also significantly inhibits NK-κB activity by stimulating PPAR-γ (Malesza et al., 2021). Oral administration of propionate to mice with myocardial injury has been shown to substantially reduce systemic inflammation and myocardial fibrosis (Tang et al., 2019). In obesity mice, butyrate supplementation has been observed to suppress the expression of inflammatory factors such as IL-1β, NLRP3 and monocyte chemoattractant protein-1 in cardiac tissues. This effect is achieved by inhibiting HDAC and negatively regulating the NLRP3 inflammasome signaling pathway, which in turn reduces the release of inflammatory factors within adipocytes (Wang et al., 2015). Given the strong association between obesity, NLRP3 inflammasomes, and the progression of HFpEF, these findings are significant. Hatahet et al. treated obese pre-HFpEF mice with tributyrin, the transcript levels protein phosphatase Mg2+/Mn2+ dependent 1K (ppm1k) were significantly increased. Ppm1k encodes protein phosphatase 2Cm, which in turn causes dephosphorylation and activation of the branched-chain alpha-keto acid dehydrogenase complex, promoting branched-chain amino acids (BCAAs) catabolism and alleviating the early cardiac mechanical dysfunction associated with the development of obesity-related HFpEF (Hatahet et al., 2023).

Collectively, these studies suggest that SCFAs play a pivotal role in preventing the development of HFpEF by cutting off the connection between inflammation and the metabolic syndrome, suggesting their significant potential in the prevention and management of HFpEF (Beale et al., 2021; Challa and Lewandowski, 2022).

TMAO is the oxygenated metabolite of trimethylamine, generated by gut microbiota catabolism of choline and L-carnitine, followed by hepatic enzymatic oxidation (Janeiro et al., 2018). In vitro screening tests indicate that 36 strains from four phyla, including Bacteroidetes, Firmicutes, Actinobacteria, and Proteobacteria, participate in these processes under anaerobic conditions (Romano et al., 2015). Studies have shown that TMAO levels significantly decrease with a diet cessation, and this is associated with reduced cardiac fibrosis (Organ et al., 2020).

TMAO has been implicated in cardiac dysfunction through multiple mechanisms (Makrecka-Kuka et al., 2017; Tang et al., 2019). Several studies have found that TMAO promotes myocardial hypertrophy and fibrosis via the TGF-β/SMAD3 signaling pathway (Li et al., 2019; Wen et al., 2022). Elevated plasma TMAO levels have been associated with cardiac inflammation and interstitial fibrosis, contributing to cardiac dysfunction in mice that consume a diet rich in choline (Chen et al., 2017). Administration of a TMAO synthesis inhibitor has shown the capacity to modulate the TGF-β/SMAD3 signaling pathway, thereby preventing myocardial hypertrophy and fibrosis (Wang et al., 2020). Other studies have revealed that TMAO inhibits sirtuin3 expression and suppresses the activity of superoxide dismutase 2 and mitochondrial aldehyde dehydrogenase 2, leading to mitochondrial reactive oxygen species accumulation and activation of NLRP3 inflammasomes and N-terminal gasdermin D production, which in turn contributes to cardiovascular endotheliitis (Sun et al., 2016; Zhang et al., 2020; Li et al., 2022). Additionally, TMAO stimulates the secretion of exosomes by hepatocytes, which activate the NF-κB signaling pathway, causing vascular endothelial diastolic dysfunction (Liu et al., 2021). In cardiomyocytes, modulation of the TMAO/NF-κB pathway inhibits myocardial hypertrophy and fibrosis, thereby ameliorating cardiac dysfunction (Wang et al., 2024). TMAO is also excreted by the kidney, and elevated levels can lead to renal interstitial fibrosis and dysfunction (Sun et al., 2017), indirectly promoting the development of HFpEF (Ter Maaten et al., 2016).

Previous studies have reported elevated plasma TMAO concentrations are associated with left ventricular diastolic dysfunction (Wilson Tang et al., 2015). While the prognostic value of TMAO levels in HF has been a subject of debated, with some suggesting its relevance is primarily for HFrEF, and not HFpEF (Wilson Tang et al., 2014; Trøseid et al., 2015; Suzuki et al., 2019). Subsequent studies have challenged this view. Notably, Salzano et al. have demonstrated elevated TMAO is associated with outcome in patients with HFpEF, especially when B-type natriuretic peptide levels are not elevated, suggesting its TMAO as a sensitive risk stratification marker for HFpEF (Salzano et al., 2020). Further studies have confirmed TMAO as an independent risk factor for HFpEF, and highly associated with HFpEF risk (Dong et al., 2021). In addition, plasma TMAO levels at discharge in individuals with HFpEF have been link to an increased risk of subsequent cardiovascular events post-discharge (Kinugasa et al., 2021).

Dysregulation of gut microbiota can affect the metabolism of AAs, increasing the risk of HF (Mahenthiran et al., 2024). Multiple metabolites derived from tryptophan have been shown to affect cardiac function. For instance, indole sulfate may increase the expression of pro-inflammatory cytokines, such as TNF-α, IL-1β in cardiomyocytes. This upregulation is mediated through activation of signaling pathways, including p38 MAPK, p42/44MAPK, and NF-κB, inducing myocardial fibrosis and hypertrophy, ultimately contributing to myocardial diastolic dysfunction (Lekawanvijit et al., 2010; Shimazu et al., 2013). In a recent study by Wang et al., exogenous supplementation of another Trp metabolite, indole-3-propionic acid supplementation reduced inflammation, oxidative stress, diastolic dysfunction, and myocardial remodeling in HFpEF. This beneficial effect is thought to be mediated by inhibition of nicotinamide N-methyltransferase and activation of sirtuin 3 (Wang et al., 2024).

Kynurenine is an important metabolite of AAs, has been positively associated with symptom severity, inflammation indicators, and mortality risk in patients with HF (Lund et al., 2020; Razquin et al., 2021). This may be due to kynurenic acid induction of pro-inflammatory IL-1β and IL-8 and the expression of TNF via the aromatic hydrocarbon receptor signaling (Dahlem et al., 2020). On the contrary, elevated inflammatory mediators can also mediate kynurenine pathway activation through induction of JAK-STAT signaling cascade (Grishanova and Perepechaeva, 2024). Other studies have found that circulating product concentrations resulting from gut microbiota metabolism of phenylalanine and tyrosine are associated with major cardiovascular events and mortality risk (Nemet et al., 2023). The accumulation of BCAAs and their intermediary metabolites in the myocardium is also associated with the development of HF (McGarrah and White, 2023). A BCAA-free diet attenuates the proliferation of cardiac fibroblasts and the expression of collagen 1a1, thereby mitigating cardiac hypertrophy induced by workload (Yang et al., 2023). Although dietary intake is the primary source of BCAAs, these indispensable amino acids are also synthesized by the gut microbiota (Gojda and Cahova, 2021). Although studies have established the effect of amino acids on the heart, further research is warranted to confirm the role of gut microbiota-mediated AAs metabolism in the pathogenesis of HFpEF.

BAs are organic acids synthesized by the liver and released into the small intestine, where they promote the absorption of dietary fat-soluble molecules (Wan et al., 2020). Bacteroides, Clostridium, Bifidobacterium, Lactobacillus, and other genera modulate the expression of bile salt hydrolase genes. Elevated levels of total and unconjugated BAs correlate with an increased abundance of the associated intestinal microbiota. Besides, the concentration of secondary BAs, including deoxycholic acid, is positively correlated with the relative abundance of Bacteroides (Yokota et al., 2012). This association suggests a significant interaction between the gut microbiota and BAs.

Recent research suggests that BAs act as signaling molecules mediating cardiac function (Yntema et al., 2023). BAs are divided into two categories based on molecular structure: hydrophilic and hydrophobic. These distinct types of BAs have differential effects on cardiac health. Hydrophobic BAs are associated with QT interval prolongation, cardiac hypertrophy, cardiomyocyte apoptosis, and dysregulation of cardiac hemodynamics (Vasavan et al., 2018).

Conversely, hydrophilic BAs, acting as steroidal signaling molecules, confer cardiac protection by modulating cardiac inflammation through several pathways. These include activation of the nuclear receptor FXR and the membrane G protein-coupled receptor 5 (TGR-5) in the myocardium, and the inhibition of NF-κB signaling (Xiaoli et al., 2020; Zhou and Anakk, 2022; Shi M. et al., 2023; Zhang et al., 2023). Activation of TGR-5 activates the AKT pathway in downstream cardiomyocytes, which reduces cardiomyocyte inflammation and oxidative stress and increases cardiomyocyte survival (Eblimit et al., 2018). Treatment with taurine deoxycholic acid attenuated hypertensive-induced cardiac inflammation and myocardial remodeling in animal experiments (Bal et al., 2019). In cell culture studies, activation of TGR-5 by lithocholic acid ameliorates hyperglycemia-induced cardiac hypertrophy in cardiomyocytes (Cheng et al., 2019). Vitamin D receptors (VDRs) are expressed within the T tubules of cardiomyocytes, with lithocholic acid (LCA) being one of its ligands (Tishkoff et al., 2008). Targeted ablation of VDR in cardiomyocytes leads to cellular hypertrophy, cardiac enlargement, and impairments in both systolic and diastolic function (Chen et al., 2011). However, it remains to be determined whether LCA ligands can modulate VDR to elicit similar effects. These studies revealed an important role for BAs in the pathophysiology of HFpEF.

The significant contribution of probiotics in maintaining human health and regulating immune function is significant (Wang et al., 2022b; Cristofori et al., 2021; Gavzy et al., 2023; Mazziotta et al., 2023; Bai et al., 2024). Clinical trials have demonstrated that individuals with cardiovascular disease can achieve significant improvements in cardiovascular health following approximately 12 weeks of probiotic therapy. Supplementation with Lactobacillus paracaseis is effective in controlling low-density lipoprotein cholesterol levels and delaying atherosclerosis progression in hyperlipidemic patients (Khongrum et al., 2023). In trials of probiotic interventions for coronary heart disease, inflammatory indicators such as LPS, IL-1β, hs-CRP, and TNF-α were significantly decreased in the probiotic group compared to the control group after 12 weeks of probiotic intervention (Moludi et al., 2021; Moludi et al., 2022).

Several studies have indicated that probiotics can enhance plasma markers indicative of inflammation and oxidative stress in HF, and significantly ameliorate HF-related comorbidities (Karim et al., 2022; Pourrajab et al., 2022; Cui et al., 2023). However, it is important to acknowledge that trials didn’t differentiate based on ejection fraction categories. Probiotics can also capable of modulating the metabolite production of gut microbiota (Sadeghi et al., 2023), which could have a positive impact on HFpEF and its complications. For instance, in obese mice on a high-fat diet, the administration of Clostridium tyrobutyricum notably improved gut microbiota dysregulation, increased the production of SCFAs, and reduced the intestinal expression of TNF-α, IL-6, IL-1β (Luo et al., 2024). Given that obesity is an important comorbidity in HFpEF, and a key factor in the pathogenesis of HFpEF, inflammation plays a central role in its development (Chirinos et al., 2020). These findings suggest that probiotics may exert beneficial effects on HFpEF by modulating the gut microbiota dysregulation. Further experimental research is needed to confirm these potential benefits.

FMT has garnered widespread attention as a therapeutic strategy for restoring gut microbiota ecological balance. Analogous to organ transplantation, FMT is the process of transplanting the fecal microbiota from a healthy donor into the gut of the patients with microbiota-related diseases (Leshem et al., 2019). The procedure aims to treat conditions by re-establishing a balanced microbial community. FMT has demonstrated the ability to improve atherosclerosis in genetically susceptible mice lacking C1q/TNF-associated protein (Kim et al., 2022), and it has shown a positive effect on cardiometabolic diseases (Pakmehr A, Mousavi et al., 2024). The therapeutic effect of FMT treatment may be associated with increased diversity of the gut microbiota in the recipient (Luqman et al., 2024).

A previous study reported that FMT in obese mice increased circulating butyrate levels (Bonomo et al., 2020). Recent studies have discovered that FMT from lean mice to obese pre-HFpEF mice augments early cardiac diastolic dysfunction and left ventricular hypertrophy, mechanisms consistent with the effects of butyrate therapy (Hatahet et al., 2023). Petrick et al. demonstrated that FMT of microorganisms from nitrate-fed mice into mice with cardiac dysfunction led to significant improvements in myocardial remodeling, oxidative stress, glucose intolerance, serum dyslipidemia and cardiac dysfunction. Additionally, this study indicated that cardiac protective effects of nitrates are associated with the remission of intestinal dysbiosis by FMT (Petrick et al., 2023). These experiments highlight the potential effect of gut microbiota modification to improve outcomes in HFpEF. The current effects demonstrated by FMT in animal trials may provide a novel approach for future clinical research.

Prebiotics are non-digestible food ingredient that stimulate microbial growth, maintain a balanced gut ecosystem, thereby contributing to human health through the modulation of gut microbiota composition and function. Non-digestible plant oligosaccharides are the most popular prebiotics (Farias D de et al., 2019). Many studies have investigated the effects of prebiotics intake, including cellulose, on cardiovascular disease. Evidence suggests that prebiotics can inhibit the inflammatory response in healthy individuals by inducing the production of immunoregulatory molecules and lactic acid by the Bifidobacterium and Lactobacillus genera (Davani-Davari et al., 2019). Furthermore, dietary involving polysaccharide, flavonoid, and polyphenols can modulate gut microbiota, potentially affecting cardiovascular health (Feng et al., 2022). These processes may be related to metabolites such as BAs, SCFAs, AAs, etc (Xu and Yang, 2021).

Exogenous supplementation of gut microbiota metabolites, particularly SCFAs, may be an effective therapeutic strategy for HFpEF. Studies shown that SCFAs supplementation can increase acetic acid levels in the colon, and elevate the relative abundance of beneficial microbes such as Akkermansia, Turicibacter, Ruminococcus, and Prevotella, while also enhancing gut barrier function. SCFAs supplementation has been demonstrated to reduce the levels of NLRP3 and IL-1β, promote the IRF4/STAT3 complex and attenuate systemic inflammatory responses (Su et al., 2022), which may improve HFpEF. However, there is currently a lack of relevant clinical researches, and in light of the safety of approach, the large cohorts of clinical research should be conducted.