From Dysbiosis to Cardiovascular Disease: The Impact of Gut Microbiota on Atherosclerosis and Emerging Therapies

Abstract

The gut microbiota consists of trillions of microorganisms, mostly bacteria, which establish a symbiotic relationship with the host. The host provides a favourable environment and the essential nutrients for their proliferation, while the gut microbiota plays a key role in maintaining the host’s health. Therefore, imbalances in its composition, a state known as dysbiosis, can contribute to the onset or progression of various pathological conditions, including atherosclerosis. Atherosclerosis is a chronic, slow-progressing inflammatory disease characterised by the formation and potential rupture of atheromatous plaques in medium- and large-calibre arteries. It underlies major cardiovascular events, such as stroke and myocardial infarction, and remains a leading cause of global morbidity and mortality. The modulation of the gut microbiota using prebiotics, probiotics, and faecal microbiota transplantation (FMT) has emerged as a promising approach for preventing and managing atherosclerosis. Although numerous studies have explored these strategies, further research is needed to establish their efficacy and mechanisms. This review explores the pathophysiology of atherosclerosis, its main risk factors, and the interplay between the gut microbiota and atherosclerosis, with a particular focus on the mechanisms by which microbiota-targeted interventions, including prebiotics, probiotics, and FMT, may serve as therapeutic adjuvants in the prevention and treatment of atherosclerosis.

1. Introduction

Atherosclerosis is a chronic inflammatory disease of the arteries characterised by alterations in lipid profile and other metabolic parameters, including blood glucose levels, insulin sensitivity, body weight, and markers of systemic inflammation, and it is the main risk factor associated with cardiovascular diseases (CVD) with complex underlying pathophysiological mechanisms [1,2].

Recently, the gut microbiota has been identified as a relevant factor in the progression of atherosclerosis [3,4]. Therefore, changes in its composition and function appear to influence cardiovascular risk through the modulation of inflammatory and metabolic processes [5].

The human microbiota consists of trillions of microorganisms that inhabit the human body, organised into complex and specific ecosystems depending on the different body regions, such as the mouth, skin, vagina, and, in particular, the gastrointestinal tract (GIT) [6]. The GIT harbours the largest, most complex, and diverse bacterial community that maintains a symbiotic and mutualistic relationship with the host’s cells. Beyond their immune and homeostatic roles, commensal gut bacteria are key regulators of digestion, being essential for nutrient extraction, synthesis, and absorption, as well as contributing to the production and modulation of some metabolites [7].

As a whole, the gut microbiota is essential for the proper functioning of the body. Disruptions in its composition and, consequently, in its function—a condition known as dysbiosis—may impair the host’s health status, and it has been linked to various pathological conditions, including atherosclerosis [8]. Strategies such as the use of prebiotics, probiotics, and faecal microbiota transplantation (FMT) have been investigated for their potential to modulate this ecosystem, contributing to the prevention and regression of atherosclerotic lesions [9].

This review aims to explore the interplay between the gut microbiota and atherosclerosis, as well as to highlight the therapeutic targets and potential mechanisms that may contribute to the prevention and treatment of this disease. To achieve this objective, this review provides a concise overview of the gut microbiota and the pathophysiology of atherosclerosis, as well as the link between them both. Furthermore, this review summarises the latest evidence on the use of prebiotics, probiotics, and FMT in the prevention and management of atherosclerosis.

2. Overview of the Human Gut Microbiota

The human gut microbiota comprises over 10 trillion microorganisms, including bacteria, archaea, viruses, protozoa, and fungi [10]. These microorganisms form a highly complex ecosystem, engaging in interactions both among themselves, as well as with the host, in a symbiotic relationship that is essential for metabolic and immune system development [11].

A balanced microbiota is composed mainly of bacteria from four major phyla that adapt to constant changes in lifestyle, Actinomycetota, Pseudomonadota, Bacillota, and Bacteroidota, with the latter two representing approximately 90% of the total composition [12].

While most microbiota studies focus on the bacterial component, the human gut is also colonised by archaea, viruses (including bacteriophages), unicellular eukaryotes, and fungi, which, despite their lower abundance, play important functional roles in maintaining microbial homeostasis [13]. Bacteriophages, in particular, exert a strong influence on bacterial diversity, survival, and activity, and altered virome profiles have been associated with conditions such as inflammatory bowel disease [14,15]. Furthermore, certain gut viruses, such as Siphoviridae infecting Streptococcaceae and Enterobacteriaceae, have been implicated in systemic disorders like atherosclerosis, where viral infections may promote vascular inflammation and plaque formation [16,17].

The fungal fraction of the gut microbiota, known as the mycobiota, accounts for approximately 0.1–0.3% of the total microbial community and includes genera such as Aspergillus, Candida, Cryptococcus, Saccharomyces, Malassezia, and Penicillium [18]. Fungal biodiversity appears to be diet-dependent, with Candida spp. being positively associated with carbohydrate intake and Aspergillus spp. being negatively associated with levels of short-chain fatty acids (SCFAs) [19]. Altered fungal profiles have been reported in various disease conditions, including GIT, neurological, and liver diseases, as well as in atherosclerosis [18,20,21].

This delicate and dynamic equilibrium is often referred to as eubiosis, a state in which microbial diversity and function support the host’s health. Eubiosis is not defined by a fixed composition, but by the maintenance of beneficial host–microbe interactions and essential physiological functions. Disruption of this balance, known as dysbiosis, may result from antibiotic use, infections, dietary shifts, or chronic inflammation. Dysbiosis is typically associated with reduced microbial diversity, the loss of beneficial species, and the expansion of opportunistic taxa. Functionally, it can impair intestinal barrier integrity and trigger inappropriate immune responses, contributing to gastrointestinal and systemic disorders [22,23,24,25,26,27].

Beyond taxonomic and functional shifts, it is also important to distinguish between the related concepts of the microbiota and the microbiome, sometimes misused as synonyms. While the microbiota refers exclusively to the community of microorganisms, the microbiome encompasses these microorganisms along with their collective genetic material [7]. The human microbiome is estimated to harbour approximately 3.3 million active genes, in contrast to the 22,000 active human genes [28].

2.1. Development and Maturation of the Gut Microbiota

The composition of the gut microbiota in adulthood is the result of a complex developmental trajectory that begins at birth. Early-life microbial colonisation is increasingly recognised as a key determinant of immune and metabolic programming, both of which are closely linked to cardiovascular health [29]. During gestation, while the amniotic fluid contains trace amounts of bacteria, it seems insufficient to influence the colonisation process of the foetus [30].

The initial composition of the neonatal gut microbiota is primarily determined by the mode of delivery. In vaginal delivery, the newborn is predominantly colonised by Lactobacillus spp., Sneathia spp., and Prevotella spp. through contact with the mother’s vaginal and faecal microbiota. Conversely, infants delivered by caesarean section tend to be colonised by microorganisms present on the mother’s skin, on surfaces of the hospital environment, and possibly in the mother’s respiratory tract. As a result, the initial colonisation is predominantly by bacteria belonging to the genera Corynebacterium spp., Staphylococcus spp., and Propionibacterium spp. [30,31]. Other important factors include the introduction of either breastfeeding or formula feeding. Breastfeeding promotes the proliferation of bacteria from the Bifidobacterium spp., which are capable of degrading particular oligosaccharides in breast milk and are known to have several benefits, including the modulation of the immune system and the production of vitamins [32]. Conversely, formula feeding tends to stimulate a more diverse microbiota, with a higher prevalence of opportunistic pathogenic bacteria belonging to the genera Enterococcus spp., Bacteroides spp., and Clostridium spp. [32,33].

The introduction and diversification of food is an important dietary event in childhood that causes profound shifts in the gut microbiota. This period is characterised by the diversification of the gut microbiota and by the dominance of bacterial species belonging to the phyla Bacteroidota and Bacillota [33,34,35,36].

The gut microbiota undergoes significant development until approximately three years of age, as this is the stage when food introduction is completed and weaning from breast or formula milk occurs. The microbial community evolves from a simple community with low richness and diversity to one that progressively acquires the composition of the adult gut microbiota [37]. Understanding how early microbial exposures shape the long-term composition and function of the gut microbiota provides valuable context for designing effective microbiota-targeted interventions aimed at preventing or mitigating atherosclerosis later in life [29,38].

2.2. Composition and Functional Organisation of the Human Gut Microbiota

The GIT is anatomically and functionally divided into the stomach, the small intestine, and the large intestine. The physiological characteristics of each region shape the growth of distinct microbial communities, resulting in variations in both microbiota density and diversity throughout the GIT (Figure 1) [39].

The stomach contains a relatively low number of microorganisms (<10³ CFU/mL) compared to the distal regions of the GIT, due to its motile activity, gastric acid secretion, and low pH (<4). Gastric lumen harbours bacteria from the phyla Bacteroidota, Actinomycetota, Fusobacteria, and Pseudomonadota, while the mucosa is colonised primarily by Bacillota and Pseudomonadota [40]. Despite the hostile environment, acid-tolerant or transient species belonging to the genera Neisseria, Haemophilus, Fusobacterium, Prevotella, Veillonella, and Streptococcus have been detected in the gastric microbiota of healthy individuals. This microbial presence is likely due to the constant influx of bacteria originating from the oral cavity via swallowing, as well as reflux from the duodenum. Indeed, more than 65% of bacterial phylotypes identified in the stomach correspond to those found in the oral microbiota, suggesting that species, such as Veillonella, Lactobacillus, and Clostridium, detected in gastric samples may largely represent transient populations rather than established residents [41,42].

The duodenum, the first segment of the small intestine, is characterised by the presence of bile acids, pancreatic secretions, and antimicrobial agents, including bile salts (which can reach concentrations up to 11.2 mM postprandially); digestive enzymes, such as proteases and lipases, which can generate antimicrobial compounds; and innate immune molecules like defensins and lysozyme [43]. The rapid transit and the presence of an oxygen gradient along the GIT, with pO₂ levels in the duodenum estimated between 10 and 30 mmHg and progressively decreasing to near-anoxic conditions (<3 mmHg) in the colon, limit both bacterial density (10³–10⁴ CFU/mL) and biodiversity. The microbial community is mainly composed of facultative anaerobes and transient oral-derived species, mainly from the Bacillota and Actinomycetota phyla, including species such as Streptococcus salivarius, Lactobacillus rhamnosus, Lactobacillus plantarum, Enterococcus faecalis, and Enterococcus faecium [44,45,46].

In contrast, the jejunum provides a more suitable environment for the growth of predominantly aerobic Gram-positive and facultative anaerobic and aerotolerant bacteria, with bacterial counts ranging from 10³ to 10⁷ CFU/mL. The most represented genera include Lactobacillus, Enterococcus, and Streptococcus. The most frequently detected species are oral-associated taxa, including Streptococcus spp. (particularly from the mitis and sanguinis groups), Granulicatella adiacens/para-adiacens, Gemella haemolysans/taiwanensis, and Schaalia odontolytica complex, suggesting that the jejunal microbiota may largely reflect transient colonisation [44,47].

In the transition to the ileum, there is a higher population of bacteria, around 10⁹ CFU/mL, and the microbiota of this region is dominated by aerobic species, whereas the more distal part shows a distinctive microbial profile that resembles the oral microbiota. The most frequently detected genera include Streptococcus, Granulicatella, Actinomyces, Solobacterium, Rothia, Gemella, and members of the candidate phylum TM7 (G-1). Dominant species comprise streptococci from the mitis and sanguinis groups, Streptococcus salivarius, Rothia mucilaginosa, and Actinomyces spp., particularly from the A. meyeri/odontolyticus group. In contrast, strict anaerobes and Pseudomonadota are present in low abundance. This composition reflects a microbial continuity between the oral cavity and distal small intestine, which markedly differs from the colonic microbiota [48,49].

The large intestine is distinguished by a substantial bacterial density, reaching up to 10¹² CFU/mL, with a preponderance of the Bacillota and Bacteroidota phyla [39]. The most abundant bacterial families include Bacteroidaceae, Prevotellaceae, and Rikenellaceae from Bacteroidota, as well as Lachnospiraceae and Ruminococcaceae from Bacillota. These families play key roles in the fermentation of complex polysaccharides and the production of SCFAs, which are essential for colonic health and host metabolism [7,50].

Understanding this close relationship and microbial transition from the oral cavity to the gut is essential, as disturbances in the oral microbiota may influence gastrointestinal health and systemic diseases through their impact on the gut microbiome [4].

Despite its complexity and inter-individual variability, the gut microbiota exhibits well-conserved functional capacities, referred to as core functions, which are consistently present in the host. These include metabolic processes, the modulation of the immune system, and protection against pathogens [51,52].

2.2.1. Metabolic Functions

The gut microbiota regulates numerous metabolic processes, including the glucose and lipid metabolisms, and contributes additional functions to the host’s metabolism, such as the fermentation of non-digestible substrates (e.g., dietary fibres) and vitamin synthesis. Consequently, by participating in the digestion and fermentation processes that produce energy, the microbiota also ensures energy homeostasis [53,54]. These processes lead to the production or modulation of metabolites by the microbiota, which act as metabolic substrates and signalling molecules, with important implications for the host health. These include trimethylamine N-oxide (TMAO) and SCFAs, such as acetate, butyrate, and propionate, derived from the fermentation of dietary fibres in the colon [53,55].

Several bacterial genera are involved in SCFA production. For instance, Faecalibacterium prausnitzii, Roseburia spp., and Eubacterium rectale are prominent butyrate producers, whereas Bacteroides spp., Prevotella spp., Bifidobacterium spp., and Ruminococcus spp. are key contributors to acetate and propionate synthesis [56,57]. Butyrate and propionate are particularly vital in the regulation of glucose metabolism. They have been shown to stimulate the release of gut hormones, such as glucagon-like peptide (GLP-1) and peptide YY (PYY), which, in turn, increase satiety via the gut–brain axis and regulate blood glucose levels by increasing pancreatic insulin secretion and inhibiting glucagon production. Additionally, these SCFAs enhance glucose uptake in skeletal muscle and adipose tissue by upregulating GLUT-4 expression through AMP-activated protein kinase (AMPK) activation [58].

The gut microbiota is also involved in the synthesis of trimethylamine (TMA) from dietary choline and L-carnitine, lecithin, phosphatidylcholine, and betaine. TMA production has been associated with species such as Anaerococcus hydrogenalis, Clostridium asparagiforme, Clostridium hathewayi, Clostridium sporogenes, Edwardsiella tarda, Escherichia fergusonii, Proteus penneri, and Providencia rettgeri [59,60]. Once absorbed, TMA is transported to the liver, where it is oxidised into TMAO by hepatic flavin-containing monooxygenases, particularly FMO3. TMAO has been shown to modulate the cholesterol and bile acid metabolisms by inhibiting reverse cholesterol transport and altering bile acid synthesis and composition. These changes may promote lipid accumulation in macrophages and vascular inflammation, thereby contributing to atherosclerosis development [57,59,61,62].

Simultaneously, the gut microbiota constitutes an intrinsic source of micronutrients, including vitamins. Vitamin B biosynthesis is attributed to bacteria such as Lactobacillus spp., Bifidobacterium spp., Bacteroides fragilis, and Escherichia coli, which are responsible for the synthesis of water-soluble B vitamins such as biotin, riboflavin, folate, thiamine, and pantothenic acid. These contribute to microbial stability and energy metabolism by acting as coenzymes in redox reactions [63,64,65]. Furthermore, the gut microbiota is responsible for the synthesis of the fat-soluble vitamin K, primarily menaquinones (vitamin K2), by species such as Enterococcus, Enterobacter, Eubacterium lentum, Veillonella, and Bacteroides, which is essential for the activation of clotting factors and free radical neutralisation, thereby protecting cells against oxidative damage [11,63,66].

2.2.2. Regulation of Immune Response and Protection Against Pathogenic Organisms

The microbiota also plays a crucial role in the induction, formation, and function of the host’s immune system. In turn, the immune system, composed of innate and adaptive components, develops mechanisms to maintain its symbiotic interaction with the microbiota, promoting its tolerance to beneficial commensal microorganisms and generating protective responses against pathogens [67].

Considering that the intestine is the most extensive interface between the external environment and the human body, the strategy adopted to ensure the continuity of this interaction is the maintenance of a stable and semi-permeable physical barrier that regulates host–microbe interactions [22].

Additionally, the intestinal epithelial barrier must maintain a balance between selective permeability for the passage of nutrients, electrolytes, and water from the intestinal lumen to the systemic circulation via intercellular or transcellular transport and protection against harmful components from the external environment, which are ultimately eliminated in faeces [22,68]. This barrier comprises multiple layers, including the mucus layer (which traps pathogens), the epithelial cell monolayer (sealed by tight junctions regulating paracellular permeability, which allow for the selective absorption of nutrients, electrolytes, and water, while preventing the translocation of both commensal and pathogenic microorganisms and harmful antigens), and the underlying immune elements (such as secretory immunoglobulin A (IgA), dendritic cells, and macrophages) that coordinate immune defence while maintaining immune homeostasis [69,70,71]. Various stimuli, such as microbial metabolites (e.g., SCFAs), dietary components, cytokines, and pattern recognition receptor signalling, modulate the function of this barrier. These stimuli contribute to the maintenance of barrier integrity, defined as the structural and functional cohesion of the epithelial and mucosal interface, by promoting epithelial cell renewal, upregulating the expression of tight-junction proteins, stimulating mucus secretion, and modulating immune tolerance, without compromising its semi-permeable nature, which refers to selective molecular trafficking while preventing microbial translocation [22,68,72,73].

The mucus layer, secreted primarily by Globet cells, consists of a physical barrier that harbours beneficial commensal microorganisms, such as Bacteroides spp., Faecalibacterium prausnitzii, and Akkermansia muciniphila, and contains defence proteins with immune-regulatory functions, including antimicrobial peptides (AMPs) and secretory IgA, which are secreted primarily by Paneth cells and plasma cells, respectively [74,75]. Beneath this layer lies a continuous and polarised monolayer of intestinal epithelial cells, under which the lamina propria contains immune cells such as T cells, B cells, macrophages, and dendritic cells, which provide both innate and adaptive immune responses that reinforce intestinal defence [68,76]. A schematic representation of the intestinal barrier, including the mucus layer, epithelial cell monolayer, and immune cell populations in the lamina propria, is provided in Figure 2 to illustrate these components and their spatial organisation.

On the other hand, microbiota-derived SCFAs regulate immune cells by binding to G-protein-coupled receptors (GPCRs) and by altering gene expression through the reduction in histone deacetylase activity. Although the intestinal epithelial barrier separates microbes from the immune cells in the lamina propria, SCFAs, being small and soluble, can diffuse through epithelial cells via passive diffusion or be transported by specific monocarboxylate transporters (e.g., MCT1, SMCT1). Once in the lamina propria, they act on various immune cell populations. This results in decreased local inflammation, protection against pathogen infiltration, and the maintenance of the integrity of the intestinal barrier [8,77]. This process is illustrated in Figure 3.

Thus, the gut microbiota’s influence on the host’s immune system is evident, playing a critical role in maintaining immune homeostasis through the modulation of immune cell function and inflammatory responses. Disturbances in this symbiotic relationship, defined as dysbiosis, can disrupt immune tolerance and trigger chronic inflammation, which has been linked to the development of various autoimmune diseases. Understanding these mechanisms is essential to unravelling how microbiota imbalances contribute to immune dysregulation and disease pathogenesis [50,78].

2.3. Factors Influencing the Gut Microbiota

Gut microbiota composition is dynamic and can be shaped by various intrinsic and extrinsic factors, including exposure to antibiotics, dietary patterns, genetics, smoking, and physical activity, ultimately influencing the host’s health status [79].

2.3.1. Exposure to Antibiotics

The effects of antibiotic use depend on their spectrum of activity, pharmacokinetics, and pharmacodynamics, as well as the route of administration, dosage, and duration of treatment. The oral administration of antibiotics is often the preferred route, directly exposing the intestinal lumen and microbiota to their bactericidal and/or bacteriostatic action [80]. Antibiotics’ effects include alterations in the composition and function of the gut microbiota, increased susceptibility to infections, and the development of antibiotic resistance [51].

Disturbances in the composition and functions of the microbiota are often characterised by an imbalance, typically associated with a reduction in microbial diversity. This does not necessarily imply a decrease in the total number of microorganisms; in fact, microbial load may increase as antibiotic-susceptible bacteria, such as members of the Bacillota phylum, are eliminated, providing an opportunity for resistant bacteria, including some Pseudomonadota species, to proliferate. Additionally, the administration of a particular antibiotic can impair the metabolic functions of the microbiota by reducing the production of key microbial metabolites, such as SCFAs, which play a role in maintaining gut homeostasis. This disruption can lead to immune dysregulation, characterised by a decreased frequency and function of regulatory T cells (Tregs) and an increased differentiation of pro-inflammatory effector T cells, such as T helper 17 (Th17) cells, tipping the balance towards inflammation and impaired immune tolerance [81,82,83].

In terms of susceptibility to new infections, antibiotic use can lead to a reduction in the thickness of the mucus layer, facilitating bacterial invasion by newly acquired pathogens, or from the overgrowth and pathogenic behaviour of opportunistic organisms present, as observed in antibiotic-associated diarrhoea, the most severe form caused by Clostridioides difficile [81,84].

2.3.2. Diet

The composition and diversity of the gut microbiota, as well as the relative abundance of microbial metabolites, are profoundly shaped by diet, as specific nutrients exert selective pressures that favour bacterial species capable of metabolise them. As a result, changes in eating habits, as well as food scarcity or excess, can lead to transient alterations of the microbiota [85].

A diet rich in animal proteins and amino acids, such as Western diets, has pronounced effects at the level of microbial metabolism, leading to the increased production of branched-chain fatty acids and potentially toxic substances such as sulphides, ammonia, and N-nitroso compounds [86]. This type of diet also increases luminal bile acid content, favouring bile-tolerant anaerobic bacteria such as Bacteroides spp., Alistipes spp., and Bilophila spp., leading to an increase in TMAO. Additionally, this intake stimulates the synthesis of nitric oxide (•NO), a free radical with antimicrobial properties that can influence gut microbiota composition [87]. Gut bacteria, such as Lactobacillus and Bifidobacterium species, are generally susceptible to the antimicrobial effects of •NO, resulting in their decreased abundance under high-•NO conditions. Conversely, certain facultative anaerobes and nitrate-reducing bacteria, including Enterobacteriaceae family members like E. coli, are more resistant to •NO and can survive or even flourish, potentially contributing to dysbiosis [87,88,89]. Moreover, •NO in excessive concentrations may induce oxidative stress and inflammation, further impacting the gut microbial balance [90].

In contrast, the intake of plant-based proteins has been associated with an increase in beneficial bacteria such as Bifidobacterium spp. and Lactobacillus spp., along with a decrease in potentially pathogenic species such as Bacteroides fragilis and Clostridium perfringens [91].

Similarly, microbial composition and diversity are significantly affected by increased fat intake, which alters the Bacillota/Bacteroidota ratio by increasing the relative abundance of the former while reducing the latter [92]. It should also be noted that a high-fat diet indirectly influences microbiota composition by promoting an increase in bile acid secretion [93,94]. In the distal small intestine, luminal pH rises progressively from approximately 6.5 in the proximal segments to around 7.5 near the ileum. Upon reaching the large intestine, pH temporarily drops to approximately 6.0 near the caecum and then increases again toward the rectum, approaching pH 7.0 [95]. These pH shifts, along with the antimicrobial properties of unabsorbed bile acids, contribute to an inhospitable environment for acid-sensitive commensals such as Bifidobacterium spp. and Lactobacillus spp. Conversely, they favour the expansion of bile- and acid-resistant taxa, including Bilophila wadsworthia, Bacteroides spp., and Alistipes spp., which are frequently associated with pro-inflammatory profiles and metabolic dysregulation [96,97].

Conversely, a fibre-rich diet, particularly one high in plant polysaccharides, promotes the growth of Bacteroidota at the expense of Bacillota [98,99]. Bacteroidota, especially Bacteroides spp., are known for producing a variety of carbohydrate-active enzymes (CAZymes), including xylanases and pectinases, which degrade complex plant fibres. In contrast, members of the Bacillota phylum, such as Clostridium spp. and Ruminococcus spp., produce enzymes like cellulases and amylases, which also contribute to polysaccharide breakdown [100]. A higher prevalence of Bacillota compared to Bacteroidota ensures a the presence of a greater number of enzymes capable of breaking down dietary polysaccharides, leading to increased monosaccharide absorption and, consequently, greater energy extraction from the diet [87].

2.3.3. Other Factors

Other factors that have been reported to also influence the gut microbiota include genetic factors, smoking, and physical exercise [101,102,103].

The host’s genetic composition is a determining factor in how certain bacteria influence the metabolism and functions of the microbiota. Members of the same family, by sharing similar genetics, tend to have a more similar microbial community compared to unrelated individuals. For example, hosts carrying the rs651821 variant of the APOA5 gene are more likely to be colonised by the genera Lactobacillus, Sutterella, and Methanobrevibacter, which haves been associated with a higher risk of developing metabolic diseases [101].

Smoking is another relevant factor. Cigarette smoke is a complex chemical mixture, with nicotine as its main active substance. Nicotine is inhaled into the lungs, primarily absorbed by the pulmonary alveoli, but also by the GIT. Consequently, it can influence the composition of the gut microbiota by increasing the abundance of Bacteroidota while decreasing Bacillota and Pseudomonadota. On the other hand, it increases intestinal pH, which promotes the development of some pathogenic microorganisms, such as Peptococcaceae [102].

Finally, regular physical exercise, particularly moderate-intensity aerobic activities performed for 30–90 min 3–5 times per week, is also positively correlated with the composition and functions of the gut microbiota. Such exercise regimens promote increased microbial diversity and a balance between beneficial commensal microorganisms and pathogens, including the growth of beneficial bacteria such as A. muciniphila, Oscillospira, Faecalibacterium prausnitzii, Methanobrevibacter, Veillonella atypica, and Prevotella spp. Mechanistically, physical exercise enhances the production of SCFAs like butyrate, modulates immune responses by increasing IgA levels and reducing Toll-like receptor signalling, and decreases intestinal transit time, collectively contributing to improved intestinal barrier function and reduced inflammation [103,104,105].

3. Cardiovascular Diseases

The term CVD is a collective term that encompasses all types of conditions that affect the cardiovascular system, namely the heart and blood vessels [106]. CVDs remain the leading cause of mortality and morbidity worldwide. In 2019, they were responsible for an estimated 17.9 million deaths, representing 32% of global deaths, making them a major public health concern [107].

In this complex context, it is important to highlight that atherosclerosis, an often asymptomatic condition, is the underlying pathological process in most CVDs [2].

3.1. Atherosclerosis

Atherosclerosis is a chronic, slow-progressing, immune-inflammatory, and fibroproliferative disease that begins with the activation of the endothelium, a process in which endothelial cells (ECs) begin to express adhesion molecules and pro-inflammatory cytokines, facilitating the adhesion and migration of leukocytes. A cascade of events follows, resulting in the narrowing of vessels and the activation of inflammatory pathways, ultimately resulting in the formation of atherosclerotic plaques. These plaques are characterised by the accumulation of lipids, fibrous elements, and calcification within the walls of medium and large arteries, which can lead to further cardiovascular complications [2,108].

Hypertension, obesity, smoking, age, gender, family history, and dyslipidaemia are some of the risk factors for atherosclerosis. Among these, dyslipidaemia plays a particularly important role in the incidence and progression of atherosclerosis [109]. It is characterised by high plasma levels of cholesterol, triglycerides, or both, which are transported by lipoproteins. Based on their density and size, lipoproteins are classified into five categories: very-low-density lipoproteins (VLDLs), low-density lipoproteins (LDLs), intermediate-density lipoproteins (IDLs), high-density lipoproteins (HDLs), and chylomicrons. Each category has specific functions in the lipid metabolism, and together they contribute to maintaining a healthy vasculature [110]. In this context, LDLs are considered to be the most atherogenic lipoproteins, as they alter the properties of the endothelium, upregulate adhesion molecules, promote monocyte recruitment and differentiation into macrophages, and induce the proliferation of smooth muscle cells [2]. To provide a better understanding of the role of circulating lipoproteins in atherogenesis, Figure 4 summarises the main metabolic pathways of lipoproteins, including the exogenous, endogenous, and reverse cholesterol transport pathways.

The progression of atherosclerosis can be divided into four distinct phases, namely endothelial dysfunction, the formation of fatty streaks, lesion progression, and, finally, plaque rupture and thrombus formation, which can lead to acute cardiovascular events such as myocardial infarction and stroke (Figure 5) [110].

3.1.1. Endothelium

The vascular endothelium consists of a monolayer of specialised cells, known as ECs, connected by tight junctions. These ECs are covered by a dense layer of glycoproteins and/or glycolipids, known as the glycocalyx, forming a barrier that lines the lumen of blood vessels. This barrier serves as the first line of defence against circulating molecules, cells, and pathogens in the bloodstream. In the large vessels, the endothelium, together with collagen and elastic fibres, forms the tunica intima. The tunica media lies beneath, and it is predominantly composed of vascular smooth muscle cells (VSMCs), collagen, and elastic tissue, providing elasticity and contractility to the vessels. Surrounding this layer is the tunica adventitia, which is mainly composed of a dense connective tissue matrix, offering structural support and protection to the blood vessel [108,111,112].

Given its location, the endothelium, specifically the ECs, continuously senses hemodynamic and biochemical changes in the bloodstream and transmits signals to the underlying layers of the vascular wall. Through their ability to produce effector molecules that regulate thrombosis, vascular tone, vascular remodelling, and inflammatory response, ECs act as key regulators of vascular homeostasis [112,113]. A compromise of this regulatory capacity, triggered by several factors, including turbulent and oscillatory blood flow (low shear stress), oxidised LDL (ox-LDL), and oxidative stress, results in endothelial dysfunction, a crucial event in the pathogenesis of atherosclerosis [112,114].

3.1.2. Endothelial Dysfunction

Among the several factors implicated in endothelial dysfunction, low shear stress has particular relevance. In fact, the atherosclerotic lesions mainly develop in regions of arterial branching or curvature, where blood flow is reduced, being turbulent and oscillatory with low shear stress [115,116,117]. In these regions, ECs adopt a polygonal shape, lose their typical alignment with the direction of flow, and undergo an increase in apoptosis rate. This leads to an increase in the ECs’ turnover and promotes the accumulation of senescent cells, which are more adhesive to monocytes, produce less ^•NO, and generate more anion superoxide (O₂^•−). Together, these alterations compromise the functionality and integrity of the endothelial barrier [116]. Furthermore, the haemodynamic conditions in these regions of the vasculature increase the residence time of the circulating molecules, LDL and blood cells, favouring their interaction with the endothelium. This fact, combined with the increased permeability of endothelium, promotes the infiltration and accumulation of LDLs in the subendothelial matrix, where they form insoluble complexes with proteoglycans and are oxidised by reactive oxygen species (ROS) to ox-LDL [118]. The accumulation of LDL is greater the higher its plasma levels are. Ox-LDL further aggravates endothelial dysfunction [117].

3.1.3. Formation of Fatty Streaks

Endothelial stimulation, also known as type II endothelial activation, occurs in response to ox-LDL and involves the overactivation of the NF-κB inflammatory pathway within ECs. This leads to the increased expression of pro-inflammatory cytokines and chemokines (MCP-1 and IL-8), vascular (VCAM-1) and intercellular (ICAM-1) adhesion molecules for monocytes, and E-selectin and P-selectin [108,119].

The recruitment of monocytes from the bloodstream to the affected area of the endothelium is the first step in a cascade of subsequent events. Initially, monocytes are captured and roll along the endothelium surface, a process mediated by P-selectin, followed by their tight adhesion through their interactions with the endothelial VCAM-1 and ICAM-1. Chemokines, including CXCL1, CXCL2, CXCL4, and CCL5, present in the endothelial surface, activate monocytes as they roll, increasing their adhesiveness [108]. Subsequently, MCP-1 and IL-8 induce the transmigration of monocytes to the subendothelial space, where they mature and differentiate into macrophages, which can polarise into M1 (pro-inflammatory) or M2 (anti-inflammatory) phenotypes, with the M1 predominating when the disease progresses. However, due to their sensitivity and plasticity, these macrophages can switch between M1 and M2 phenotypes in response to new signals [120].

M1 macrophages produce huge amounts of pro-inflammatory cytokines and chemokines, as well as reactive oxygen and nitrogen species (RONS) such as ^•NO and O₂^•−, contributing to an oxidative and inflammatory environment. They also express scavenger receptors, including CD36, SR-AI, and LOX-1, which recognise and internalise the modified forms of LDL, particularly ox-LDL. It is important to note that the activity of these receptors is not negatively regulated by the intracellular concentration of cholesterol, thus leading to a deregulated and exacerbated uptake of ox-LDL. Once internalised, ox-LDLs are degraded in lysosomes, and excess free cholesterol is transported to the endoplasmic reticulum (ER), where it is esterified by acyl-CoA acyltransferase (ACAT) into cholesterol esters, which are stored in lipid droplets in the cytoplasm or which are associated with the ER, a characteristic feature of foam cells [119,121]. Cholesterol esters are hydrolysed, and the resulting free cholesterol suffer efflux from macrophages to HDL via transporters like ABCA1. However, in the atherosclerotic lesions, the pro-inflammatory environment impairs these efflux systems. Therefore, the deregulated uptake of ox-LDL by macrophages, associated with the impaired efflux of cholesterol, promotes the formation and accumulation of foam cells, a key feature in atherosclerosis [122,123].

The excessive lipid uptake by macrophages sustains the inflammatory response, which is further exacerbated by the continuous activation of NF-κB signalling pathways by ox-LDL. This vicious cycle sustains EC activation, monocyte recruitment, and foam cell accumulation, ultimately leading to the development of fatty streaks [124]. In addition, VSMCs in the tunica intima can also internalise ox-LDL through their scavenger receptors (SR-A, CD36, LOX-1), further contributing to the growth of fatty streaks [125].

Fatty streaks are an important precursor; however, they generally do not cause immediate clinical complications and may even spontaneously regress [119].

3.1.4. Lesion Progression

The progression of fatty streaks into more advanced fibrolipidic lesions, whose regression is less likely, occurs through the infiltration and proliferation of VSMCs from the tunica media into the tunica intima. This migration is induced by growth factors secreted by foam cells derived from macrophages, as well as by the activated VSMCs themselves. These include platelet-derived growth factors, fibroblast growth factors, heparin-binding epidermal growth factors, and matrix metalloproteinases (MMPs) [121].

Within the tunica intima, VSMCs proliferate and synthesise an extracellular matrix composed of collagen, proteoglycans, and elastin, which forms a fibrous cap over the foam cells. This cap provides structural support to the plaque and separates the necrotic core, a hypocellular and lipid-rich region, from the vessel lumen, where coagulation factors and circulating platelets are present. The thickness of this fibrous cap is directly correlated with plaque vulnerability [108,126].

As the plaque develops, there is an increase in apoptosis among both macrophages and VSMCs in response to the oxidative and inflammatory microenvironment and activation of death-signalling receptors [127]. Due to the excessive apoptosis, the clearance process (efferocytosis) becomes insufficient, leading to secondary necrosis. The necrotic cells release intracellular inflammatory and oxidative contents, contributing to the expansion of the necrotic core. In addition, apoptotic and necrotic cells release tissue factor, which, together with oxidised lipids, increases the thrombogenicity of the necrotic core [108,127].

3.1.5. Plaque Rupture and Thrombus Formation

Inflammation plays a critical role throughout all stages of atherosclerotic plaque development, and plaque rupture is no exception. Inflammatory processes promote the instability of the fibrous cap that overlays the foam cells [128].

Some pro-inflammatory cytokines, such as interferon-gamma, inhibit collagen synthesis. In addition, the apoptosis of VSMCs reduces the production of the extracellular matrix, further decreasing collagen levels while simultaneously increasing the expression of MMPs. These combined factors contribute to plaque vulnerability. Consequently, a vulnerable plaque is more prone to rupture due to the action of ROS, which promote MMP release and the subsequent degradation of the fibrous cap. When the cap fissures or ruptures, the subendothelial space is exposed to circulating blood, triggering a coagulation process [129].

Initially, platelets adhere to exposed subendothelial collagen and promote the recruitment of additional platelets to this area to facilitate healing. Simultaneously, pro-thrombotic components within the plaque core, particularly tissue factor, come into contact with plasma coagulation factor VII, activating the coagulation cascade. This cascade results in the formation of the intermediate product, thrombin, and ultimately leads to the production of fibrin [130].

Fibrin is an insoluble protein that forms a meshwork that binds to platelets, creating a thrombus, which consists of a stable and organised structure whose purpose is to protect the site of injury. However, it is important to note that if the thrombus detaches from the arterial wall, it can lead to serious cardiovascular complications [128,130]. These include myocardial infarction, stroke, and peripheral artery disease, all of which highlight the clinical significance of atherosclerosis and the importance of identifying modifiable risk factors, such as those related to gut microbiota [108,131,132,133,134].

4. Interaction Between the Gut Microbiota and Cardiovascular Diseases

The gastrointestinal system is considered to be the largest endocrine organ in the human body due to its ability to produce a wide range of bioactive metabolites, such as SCFAs, TMA—which is converted by hepatic FMO3 into TMAO—secondary bile acids (e.g., deoxycholic acid, lithocholic acid), and tryptophan-derived indoles. These compounds can either be metabolised by the host’s enzymes, such as sulfotransferases, monooxigenases, or conjugating enzymes, into signalling molecules, or directly act as such [135]. For instance, SCFAs act via G protein-coupled receptors (e.g., GPR41 and GPR43), while indole derivatives modulate the aryl hydrocarbon receptor (AhR) pathway [135,136,137,138]. These molecules are absorbed by the intestinal epithelium through various mechanisms, including passive diffusion (e.g., for lipophilic bile acids), active transport (e.g., via monocarboxylate transporters for SCFAs), and carrier-mediated uptake. Upon entering the bloodstream, they can exert effects on various organs and systems, thereby influence multiple host metabolic pathways [135].

Shifts in microbial composition, characterised by an increased abundance of pro-inflammatory taxa, such as members of the Enterobacteriaceae family (including Escherichia-Shigella), Desulfovibrio spp., Collinsella spp., Eggerthella spp., and Streptococcus spp., along with a depletion of beneficial butyrate-producing genera, such as Faecalibacterium prausnitzii, Roseburia spp., Subdoligranulum spp., and Eubacterium rectale, have been strongly associated with the pathogenesis of CVDs. These alterations are frequently accompanied by reduced microbial diversity and changes in the relative proportions of dominant bacterial phyla. In particular, shifts in the Bacillota/Bacteroidota ratio have been linked to metabolic disturbances, such as increased energy harvest, low-grade inflammation, and adiposity, which collectively contribute to cardiovascular risk. Although some studies report a higher Bacillota/Bacteroidota ratio in obese and hypertensive individuals, findings are not universally consistent, highlighting the complexity of microbiota–host interactions in CVD development. Conversely, protective genera, such as Akkermansia, Blautia, and Lactobacillus, are often depleted in individuals with CVD. Such compositional shifts may modulate cardiovascular risk by influencing lipid metabolism, gut barrier integrity, immune activation, and the production of bioactive metabolites like TMAO, ultimately contributing to vascular dysfunction and atherogenesis [90]. Furthermore, bacterial components, such as lipopolysaccharide (LPS), a constituent of the outer membrane of Gram-negative bacteria, can trigger immune activation via the NF-κB pathway, promoting foam cell formation and atherogenesis [135,139]. The accompanying endothelial dysfunction, aggravated by elevated levels of ROS, further amplifies vascular inflammation, contributing to plaque formation and thrombosis [135].

While bacterial-derived metabolites have been extensively studied in the context of atherosclerosis, emerging evidence suggests that other non-bacterial components of the gut microbiota, including fungi, viruses, and bacteriophages, may also contribute to cardiovascular pathology, albeit through distinct mechanisms [16,20,140,141,142].

4.1. Trimethylamine N-Oxide

TMAO is the main diet-derived metabolite, linked to an increased risk of developing atherosclerosis, thrombosis, and stroke. This compound is formed from TMA, a precursor produced in the colon by the action of bacteria from the phylum Bacillota, Pseudomonadota, and Actinomycetota, whose species have been previously mentioned [143]. These bacteria convert dietary quaternary amines, such as L-carnitine and choline, present in foods like red meat and eggs, into TMA. Once produced, TMA is absorbed and transported via the portal vein to the liver, where it is converted into TMAO by the FMO3 enzyme. In addition, this metabolite is found in fish and seafood, so their consumption represents an exogenous and direct source of TMAO [144,145]. Once in circulation, TMAO contributes to endothelial dysfunction, promotes foam cell formation, and enhances platelet activity, which, together, increase the risk of thrombus formation [143].

Endothelial dysfunction is an early and critical step in atherosclerosis. In this context, TMAO is known to activate the NF-κB signalling pathway, partly through a mechanism involving high-mobility group box 1 (HMGB1), a key inflammatory mediator [146,147]. HMGB1 can be released extracellularly and interacts with the receptor for advanced glycation end-products (RAGE), initiating downstream pro-inflammatory signalling cascades. This interaction promotes the activation of NF-κB, leading to the upregulation of pro-inflammatory genes that encode adhesion molecules, chemokines, and inflammatory cytokines. Additionally, HMGB1/RAGE signalling can activate the NLRP3 inflammasome, further exacerbating the inflammatory response. Thus, TMAO-induced HMGB1 release, and the subsequent RAGE activation, constitute an important pathway, contributing to endothelial inflammation and dysfunction in atherosclerosis [146,147,148].

This also decreases the expression of tight junction proteins, such as occludin and zonula occludens-2 (ZO-2), as well as vascular endothelial cadherin in ECs, which increases permeability to LDL, a particle formed in the bloodstream from the metabolic conversion of VLDL secreted by the liver, as shown in Figure 4. Additionally, TMAO upregulates VCAM-1 and ICAM-1 expression, promoting monocyte adhesion [146].

Concerning foam cell formation, macrophages play a central role by accumulating lipoproteins. In this context, several studies support a crucial role of TMAO in promoting foam cell development [149,150]. TMAO, which structurally resembles ox-LDL, promotes monocyte differentiation into macrophages and increases the expression of scavenger receptors like CD36 and SRA-1. More recently, Luo and colleagues demonstrated that TMAO downregulates the expression of Nrf2 and its downstream effectors, heme oxygenase-1 and glutathione peroxidase 4, leading to oxidative stress and the reduced expression of cholesterol efflux proteins, such as ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1), thereby promoting lipid accumulation in foam cells [151]. In addition, TMAO has been associated with an inhibition of the reverse cholesterol transport, a key protective mechanism against atherosclerosis, as depicted in Figure 4 [138,151,152,153]. Reverse cholesterol transport involves the efflux of excess cholesterol from peripheral cells, such as macrophages, to apolipoprotein A-I (ApoA-I), initiating the formation of nascent HDL particles via their interactions with ABCA1 and ABCG1. These HDL particles mature and deliver cholesterol to the liver through scavenger receptor class B type 1 (SR-B1), either directly or indirectly, via the cholesteryl ester transfer protein (CETP). TMAO interferes with this pathway by downregulating the expression of LXRα, a transcriptional regulator of ABCA1 and ABCG1, thereby impairing cholesterol efflux from macrophages and HDL biogenesis. This disruption compromises reverse cholesterol transporter efficiency and contributes to lipid accumulation and foam cell formation within atherosclerotic plaques [138,153,154].

Finally, platelet activation and aggregation precede thrombus formation. TMAO enhances platelet reactivity by increasing intracellular calcium release and platelet activation. Moreover, TMAO exerts a pro-thrombotic effect by activating the pathway that leads to the expression of endothelial tissue factor, the membrane receptor for factor VII, which initiates the extrinsic coagulation cascade [155,156].

4.2. Bile Acids

Bile acids are amphipathic molecules synthesised in the liver from cholesterol and which, subsequently, are modified by the gut microbiota into secondary bile acids. They are stored in the gallbladder and are released into the small intestine after food intake, with their main functions being the emulsification of dietary fats and the promotion of intestinal absorption of lipids and fat-soluble vitamins. Bile acids can be classified into primary and secondary acids based on their structure, with primary bile acids being metabolised into secondary bile acids by enzymes derived from the gut microbiota [157,158].

Primary bile acids, such as cholic acid and chenodeoxycholic acid, result from cholesterol oxidation by cholesterol 7α-hydroxylase (CYP7A1), followed by conjugation with glycine or taurine by bile acid-CoA:amino acid N-acyltransferase (BAAT). Subsequently, through the action of bile salt hydrolase (BSH), primary bile acids are hydrolysed and deconjugated, ready to be metabolised by the gut microbiota through various mechanisms, including 7α-dehydroxylation, dehydrogenation, and epimerisation, leading to the formation of secondary bile acids such as deoxycholic acid and lithocholic acid (Figure 6) [159,160].

Bile acids play a protective role in the development of atherosclerosis, as they act as signalling molecules for a range of physiological processes mediated by the farnesoid X receptor (FXR) and the G-protein-coupled bile acid receptor (TGR5) [161]. FXR plays a central role in the regulation of the glucose, lipid, and bile acid metabolisms. Given its high expression in the GIT, it acts as a sensor of bile acid levels in hepatocytes and enterocytes, thus regulating the transcription of genes involved in bile acid synthesis, conjugation, and transport, including CYP7A1. The activation of FXR in ECs positively regulates endothelial nitric oxide synthase, enhancing nitric oxide production, which promotes vasodilation, reduces endothelin-1 expression, a potent vasoconstrictive peptide, and modulates angiotensin II receptors, inhibiting the inflammation and migration of VSMCs, a key factor limiting the progression of atherosclerotic lesions, as these cells in the intima produce the extracellular matrix (Figure 6) [137,160,162].

Additionally, FXR activation also increases faecal cholesterol excretion, thereby lowering plasma cholesterol levels. This contributes to reducing the amount of circulating LDL, limiting their retention in the endothelial barrier and, subsequently, decreasing ox-LDL formation, as shown in Figure 6 [163,164].

Furthermore, there is a reduction in pro-inflammatory cytokines, such as TNF-α, IL-1β, and IL-6, which initiate monocyte recruitment, inhibiting the formation of fatty streaks [165,166,167]. On the other hand, as illustrated in Figure 6, FXR expression in hepatocytes, which can be upregulated by bile acids, regulates FMO3, the key enzyme in TMAO synthesis, the metabolite that actively contributes to the formation and development of atherosclerotic lesions, as previously explained [168].

The activation of the bile acid membrane receptor TGR5 also plays a protective role in atherosclerosis by interfering with the crucial mechanisms underlying the formation of fatty streaks. In this context, TGR5 receptor activation can counteract this process by acting in both ECs and macrophages. In ECs, it inhibits the expression of VCAM-1 and ICAM-2, thereby limiting the recruitment and adhesion of monocytes, while in macrophages, it inhibits the NF-κB inflammatory pathway, thus reducing the production of pro-inflammatory mediators (TNF-α, IL-1β, IL-6) and downregulates the expression of CD36 and SR-A receptors, which are involved in lipid uptake. As a result, macrophages take up fewer lipids, limiting their accumulation in lipid droplets and ultimately preventing foam cell formation and the development of fatty streaks, as represented in Figure 7 [159,169].

4.3. Short-Chain Fatty Acids

The human digestive system cannot break down complex carbohydrates, particularly dietary fibres, which are not digested or absorbed in the small intestine. Thus, in the colon, the gut microbiota is responsible for fermenting these carbohydrates, resulting in the production of SCFAs. SCFAs consist of carboxylic acids with aliphatic chains ranging from one to six carbon atoms, with the three main ones being acetic acid (C2), propionic acid (C3), and butyric acid (C4), while valeric acid (C5) and hexanoic acid (C6) are less abundant. Bacteria belonging to the Bacteroidota phylum are primarily responsible for producing acetic acid and propionic acid, whereas butyric acid mainly arises from the metabolism of bacteria of the Bacillota phylum [170,171]. The main precursor of SCFAs is pyruvate, which is converted into acetyl-CoA. As illustrated in Figure 8, pyruvate is generated through the glycolytic pathway from hexoses and deoxy-hexoses, as well as through the pentose–phosphate pathway from pentoses. These monosaccharides are the products of the microbial hydrolysis of indigestible carbohydrates [172].

Butyric acid is the main energy source for colonocytes and has two endogenous metabolic production pathways. The first pathway involves the phosphorylation of butyryl-CoA, resulting from the reduction in acetoacetyl-CoA, a combination of two acetyl-CoA molecules, to butyryl-phosphate, which is then converted into butyrate by the action of butyrate kinase (BK), a pathway characteristic in certain Coprococcus species. In the second and main pathway, the CoA fraction of butyryl-CoA is transferred to exogenous acetate via butyryl-CoA:acetate CoA transferase (BCoAT), leading to the formation of butyrate and acetyl-CoA. This pathway is utilised by bacteria such as Faecalibacterium prausnitzii, Eubacterium rectale, Eubacterium hallii, Ruminococcus bromii, and Roseburia spp. (Figure 8) [173,174].

Propionic acid, like butyric acid, can be synthesised via three main pathways (Figure 2). One is the propanediol pathway, which converts deoxy-hexoses into propionic acid through the action of propionaldehyde dehydrogenase (PduP)—a route commonly found in bacteria of the Lachnospiraceae family. The second is the succinate pathway, which is considered to be the predominant route for propionate production in the human colon. It is mainly carried out by bacteria from the Bacteroidota phylum and involves the conversion of phosphoenolpyruvate into succinate, which is then metabolised into propionate through intermediate steps involving methylmalonyl-CoA and propionyl-CoA [175]. The third is the acrylate pathway, in which lactate is converted into propionate through a series of steps involving lactoyl-CoA and acryloyl-CoA intermediates, catalysed by enzymes such as acryloyl-CoA reductase (Acr), a mechanism characteristic of Coprococcus catus [176].

Acetate production can also occur in two ways: through the Wood–Ljungdahl pathway, by which acetogenic bacteria synthesise it from hydrogen, carbon dioxide, or formic acid, or, more predominantly, through the decarboxylation of pyruvate into acetyl-CoA, which is then hydrolysed into acetate [177].

SCFAs, by binding to G-protein-coupled receptors such as free fatty acid receptors (FFAR) 2 and 3 (also known as GPR41 and GPR43, respectively), play regulatory functions in the cellular metabolism of fatty acids, glucose, and cholesterol. FFAR2 and FFAR3 are primarily expressed in intestinal epithelial cells, immune cells (such as neutrophils and macrophages), and adipose tissue, where they mediate anti-inflammatory and metabolic effects. Therefore, a dysbiotic gut microbiota, characterised by reduced SCFA production, is associated with impaired signalling through these receptors [135,178].

These carboxylic acids also play an important role in regulating immune cell activity within the intestinal mucosa and in preserving epithelial integrity. This is achieved by reducing the pH of the colon, which inhibits the growth of pathogenic bacteria, and by the expression of tight junction proteins. In this way, butyric acid is considered to be the primary positive regulator of genes encoding these proteins. It also stimulates the expression of mucin 2 (MUC2), which strengthens the mucus barrier against pathogenic agents, and enhances AMP production, both essential components of the host’s first defence line [179].

In the case of atherosclerosis, SCFAs, particularly butyrate and propionate, exhibit protective effects through multiple mechanisms, as demonstrated in both in vitro and in vivo studies. In vitro experiments using human umbilical vein endothelial cells (HUVECs) have shown that these SCFAs inhibit NF-κB activation and decrease the expression of pro-inflammatory cytokines such as TNF-α and IL-6, primarily through the inhibition of histone deacetylases (HDACs). This modulation leads to the expression of IL-10, an anti-inflammatory cytokine, and reduced expression of vascular adhesion molecules like VCAM-1 and ICAM-1, which are critical for monocyte adhesion and early plaque development. In addition to this HDAC-dependent mechanism, SCFAs, particularly butyrate, can also activate intracellular signalling pathways by binding to FFAR2 on leukocytes. This interaction leads to AMPK activation, which stimulates PGC-1α and its downstream effector PPAR-α, further contributing to NF-κB inhibition and the suppression of ICAM-1 and VCAM-1 expression. These converging anti-inflammatory pathways underscore the multifaceted protective role of SCFAs in atherosclerosis [159,178,180,181,182]. In vivo studies using apolipoprotein E-deficient (ApoE^−/−) mice, a well-established model for atherosclerosis, have shown that dietary supplementation with butyrate attenuates atherosclerotic lesion development, decreases macrophage infiltration, and improves endothelial function. These outcomes are associated with the downregulation of oxidative stress pathways, including the suppression of NADPH oxidase activity and inhibition of the NLRP3 inflammasome [159,182,183,184,185].

Regarding cholesterol metabolism, propionic acid is a potent inhibitor of cholesterol synthesis, while the three main SCFAs increase hepatic cholesterol uptake from the blood and accelerate its excretion, contributing to a reduction in plasma cholesterol levels. Additionally, SCFAs can regulate lipolysis and adipogenesis. Specifically, acetic acid and propionic acid inhibit endogenous lipolysis, while propionic acid regulates extracellular lipolysis by promoting lipoprotein lipase expression, resulting in decreased plasma lipid levels. Both of these metabolic processes contribute to reducing the risk factors associated with the development and progression of atherosclerosis [186].

4.4. Tryptophan Metabolites

Tryptophan is an essential amino acid that cannot be synthesised endogenously and must, therefore, be acquired through dietary protein intake [187]. The conversion of tryptophan to indole is an important metabolic process, mediated mainly by the enzyme tryptophanase, which is expressed in many Gram-negative, as well as Gram-positive, bacterial species, including E. coli, Clostridium spp., and Bacteroides spp. [188].

While this section focuses on the microbiota-dependent tryptophan metabolism, it is important to acknowledge that host-mediated pathways, such as the kynurenine pathway, also contribute to the pathogenesis of atherosclerosis. The enzyme indoleamine 2,3-dioxygenase (IDO1), which catalyses the first step in this pathway, is induced by inflammation and has been implicated in both pro- and anti-atherogenic processes. Although the microbiota may influence systemic tryptophan availability and thus indirectly affect the kynurenine metabolism, the mechanistic interactions between these axes remain incompletely understood and lie beyond the scope of the present discussion [189]. In this context, several studies have demonstrated that, through several other metabolic pathways, intestinal microbial species produce not only indole, but also other tryptophan catabolites. For instance, Clostridium sporogenes converts tryptophan into tryptamine, indolelactic acid (ILA), and indolepropionic acid (IPA). Similarly, some species of the genus Peptostreptococcus, including P. russellii, P. anaerobius, and P. stomatis, are able to convert tryptophan into indoleacrylic acid (IA) and IPA. Additionally, Lactobacillus species convert tryptophan into indole-3-aldehyde (IAld) and ILA via the action of aromatic amino acid aminotransferase and indolelactic acid dehydrogenase. Ruminococcus gnavus also metabolises tryptophan, producing tryptamine through the enzyme tryptophan decarboxylase. Several Bacteroides species, along with Clostridium bartlettii, are capable of producing ILA and indole-3-acetic acid (IAA), whereas Bifidobacterium species primarily generate ILA. Moreover, the metabolite 3-methylindole (skatole), known for its strong odour, is produced by Bacteroides and Clostridium species via the decarboxylation of IAA [187,190]. These compounds have emerged as key modulators of vascular inflammation, oxidative stress, and endothelial function [189], primarily through their interaction with host receptors such as the AhR, by modulating the expression of genes involved in xenobiotic metabolism, inflammation, and barrier integrity [191].

AhR is a ligand-dependent transcription factor expressed in a variety of cell types, including endothelial cells, epithelial cells, and several subsets of immune cells, that binds a variety of endogenous and exogenous compounds, including indole metabolites produced by the gut microbiota. In the context of atherosclerosis, AhR activation by tryptophan metabolites can have both positive and negative effects, depending on the specific metabolite, its concentration, and the cells or tissue involved [136]. Therefore, tryptophan metabolites, such as IAld, ILA, and IPA, have been shown to have atheroprotective effects, contributing to the maintenance of vascular homeostasis, since they seem to promote anti-inflammatory responses, preserve endothelial barrier function, and supress the production of ROS [189,192,193].

Conversely, other compounds derived from tryptophan, namely indoxyl sulphate (IS), a hepatic metabolite derived from microbial indole, act as uremic toxins and have been associated with vascular calcification, endothelial dysfunction, and pro-inflammatory signalling through the AhR and other pathways [194]. Since its clearance is impaired in patients with chronic renal disease, IS accumulation is especially significant in these patients, aggravating atherosclerotic processes [195,196].

The dual role of AhR ligands derived from microbiota is supported by data from animal models of atherosclerosis. A study in Apoe-deficient mice show that the administration of beneficial AhR ligands, such as IPA or IAld, ameliorated vascular inflammation and reduced atherosclerotic lesion formation [192,193]. In contrast, exposure to IS aggravated vascular damage, promoted oxidative stress, and impaired cardiac function [194,197]. These findings suggest that AhR acts as a molecular sensor that combines microbial signals to modulate the cardiovascular physiology of the host, with the nature of the ligand and the inflammatory context influencing the outcome [136].

Altogether, these insights highlight the therapeutic potential of modulating the microbiota–tryptophan–AhR axis as a novel avenue for preventing or attenuating atherosclerosis.

4.5. Non-Bacterial Microbiota and Atherosclerosis

Recent research has begun to investigate how non-bacterial components of the gut microbiota, such as fungus, viruses, and bacteriophages, may affect CVD by indirect but biologically significant routes, in addition to the known functions of microbial metabolites. These organisms seem to modify the host’s physiology via modifying barrier integrity, microbial community structure, and mucosal immunity, all of which are increasingly acknowledged as significant factors in the pathophysiology of atherosclerosis, rather than acting through particular circulating metabolites [16,198].

Fungi represent a small but metabolically active component of the gut ecosystem, collectively known as the mycobiome. Although Candida albicans is the most frequently detected species among the fungal elements of the gut mycobiota, other genera, including Malassezia, Saccharomyces, and Mucor, have also been implicated in human disease and may play distinct roles in gut–immune–vascular interactions [199].

A preliminary study exploring the role of the gut mycobiota in subclinical atherosclerosis found that the relative abundance of Mucor racemosus in the gut was negatively associated with both carotid intima-media thickness (cIMT) and the Framingham Risk Score (FRS), a clinical estimate of 10-year cardiovascular risk based on traditional factors. The results of the study point to a potential preventive effect of some fungal species in cardiovascular health, even though the underlying processes are currently unknown [200]. While M. racemosus may exert a protective influence, other fungal taxa appear to have the opposite effect. A recent study suggests that C. albicans is implicated in promoting pro-inflammatory responses and exacerbating atherosclerotic processes. The study showed that, in patients with dyslipidaemia, elevated levels of C. albicans were positively correlated with total cholesterol and LDL cholesterol, and its colonisation in murine models aggravated atherosclerosis. From a mechanistic point of view, a metabolite of C. albicans, formyl-methionine, activates the intestinal HIF-2α–ceramide signalling pathway, leading to increased ceramide synthesis and accelerated plaque development, an effect that can be mitigated by the pharmacological inhibition of HIF-2α [20].

The composition of the gut mycobiota in patients with atherosclerotic cardiovascular disease (ACVD) differs markedly from that of healthy individuals, according to a recent study. Shotgun metagenomic analyses revealed an increase in opportunistic fungal pathogens, including C. albicans, Exophiala spinifera, and Malassezia restricta. Notably, network analysis revealed that the fungal–bacterial ecosystem in these individuals displayed greater dysregulation and lower interconnectivity, with species such as Malassezia globosa, Nakaseomyces glabratus, and Penicillium sumatraense emerging as central nodes [140]. Similarly, a cross-sectional study that examined the bacterial and fungal communities in patients with coronary artery disease found that the severity of the condition was correlated with increasing changes in gut fungal composition [141].

In addition to fungi, viruses and bacteriophages, collectively known as the gut virome, represent another important non-bacterial component of the intestinal microbiota with potential implications in cardiovascular health. The virome is mainly composed of bacteriophages, and has increasingly been recognised as a potential regulator of the bacterial community in the gut and can indirectly impact the cardiometabolic pathways that influence the development and progression of atherosclerosis. Phages can alter bacterial communities through lytic or lysogenic cycles, and this can result in the transmission of auxiliary metabolic genes by transduction, a horizontal gene transfer mechanism. These genes may modulate bacterial metabolism in mechanisms that ultimately influence the host’s immunity, intestinal permeability, and the production of inflammatory mediators. In fact, individuals with cardiometabolic diseases, including obesity, type 2 diabetes, and atherosclerosis, have been found to have altered gut phageome composition. For instance, enrichment in phages infecting Enterobacteriaceae and Streptococcus species has been reported among these individuals [17,142].

In parallel, although through different pathways, eukaryotic viruses have also been implicated in the pathogenesis of atherosclerosis. Various human viruses, including influenza virus, herpes simplex virus (HSV), hepatitis viruses (HAV, HBV, HCV), human cytomegalovirus (HCMV), human papillomavirus (HPV), human immunodeficiency virus (HIV), and SARS-CoV-2, have been associated with this disease through direct and indirect mechanisms, such as chronic systemic inflammation, immune activation, endothelial dysfunction, and oxidative stress. For example, influenza virus can increase LDL uptake by desialylation and induce endothelial permeability and thrombosis. In arterial walls, HCMV has been detected, and this virus has been implicated in the formation of foam cells, the proliferation of smooth muscle cells, and endothelial injury. Finally, cytokine storms and persistent inflammation can be triggered by both HIV and SARS-CoV-2, further contributing to vascular damage and plaque formation [16].

Collectively, these findings suggest that cardiovascular health, and, in particular, atherosclerosis, is actively influenced by the non-bacterial gut microbiota and may act synergistically with bacteria derived effects. To fully understand their involvement in atherogenesis and their potential as therapeutic targets, further studies are required.

4.6. Microbiota-Induced Immune and Inflammatory Pathways in Atherosclerosis

The continuous communication between the gut microbiota and the immune system is essential for maintaining systemic homeostasis. However, under conditions of intestinal dysbiosis, this balance is disrupted, triggering chronic low-grade inflammation, characterised by elevated levels of pro-inflammatory cytokines such as TNF-α and IL-6, implicated in the pathophysiology of atherosclerosis, since they promote an inflammatory state that damages the endothelium and facilitates the formation of atherosclerotic plaques [201,202].

In parallel, increased intestinal permeability, also known as “leaky gut”, another common alteration associated with dysbiosis, allows for the translocation of bacterial products, such as LPS, into the systemic circulation [203]. These components, in turn, activate Toll-like receptors, notably TLR4, present on endothelial and immune cells, thereby promoting a systemic inflammatory response. This process contributes to atherosclerosis progression by stimulating monocyte adhesion to the endothelium and promoting foam cell formation within atherosclerotic plaques [204,205]. Bacterial translocation is also associated with the activation of the NF-κB signalling pathway, which is crucial in regulating vascular inflammation [206].

In addition, the activation of TLR4 and TLR2 receptors, which recognise pathogen-associated molecular patterns (PAMPs) such as LPS, peptidoglycans, and bacterial lipoproteins, leads to the activation of pro-inflammatory intracellular signalling pathways, including the NF-κB pathway. This promotes the expression of inflammatory cytokines and endothelial adhesion molecules, facilitating the infiltration of monocytes and T lymphocytes into atherosclerotic plaques, which aggravates inflammation and accelerates the progression of the disease. These alterations also contribute to endothelial dysfunction, a critical step in the pathophysiology of atherosclerosis, as previously discussed [201,207,208,209].

On the other hand, dysbiosis favours the proliferation of pathobionts, such as members of the Enterobacteriaceae family, which have the ability to proliferate in inflammatory intestinal environments [88]. These microorganisms utilise nitrates as final electron acceptors in anaerobic respiration, giving them a competitive advantage under inflammatory conditions [88,210]. This process is associated with increased production of TMA, as discussed in Section 4.1, and contributes to alterations in the lipid composition of endothelial cells, promoting the formation of atherosclerotic plaque [211].

Beyond the innate immune response mediated by TLRs, the gut microbiota can also influence the adaptive immune response, namely the expansion of Th17 lymphocytes, which play a central role in the chronic inflammation associated with atherosclerosis [212]. Studies have shown that intestinal dysbiosis can favour the differentiation of Th17 cells, which secrete cytokines such as IL-17 that promote vascular inflammation and plaque instability. This mechanism is particularly relevant, given that Th17 lymphocytes are associated with autoimmune and chronic inflammatory processes [212,213]. Nonetheless, a study has suggested that IL-17 might also exert stabilising effects on atherosclerotic plaques by downregulating VCAM-1 expression on endothelial cells [214], highlighting the complex and context-dependent role of IL-17 in atherosclerosis. The increase in Th17 cells in atherosclerosis enhances the activation of the NF-κB pathway and the production of matrix MMPs, which degrade the extracellular matrix and weaken plaque structure, increasing the risk of rupture and thrombosis [215,216].

5. Gut Microbiota-Targeted Interventions in the Prevention and Progression of Atherosclerosis

In recent years, the gut microbiota has attracted considerable attention, aimed at understanding how alterations in its composition, structure, and metabolism can influence host metabolic and physiological processes, particularly in relation to the cardiovascular system and, more specifically, atherosclerosis [217]. In this context, the critical and multifaceted role of the gut microbiota in atherosclerosis has been evidenced in small clinical studies and some preliminary reports, highlighting the potential of novel therapeutic approaches that modulate the gut microbiota and may exert a relevant cardioprotective effect on the development and progression of the disease [218]. Accordingly, emerging treatment strategies include the use of prebiotics and probiotics, as well as FMT. These interventions have been associated with improvements in lipid profiles, a reduction in inflammatory state, and the potential benefits provided by the previously discussed active metabolites [217].

Beyond targeted microbial interventions, commonly used pharmacological agents may also influence gut microbial composition. Statins, widely prescribed for their cholesterol-lowering effects, have recently been shown to modulate the gut microbiota. A recent review reported that statin therapy can alter the relative abundance of key microbial taxa and affect the production of microbial metabolites, such as short-chain fatty acids and bile acids, potentially contributing to their pleiotropic cardiovascular benefits beyond their lipid-lowering effects. These findings highlight the relevance of considering drug–microbiota interactions when evaluating the overall metabolic and vascular effects of lipid-lowering therapies [219].

5.1. Prebiotics

Currently, a prebiotic is defined as a substrate that is selectively utilised by host microorganisms conferring a health benefit [220,221].

This definition encompasses not only nondigestible carbohydrate-based compounds, such as fructooligosaccharides (FOS) and galactooligosaccharides (GOS), but also other compounds like certain polyphenols and polyunsaturated fatty acids, provided that there is robust evidence of their selective microbial utilisation and associated health benefits. Furthermore, prebiotic effects are no longer confined to the gut, but may also occur at other body locals colonised by microorganisms [220].

The key concept of prebiotics is selectivity, as they specifically promote the stimulation, growth, and proliferation of beneficial microorganisms, including, but not limited to, Lactobacillus spp. and Bifidobacterium spp., which inhibit the proliferation of pathogenic bacteria by reducing intestinal pH [222,223]. Additionally, prebiotics stimulate changes in bacterial metabolism, such as increased production of SCFAs, namely butyric and propionic acids, which, among the other benefits already discussed, improve epithelial integrity and exert anti-inflammatory effects [224]. Moreover, prebiotics may improve lipid metabolism through microbiota-mediated mechanisms, including bile acid metabolism and SCFA production. Taken together, and as previously discussed, prebiotics exert positive effects on the prevention and development of atherosclerosis [203].

Inulin or GOS supplementation has been associated with modest reductions in IL-6, TNF-α, high-sensitivity C-reactive protein (CRP), and circulating lipopolysaccharides in patients with coronary artery disease or elevated metabolic risk, according to recent clinical research. In addition, improvements in lipid profiles and endothelial function have also been reported. However, evidence for direct effects on atherosclerotic burden remains scarce, and no trials to date have demonstrated the clear regression of vascular lesions [225,226]. While preclinical findings in animal models have provided mechanistic support, the translation of these effects to clinical outcomes in humans remains to be firmly established [227].

5.2. Probiotics

The term “probiotic”, derived from the Greek meaning “promotion of life”, refers to “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host”. Ideally, these microorganisms should be of human origin, safe, and free from genes conferring antibiotic resistance, virulence, pathogenicity, or toxicity, and should not exhibit haemolytic activity [228,229].

Functionally, probiotics can be divided into three main groups: lactic acid bacteria, with bacteria belonging to the genera Lactobacillus and Bifidobacterium; non-lactic acid bacteria, such as Bacillus spp., Escherichia spp., and Propionibacterium spp.; and yeasts and fungi, mainly Saccharomyces spp. and Aspergillus spp. Nevertheless, Lactobacillus spp. and Bifidobacterium spp. have been most extensively used as probiotics in recent decades [230].

To be suitable for human use, probiotics must tolerate gastric acid and bile salts to reach the intestine in a viable and stable form. Once this chemical barrier is overcome, they must adhere to the intestinal ECs, where they exert their functions. In addition, they should exhibit antimicrobial activity to inhibit the growth of pathogenic bacteria. Probiotics should also benefit the host by strengthening the immune system, preventing infections, promoting intestinal homeostasis, and reducing plasma cholesterol levels [230,231].

Considering the benefits associated with lowering plasma cholesterol in the context of atherosclerosis, the hypocholesterolaemic and anti-atherosclerotic effects of probiotics may be explained through different mechanisms. These include the activity of bile salt hydrolase (BSH), the production of SCFAs, the co-precipitation of cholesterol with deconjugated bile salts, and the assimilation of cholesterol during bacterial growth, with the subsequent incorporation into the cell membrane [232].

Several studies have shown that many Bifidobacterium and Lactobacillus species produce BSH, an enzyme whose activation may reduce cholesterol levels by increasing the excretion of free bile acids and, consequently, decreasing their intestinal reabsorption. In response, the liver increases cholesterol uptake to synthesise new bile acids, ultimately contributing to reduced circulating cholesterol levels. In addition, certain strains of these bacteria have the capacity to bind cholesterol and either incorporate it into their cell membranes or sequester it on their surface, further contributing to plasma cholesterol reduction. Another important metabolic pathway involves SCFAs, which lower cholesterol via two different mechanisms: the inhibition of the enzyme HMG-CoA reductase, the rate-limiting enzyme for hepatic cholesterol synthesis; and the stimulation of 7α-hydroxylase activity by propionic acid, thereby enhancing hepatic bile acid synthesis via cholesterol oxidation [217,233,234,235].

Probiotics also contribute to intestinal homeostasis by improving epithelial barrier integrity through the promotion and maintenance of tight junctions. This provides an additional mechanism of protection against atherosclerosis by preventing the absorption of microbial metabolites and components such as TMAO and LPS, which are key contributors to its pathogenesis [236]. In this context, Lactobacillus plantarum ZDY04 has been shown to reduce plasma TMAO levels through the modulation of the gut microbiota [237].

In addition, E. faecium GEFA01 shows potential as a probiotic for atherosclerosis by lowering cholesterol and modulating gut microbiota in hyperlipidaemic mice [154].

Finally, by inhibiting NF-κB activation, as observed in a study with Lactobacillus acidophilus ATCC 4356, probiotics have a proactive role in preventing atherosclerosis by promoting an anti-inflammatory action. This is demonstrated by their capacity to decrease levels of the pro-inflammatory cytokine TNF-α and increase expression of the anti-inflammatory cytokine IL-10 [238].

Recent clinical trials investigating the use of isolated probiotic strains in individuals with metabolic disorders or hypercholesterolaemia have demonstrated promising, albeit preliminary, cardiometabolic benefits. Daily supplementation with specific Lactobacillus and Bifidobacterium strains [239], such as L. plantarum ECGC 13110402 [240], L. reuteri V3401 [241] or NCIMB 30242 [242], and B. animalis ssp. lactis Lafti^®B94 [239,243], over 8–12 weeks has been associated with reductions in LDL cholesterol, systemic inflammatory markers (e.g., hs-CRP, TNF-α, IL-6), and circulating endotoxins such as LPS. Notably, the PROMISE study was among the first to employ PET-CT imaging to assess vascular inflammation in response to a probiotic intervention, offering a mechanistic link between microbial modulation and vascular health. While these interventions did not report the structural regression of atherosclerotic lesions, the observed anti-inflammatory and lipid-lowering effects suggest that specific probiotics may contribute to a favourable shift in the systemic milieu that precedes atherogenesis. Nonetheless, larger and longer-term trials are required to validate these findings and clarify their potential in cardiovascular risk reduction [243].

5.3. Faecal Microbiota Transplantation

Although it is a concept that dates back to the late 1960s, FMT is currently an attractive method that is increasingly being used to alter the composition and restore the functions of the gut microbiota in pathological states associated with dysbiosis [244]. This procedure involves the collection of filtered faecal samples from carefully selected healthy donors, or from the recipient before the onset of the pathology, in a process known as autologous FMT, followed by their transplantation into the recipient’s GIT [57]. FMT has proven to be a highly effective method in treating C. difficile infection [245]. Furthermore, FMT is considered an emerging therapy in cases of extra-intestinal diseases, including CVDs, where a shift in the recipient’s bile acid composition has been observed, bringing it closer to that of the donor [246,247]. Additionally, FMT performed from lean individuals to obese recipients has shown improvement in the sensitivity of recipients to triglycerides, resulting in a decrease in their plasma concentration [248]. These observations suggest an anti-atherosclerotic effect, considering the previously mentioned benefits of bile acids, as well as a reduction in dyslipidaemia, one of the main risk factors for the disease. However, FMT remains a limited therapeutic approach due to various associated risks, including the transfer of endotoxins and infectious agents, and its long-term effects are still not fully understood [9,217].

Current evidence for FMT in atherosclerosis is limited, but evolving. Preclinical models consistently show that FMT from healthy donors can reduce plaque development, modulate systemic inflammation, and lower pro-atherogenic metabolites such as TMAO [249,250]. In humans, only one ongoing clinical trial (NCT04410003) is directly investigating FMT in severe unexplained atherosclerosis [251]. Other small-scale studies on metabolic syndrome, including one involving 32 women [252] and another with 20 men receiving FMT from vegan donors, have demonstrated shifts in microbiota composition, but no significant clinical improvement in TMAO levels, lipid profiles, or endothelial function [253]. Larger trials targeting atherosclerotic outcomes are needed to clarify FMT’s therapeutic potential.

6. Conclusions

The gut microbiota plays an essential role in maintaining human health, influencing processes ranging from digestion to immune regulation and metabolic functions. It is increasingly recognised as an important factor in cardiovascular health. However, several factors can alter its composition and functions, leading to a state of dysbiosis, which is associated with several pathological conditions, including atherosclerosis.

Atherosclerosis, a complex and multifactorial condition, is closely associated with CVDs and represents one of the leading causes of morbidity and mortality worldwide. Therefore, a detailed understanding of the risk factors and underlying mechanisms of atherosclerosis is essential for the development of effective prevention and treatment strategies. Thus, the study of the interaction between the gut microbiota and atherosclerosis has emerged as a promising research field, exploring metabolic products, the regulation of inflammatory processes, and how restoring the balance of the gut microbiota can positively impact this pathological state.

Therapeutic interventions that target not only the modulation of the gut microbiota, but that also influence associated metabolic and inflammatory processes, such as the consumption of prebiotics, probiotics, and FMT, have been explored and show promise for the prevention and treatment of atherosclerosis.

Although benefits have been observed with the use of prebiotics and FMT, including improvements in cholesterol and triglyceride profiles, increased production of SCFAs, and improved bile acid profiles, all of which play a protective role in atherosclerosis, probiotics stand out for their proven clinical efficacy. These act through hypocholesterolaemic mechanisms, maintaining intestinal barrier integrity, reducing plasma TMAO levels, and exhibiting anti-inflammatory effects, thereby reinforcing their therapeutic potential in the prevention and management of atherosclerosis.

Nonetheless, although these three therapeutic strategies have shown promising results, further clinical studies are required to determine their efficacy, establish optimal dosages, and elucidate the precise mechanisms underlying their cardiovascular benefits. Additionally, long-term clinical trials are crucial to ensure the safety and efficacy of these interventions in the context of atherosclerosis.