Atherosclerosis (AS) serves as a major contributing factor to coronary artery disease (CAD), stroke, and peripheral artery disease [1]. Despite notable advancements in preventive measures, pharmacological treatment, and interventional procedures, the incidence and mortality rates of coronary artery disease continue to rise annually [2]. Detecting AS at an early stage and initiating timely intervention are recognized as effective strategies for both its prevention and treatment.

Recent research underscores the pivotal role of the gut microbiota in modulating a wide range of metabolic processes within the human body [3]. Specifically, the byproducts of gut microbiota activity, including metabolites such as lipopolysaccharide (LPS), short chain fatty acids (SCFA), trimethylamine-N-oxide (TMAO), total bile acid (TBA), phenylacetyl glutamine (PAGIn), tryptophan (TRP), interleukin-6 (IL-6), and high-sensitivity C-reactive protein (hs-CRP), have been implicated in the exacerbation of systemic inflammation. These compounds have been shown to influence key pathological processes such as foam cell accumulation, endothelial dysfunction, and lipid accumulation [4, 5]. These factors can also contribute to the development of AS, although the underlying mechanisms are multifactorial. Ongoing investigations into the relationship between gut microbiota derivatives and AS are essential for advancing prevention and therapeutic strategies of AS [6].

In clinical practice, coronary arteriography (CAG) and coronary computed tomography angiography (CTA) are the standard imaging modalities employed to diagnose CAD and assess its severity. For evaluating the extent of coronary lesions and overall disease burden, scoring systems such as Synergy between Percutaneous Coronary Intervention with Taxus and Cardiac Surgery (SYNTAX) and Gensiniare commonly utilized. Another important diagnostic concept is coronary flow reserve (CFR), a parameter that quantifies the ability of coronary vessels to dilate in response to stress. Introduced by Gould et al. in 1974 [7], CFR is calculated as the ratio of coronary blood flow (CBF) or myocardial blood flow (MBF) during induced stress to the baseline CBF or MBF [8].

A growing body of evidence has established that a reduced CFR is strongly associated with adverse clinical outcomes in CAD patients, including increased all-cause mortality and a higher incidence of major adverse cardiac events (MACE) [9]. Nevertheless, these prognosis methods are invasive and associated with significant financial costs. Hence, the identification of novel, minimally invasive indicators for accurately predicting the prognosis of AS and CAD holds substantial clinical importance.

As the global burden of CAD continues to rise annually, it has become a focal point of medical research and public health attention. Although significant advancements have been made in prevention strategies, pharmacological treatments, and interventional procedures, the incidence of CAD remains on an upward trajectory.

Recent evidence from numerous studies highlights the association between gut microbiota-derived metabolites and both the pathogenesis of AS and the prognosis of CAD. This investigation explored the relationship between the serum levels of IL-6, hs-CRP and TMAO, were positively correlation with the Gensini score, suggesting its potential role as a predictor of CAD severity. Furthermore, IL-6 was identified as an independent risk factor for the severity of CAD. The study also established that serum levels of IL-6 $>$45.17 pg/mL or TMAO $>$12.44 µmol/L had predictive significance for CAD diagnosis. Additionally, the serum concentration of TMAO showed a negative correlation with CFR, indicating its potential as a clinical biomarker for predicting CAD prognosis. Chronic inflammation, a critical factor in the onset and progression of AS, plays a vital role in triggering thrombosis and plaque rupture, thereby contributing to the poor prognosis of CAD [11]. This finding of this study indicate that IL-6 (r = 0.410, p 0.001) and hs-CRP (r = 0.317, p = 0.007) exhibit a positive correlation with the severity of CAD, with IL-6 identified as a contributing risk factor for CAD severity (b = 1.769, 95% CI: 0.256–3.282, p = 0.023). It was observed that serum IL-6 levels exceeding 45.17 pg/mL possess predictive utility in diagnosis CAD. Research suggests that IL-6 mediates its biological functions through the IL-6R and gp130 signal transduction receptor complex on cardiomyocytes. This pathway is linked to poor CAD prognosis, facilitated by mechanisms such as platelet aggregation and the proliferation of coronary vascular smooth muscle cells, along others [12, 13]. IL-6 can also augment inflammation by increasing the production of acute inflammatory proteins including hs-CRP in the liver. Yang et al. [14] showed that hsCRP increase the risk of CAD. Similarly, Cheng et al. [15] demonstrated a positive relationship between the Gensini score, which quantifies the extent of coronary artery stenosis, and hs-CRP. These conclusions align with the outcomes of this investigation, which further revealed that hs-CRP acts as an independent risk factor for poor CAD prognosis.

LPS, a fundamental constituent of the Gram-negative bacteria cell wall, plays a crucial role in bacterial structure and function. Once the intestinal flora is altered, LPS is released by the bacterial membrane, which destroys the intestinal vascular barrier and leads to the absorption of LPS into the blood circulation, resulting in an increased systemic inflammatory response leading to sepsis. Serum analyses revealed that levels of LPS were considerably elevated in the CAD group compared to the non-CAD group. LPS can directly or indirectly induce the adhesion reaction between platelets and the vessel wall [16]. LPS can promote the production of foam cells through Toll-like receptor 4, CD36 and CD204, leading to plaque instability and eventually lead to AS.

PAG, a metabolite resulting from intestinal flora transformation, has been linked to CAD. Studies revealed that serum PAGIn concentrations in CAD patients were notably higher than those in non-CAD patients. Fang et al. [17] identified an elevation in blood PAGln levels in CAD patients with in-stent restenosis. Furthermore, Stanley Hazen’s team established a connection between PAGIn and cardiovascular disease, as well as adverse cardiovascular events, in a cohort of 4000 participants. Research on whole blood and isolated platelets from animal models with arterial injury revealed that gut microbiota-derived PAGln metabolites could promote platelet activation-related phenotypes and enhance thrombosis [18].

TMAO in human body is mainly derived from choline, L-carnitine, betaine, phosphatidylcholine, and lecithin, in red meat, egg yolk, fish, animal liver, soybean and other foods. TMAO is produced through the action of intestinal microorganisms, and TMAO is finally formed in the liver [19, 20]. The analysis revealed a significant positive correlatation between TMAO and the severity of CAD (r = 0.311, p = 0.008), identifying TMAO as an independent determinant of CAD severity (b = 10.735, 95% CI: 4.883–16.588, p 0.001). Notably, when serum TMAO levels exceeded 12.44 µmol/L, its diagnostic value for CAD became apparent. Additionally, a negative relationship was observed between TMAO and the prognosis of CAD (r = –0.593, p = 0.009). Existing research highlights that gut microbiota influences serum TMAO levels, with evidence linking

TMAO to atherosclerotic plaque burden in healthy individuals. This association is independent of both renal function and dietary intake [21]. Multiple studies have demonstrated that TMAO serves as an independent predictor of mortality risk in CAD patients [22], and it acts as a concentration-dependent risk factor for CAD [23]. In a longitudinal study tracking 2235 individuals with stable CAD over a 5 years period, it was found that elevated TMAO levels significantly heightened the risk of cardiovascular death [24]. Research conducted by Waleed et al. [25] involving 4039 participants, further established that TMAO was independently linked to coronary atherosclerotic burden. Consistent with these findings, the present study also confirmed that TMAO is associated with both the severity of coronary artery lesions and the prognosis of coronary artery disease.

TBA is a key intermediate in the human body’s lipid metabolism pathway [26]. Bile acids affect lipid metabolism, glucose metabolism, energy metabolism, blood pressure regulation, and the inflammatory response by regulating the 15. farnesoid X receptor (FXR) and the Takeda G-protein-coupled receptor 5 (TGR5). The findings of this study revealed the serum in CAD higher than those in non-CAD patients. Clinical research indicates that the serum TBA concentrations in patients with essential hypertension are higher than those observed in healthy individuals, with a positive correlation observed between TBA concentration and the classification of hypertension. Studies have also shown that FXR agonist and taurocholate can reduce postprandial blood lipids in mice, and bile acids can promote coronary atherosclerosis by inhibiting FXR receptors [27].

GLP-1 is an incretin hormone rapidly secreted by intestinal endocrine L cells after meals. Studies have shown that the imbalance of intestinal flora can lead to the reduction of GLP-1 secretion. The results suggested that serum GLP-1 in CAD patient were lower than those in non-CAD, which is possible to be a protective factor. Helmstädter et al. [28] showed that GLP-1 and its analogues can reduce blood pressure and play a role in protecting endothelial function. GLP-1 can also play an anti-inflammatory role, regulate oxidative stress, improve energy metabolism, and ultimately play a protective role in cardiovascular disease.

In conclusion, derivatives originating from gut microbiota, such as IL-6, LPS, hs-CRP, PAGIn, TMAO and GLP-1, have a notable connection to the severity of CAD. Among these, IL-6, TMAO, and hs-CRP serve as independent risk indicators for the progression of CAD. When the serum cincentrations of IL-6 exceed 45.17 pg/mL or TMAO surpass 12.44 µmol/L, they hold predictive significance for CAD diagnosis. Additionally, TMAO is linked to the prognosis of CAD. The finding highlights that serum levels of IL-6, LPS, hs-CRP, PAGIn, TMAO and GLP-1 are markedly elevated in patients with a high risk of AS, providing a potential basis for patient risk stratification. Strategies involving probiotics and fecal transplantation are emerging as promising avenues for the prevention and mitigation of CAD. For instance, research indicates that probiotics and fecal transplantation may offer cardiovascular protection by lowering blood pressure [29, 30].

This study has several limitations. It is a single-center study and the sample size is small, which may affect the statistical power. The study sample was drawn from patients with suspected CAD who underwent coronary arteriography in the department of cardiology, rather than from a healthy population, which may have led to selection bias, although we used PSM to reduce this. In this study, only the serum levels of derivatives of gut microbiota was detected, and the lack of genetic detection was insufficient.

Serum biomarkers, including IL-6, LPS, hs-CRP, PAGIn, TMAO and GLP-1 can serve as clinical indicators for predicting the severity of CAD. Among them, IL-6, TMAO and hs-CRP are identified as independent risk factors for CAD severity. Elevated levels of IL-6 and TMAO possess predictive value for CAD diagnosis. Additionally, serum TMAO levels can act as a clinical marker to forecast the prognosis of CAD.

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

CRJ and SX designed the research study. HZ, YYJ, SX, YFB and YJY performed the research. YFB analyzed the data and wrote the manuscript. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was carried out in accordance with the guidelines of the Declaration of Helsinki and was obtained permission from the Ethics Committee of First Hospital of Shanxi Medical University (Ethics approval number: [2019] K-SK037). Consent for data use has been consented by the patients.

This project was funded by Shanxi Provincial Key Research and Development Program Project Fund “The correlation between intestinal flora and the prognosis of patients with coronary heart disease (No: 201903D321181)”.

The authors declare no conflict of interest.