Fatty liver has become the most prevalent chronic liver disease globally and it is strongly associated with an increased risk of developing serious extra-hepatic diseases such as CVD. Fatty liver is typically accompanied with insulin resistance, hypertension, atherogenic dyslipidaemia, and dysglycaemia, which are all established risk factors for CVD. A meta-analysis concludes that nonalcoholic fatty liver disease (NAFLD) confers an odds ratio (OR) of 1.64 for CVD events [1]. Large number of retrospective and population-based cohort studies have proven that patients with fatty liver have a higher rate of CVD mortality than the general population [2,3]. It is reported that persistent fatty liver is associated with an increased risk of subclinical carotid atherosclerosis development [4]. Other studies also report that fatty liver is associated with early changes in left ventricular morphology and diastolic function [5], impaired myocardial energy metabolism [6], and coronary dysfunction.

MAFLD is the term suggested in 2020 to refer to fatty liver disease related to systemic metabolic dysregulation. Since the introduction of the term, it has been demonstrated the superiority of MAFLD over the traditional NAFLD terminology in many key areas, including for the risk of liver and extrahepatic mortality, disease associations, and for identifying high-risk individuals. It has been adopted by a number of leading national societies due to its concise diagnostic criterion, removal of the requirement to exclude concomitant liver diseases, which reduces the shortcoming resulted from this condition [7].

The most important difference between MAFLD and NAFLD is, as mentioned before, MAFLD diagnosis does not require exclusion of patients with alcohol intake, or other chronic liver diseases. MAFLD is diagnosed based on a radiologically diagnosed hepatic steatosis and the presence of any one of the following three conditions [1]: namely overweight/obesity [2], presence of diabetes mellitus [3], or evidence of metabolic dysregulation. The metabolic dysregulation was defined as the presence of two or more of the following conditions: (a) Waist circumference ≥102 in men and 88 cm in women. (b) Blood pressure ≥130/85 mmHg or specific drug treatment. (c) TG ≥ 1.70 mmol/L or specific drug treatment. (d) HDL-C < 1.0 mmol/L for male and <1.3 mmol/L for female. (e) Prediabetes (ie fasting glucose levels 5.6–6.9 mmol/L, or 2-h post-load glucose levels 7.8–11.0 mmol/L or HbA1c 5.7%–6.4%). (f) Homeostasis model assessment-insulin resistance (HOMA-IR) score ≥2.5. (g) C-reactive protein (CRP) level >2 mg/L [8].

Plaque and carotid intima-media thickness (IMT) increase are both strongly associated with the presence of CVD [[9], [10], [11], [12]]. However, meta-analyses suggest that common carotid artery (CCA)-IMT alone only minimally improves predictive power. IMT measurement at the CCA, carotid bulb, and internal carotid artery (ICA) along with plaque assessment in all carotid segments are more precise in CVD risk prediction [11]. There are other cardiovascular risk prediction tools such as coronary artery calcium (CAC) scores and polygenic risk scores (PRS). But CAC has significant cost and radiation exposure, and there is a problem of reproducibility and Eurocentric bias of population cohorts in PRS. Although carotid ultrasound examination could discover MAFLD patients with CVD risk, the pathological alteration in the cardiovascular have already existed in body before the carotid lesion is detected. Hence, there is an unmet need for high-fidelity earlier detection and risk prediction approaches for MAFLD patients with CVD risk. Meanwhile, the signature of these prediction substances could be the potential therapeutic targets to prevent the CVD development in MAFLD patients.

Recent studies have proved that the gut microbiomes are a force to be reckoned with in the development of NAFLD [13,14] and CVD [15,16]. It decides what substances the host directly absorb from the intestinal into blood system, which transform into physiological modulators (e.g., short-chain fatty acids (SCFAs)), and pathogenic mediators (e.g., trimethylamine N-oxide (TMAO)). This has created novel potential diagnostic and therapeutic opportunities for improving the early prediction and treatment of MAFLD patients with CVD risk. In this study, we randomly selected MAFLD patients with carotid artery pathological changes as MAFLD patients with CVD risk, and compared their gut microbiomes and plasma metabolites with simple MAFLD patients to figure out the potential pathogenic factors in the early stage of CVD. The normal subjects are taken as negative controls and MAFLD patients with diagnosed CAD are positive controls. This will contribute to the prevention of the development of CVD in MAFLD patients in an early time.