Metabolic associated fatty liver disease better identifying patients at risk of liver and cardiovascular complications

A nomenclature of “metabolic associated fatty liver disease” (MAFLD) with a new definition was proposed in 2020 instead of the previous “non-alcoholic fatty liver disease” (NAFLD). Whether it better coheres with the clinical demand remains controversial. The participants with fatty liver on ultrasonography in Taiwan bio-bank cohorts were included. MAFLD is defined as the presence of fatty liver, plus any of the following three conditions: overweight/obesity, type 2 diabetes mellitus (DM), or metabolic dysfunction. The severity of liver fibrosis was determined using fibrosis-4 (FIB-4) index and NAFLD fibrosis score (NFS). The risk of atherosclerotic cardiovascular disease was assessed using intima–media thickness (IMT) or plaques of carotid duplex ultrasound. A total of 9,719 subjects (ages 55.9 ± 10.8; males 42.6%) were distributed among 4 groups: “overlapping group”, “MAFLD only”, “NAFLD only”, and “neither fatty liver disease (FLD)” with the percentages of 79.7, 12, 7.1, and 1.2%, respectively. Compared with NAFLD patients, MAFLD patients had a greater percentage of males, higher levels of BMI, waist circumference, HbA1c, and triglyceride. In addition, they had higher levels of serum ALT, AST, GGT, fatty liver index (FLI), NFS, and IMT, but no difference in FIB-4 index and the percentage of carotid plaques. To note, “MAFLD only group” had greater levels of AST, ALT, GGT, FLI, FIB-4, NFS, IMT and a higher percentage of carotid plaques than the “NAFLD only group”. The grand, population-based study showed MAFLD with new diagnostic criteria to aid in identifying a greater number of high-risk patients of metabolic, liver, and cardiovascular complications, suggesting MAFLD may be a better nomenclature than NAFLD in clinical practice.


Introduction
Non-alcoholic fatty liver disease (NAFLD) is the prevailing source of chronic liver disease in Western countries and its prevalence continues to rise in parallel with the epidemic of obesity and diabetes [1]. It encompasses a spectrum of histologic features ranging from simple steatosis to non-alcoholic steatohepatitis (NASH) with varying stages of liver fibrosis, and can be complicated with cirrhosis, end-stage liver disease or even hepatocellular carcinoma [2,3]. NAFLD is clinically associated with metabolic syndrome and metabolic components such as obesity, diabetes, and hyperlipidemia. Therefore, it is also regarded as liver manifestation of metabolic syndrome [4,5]. Furthermore, NAFLD patients had a greater increase in mortality than the general population.
The top three causes of death were cardiovascular disease, non-liver cancers, and liver-related disease [6][7][8]. Thus, it is a prominent health issue worldwide.
NAFLD is a heterogeneous disease with a "multiplehit" pathogenesis [9]. It is an exclusive diagnosis without mentioning the underlying causes in nomenclature. Due to the term "non alcohol", it is highly misunderstood which results in it being difficult to explain and educate patients in clinical practice. The nomenclature of metabolic (dysfunction) associated fatty liver disease (MAFLD) was proposed to replace the previous term NAFLD by international consensus in 2020 [10]. An international consortium of 32 experts from 22 countries worldwide were convened to redefine the diagnostic criteria of MAFLD and its spectrum of heterogeneity [11]. The criteria are based on the evidence of hepatic steatosis by either imaging or liver histology, including any of the following three criteria: overweight/ obesity, type 2 diabetes mellitus (DM), or metabolic dysfunction. The new disease name is an "inclusive" diagnosis mentioning the underlying cause of metabolic dysfunction. It has the benefit of simpler understanding and education in clinical practice. A patient-center, multidisciplinary approach has been proposed to improve the quality of life in patients with MAFLD plus metabolic diseases [12,13]. Furthermore, the management extends beyond just focusing on hepatic outcomes [14]. However, the disease name was not fully accepted by major societies. A study of 1,710 participants from a general United States (US) population used vibration-controlled transient elastography to assess the status of hepatic steatosis and liver fibrosis. They found that the prevalence and risk of advanced liver fibrosis was similar between NAFLD and MAFLD [15]. Due to the change of diagnostic criteria, some patients may be added (combined with other causes of chronic liver diseases) or missed (no metabolic risk) after the change of nomenclature from NAFLD to MAFLD. Whether MAFLD is superior than NAFLD remains inconclusive, a study using a cohort of Taiwan bio-bank, a representative sample of Taiwan general population was conducted to investigate the changes of metabolic profiles, severity of liver fibrosis and cardiovascular (CV) risk after the switch of disease name and diagnostic criteria from NAFLD to MAFLD.

Patients and study design
The data were collected from Taiwan bio-bank. It is a general population-based research database comprised of > 20-year-old residents. The participants were enrolled through 43 recruitment stations in Taiwan since 2008. The methodologies of data collection from all participants underwent standardized procedure and were described in previous studies [16,17]. Briefly, after obtaining informed consent, a formal questionnaire including past history, smoking, drinking, diet, work, and exercise was performed by an experienced nurse. The type (% of alcohol), and the amount and frequency of alcohol were recorded. Furthermore, the demographic, clinical, and laboratory data were collected. The samples of DNA, blood, and urine were optionally obtained. All the participants were invited to receive a follow-up at the interval of 2-4 years. At the first follow-up, the participants will receive additional examinations including abdominal ultrasound, bone density measurement, and carotid duplex ultrasound. By June 30, 2022, the number increased to approximately 172,000 participants.
In the present study, the participants without the data of liver ultrasound were excluded. NAFLD is defined as fatty liver in the ultrasound examination without the hepatitis B virus (HBV), hepatitis C virus infection, alcohol or other known causes of chronic liver disease. Participants with persistent alcohol intake > 210 g/week for male and > 140 g/ week for female with a period of at least 3 months were excluded from the diagnosis of NAFLD. The diagnosis of MAFLD was based on the evidence of hepatic steatosis on liver ultrasound including any of the following three criteria: overweight/obesity (body mass index (BMI) > 23 kg/m 2 ), type 2 DM, or metabolic dysfunction. DM is defined as having past history of DM or serum HbA1c > 6.5%.

Statistical analyses
The data were expressed as mean ± standard deviation for continuous variables and number (percentage) for categorical variables. Statistical analysis was performed using SPSS version 26.0 (SPSS Inc. Chicago, IL). The data were analyzed by Chi-square test for categorical variables and student's t test for continuous variables. A p value less than 0.05 was considered statistically significant.

Results
A total of 22,909 cases with the data of abdominal ultrasonography were recruited from Taiwan bio-bank. Of them, 9,735 (42.5%) participants had fatty liver in their ultrasounds. After excluding those without the data of hepatitis B surface antigen (HBsAg), anti-hepatitis C virus (anti-HCV) antibody, and alcohol consumption, 9,719 subjects were included for the final analysis. The percentages of "overlapping group", "MAFLD only", "NAFLD only", and "neither FLD" were 79.7, 12, 7.1, and 1.2%, respectively. After the switch of diagnostic criteria from NAFLD to MAFLD, the missed population was "NAFLD only group" and the added population was "MAFLD only group", as shown on Fig. 1. The concordance of the two diseases' diagnosis was 79.7% in our study population.

Comparison of clinical characteristics and metabolic profiles between NAFLD and MAFLD patients
Since NAFLD and MAFLD patients were selected based on different diagnostic criteria from the same population, more than 80-90% of patients in these two groups were duplicated. Therefore, it is relatively difficult to reach a statistical difference. Compared with NAFLD patients, MAFLD patients had a higher frequency of males, DM, and hypertension history. Furthermore, several metabolic components including: BMI, body fat, waist circumference (WC), glycated hemoglobin (HbA1c), triglyceride (TG), and uric acid were higher. But the high-density lipoprotein (HDL) in MAFLD patients was lower than NAFLD patients. The two groups of patients were comparable in age, hyperlipidemia history, cholesterol (CHO), and low-density lipoprotein (LDL) ( Table 1).

Comparison of liver and cardiovascular risk between NAFLD and MAFLD patients
MAFLD patients showed higher levels of alanine aminotransferase (ALT)/aspartate aminotransferase (AST), gamma-glutamyl transferase (GGT), FLI, NFS, and IMT, but showed no difference in FIB-4 index and the percentage of carotid plaque ( Table 1).

Comparison of clinical characteristics and metabolic profiles between "MAFLD only" and "NAFLD only" group
These two groups of patients were new or missed population after the switch of diagnostic criteria from NAFLD to MAFLD. Compared with "NAFLD only" group, "MAFLD only" group had a higher percentage of males, DM, hypertension, and hyperlipidemia histories. Furthermore, they had a greater age, BMI, body fat, WC, HbA1c, TG, and uric acid, but lower CHO and HDL levels. The serum LDL levels were comparable between the two groups ( Table 2). Liver and cardiovascular risk between "NAFLD only" and "MAFLD only" group "MAFLD only" group had greater serum ALT/AST, GGT, FLI, FIB-4 index, NFS, IMT, and higher percentage of carotid plaque than "NAFLD only" group (Table 2).

Discussion
In this study of 9,719 fatty liver patients, 79.7% were "overlapping group", which fulfills both diagnostic criteria of NAFLD and MAFLD. The added population ("MAFLD only" group; 12%) had more people than the missed population ("NAFLD only" group; 7.1%) after the switch of diagnostic criteria from NAFLD to MAFLD. MAFLD patients had higher frequency of DM and hypertension; higher levels of glucose and lipid profiles than NAFLD patients even with a large degree of overlapping between the two groups of patients. In addition, "MAFLD only" group had a higher frequency of metabolic diseases, poor metabolic profiles, increased severity of hepatic steatosis and liver fibrosis, and risk of atherosclerotic cardiovascular disease (ASCVD) than the "NAFLD only" group. Thus, the new diagnostic criteria of MAFLD not only includes a greater number of patients, but also high-risk patients with metabolic, liver, and cardiovascular complications, suggesting that MAFLD may be a more suitable nomenclature than NAFLD in clinical diagnosis and treatment [22].
Fatty liver patients with alcohol or other known etiologies of chronic liver disease were excluded from the diagnosis of NAFLD. In contrast, the diagnosis of MAFLD includes fatty liver plus metabolic risk and permits of other concomitant liver diseases [23]. Two large population-based studies from Korea and China showed that the added population had more people than the missed population after the switch of disease name from NAFLD to MAFLD [24,25]. However, another population-based study from NHANES III database, a representative sample of US general population had inconsistent findings. MAFLD patients were diagnosed in 31.24% participants; while NAFLD was 33.23% among the overall population [26]. Our grand, population-based study from Taiwan showed that the diagnostic criteria of MAFLD could encompass more patients than NAFLD.
The diagnostic criteria of MAFLD include two essential factors: fatty liver and metabolic dysfunction. The difference of diagnostic criteria between MAFLD and NAFLD is metabolic dysfunction and any other concomitant liver diseases. A recent study from China showed that MAFLD patients had greater proportions of DM and dyslipidemia, higher BMI, WC, blood glucose, and lipid levels than NAFLD patients [23]. Our study also found higher frequency of metabolic diseases and abnormal metabolic factors including: BMI, glucose, lipid, and uric acid in MAFLD patients, suggesting the definition of MAFLD could diagnose those with a higher degree of metabolic dysfunction than with NAFLD criteria.
The top three causes of death in NAFLD patients were CVD, non-liver cancers, and liver-related diseases. Hence, extensive research is a must to confirm whether the new diagnostic criteria of MAFLD have the advantage of selecting high-risk patients of liver or CVD. As noted, the severity of liver fibrosis was associated with overall mortality and the risk of HCC development [27]. The severity of atherosclerosis correlates with the risk of ASCVD [28]. A Taiwan study of 166 patients with pathologic diagnosis of MAFLD proved that "MAFLD only group" had a greater NAFLD activity score and percentage of advanced fibrosis than those in the "NAFLD only group" [29]. In another study of 765 Japanese fatty liver patients using shear wave elastography, they found liver stiffness was more common in MAFLD patients than those with NAFLD. In addition, the diagnostic criteria of MAFLD had a higher sensitivity of detecting significant liver fibrosis than in the previous NAFLD criteria [30]. In our study, MAFLD patients had a greater serum AST, ALT, GGT, fatty liver index, and NFS than NAFLD patients. Furthermore, the "MAFLD only group' had larger serum AST, ALT, GGT, FLI, NFS, and FIB-4 score than the "NAFLD only group". Hence, the new diagnostic criteria of MAFLD can aid in identifying those with a large degree of disease activity including hepatic steatosis, liver inflammatory, and fibrosis than with the previous NAFLD criteria.
Regarding CVD risk, a nationwide health-screening database from Korea cohort with a median follow-up of 10.1 years revealed that both NAFLD and MAFLD patients had a greater risk of CV events than in healthy controls. In addition, when "neither FLD" was used as a reference, "overlapping group" showed immense hazard ratios for CV events, followed by "MAFLD only" and "NAFLD only" groups [31]. Another study of 2,985 subjects followed for 7 years found that although NAFLD and MAFLD had similar metabolic traits and similar outcomes of CV events at baseline, "MAFLD only" patients had more risk of adverse outcomes than "NAFLD only" patients [32]. The other study Table 2 Comparison of clinical characteristics and outcomes between "MAFLD only" and "NAFLD only" groups *The statistical analysis and comparison were conducted between the "MAFLD only" and "NAFLD only" groups MAFLD metabolic associated fatty liver disease, NAFLD non-alcoholic fatty liver disease, DM diabetes mellitus, HTN hypertension, BMI body mass index, WC waist circumference, HBsAg hepatitis B surface antigen, HbA1c glycated hemoglobin, TG triglycerides, CHO cholesterol, HDL high-density lipoprotein, LDL low-density lipoprotein, AST aspartate aminotransferase, ALT alanine aminotransferase, GGT γ-glutamyl transferase, FIB-4 fibrosis-4, NFS NAFLD fibrosis score, IMT intima-media thickness  [33]. Using the data of carotid duplex ultrasound, our study found that MAFLD patients had greater IMT than those with NAFLD. In addition, "MAFLD only" group had higher IMT and percentage of carotid plaques than the "NAFLD only" group after the switch of disease name from NAFLD to MAFLD. Our data consistently confirm that MAFLD is more suitable in identifying high-risk patients of ASCVD than the previous term NAFLD. The missed population after the switch of diagnostic criteria from NAFLD to MAFLD includes fatty liver patients who fulfilled the definition of NAFLD, but had no metabolic risk (MR) ("NAFLD only" group; non-MR NAFLD). Since these patients included no metabolic dysfunction and other concomitant liver diseases, the risks of liver and CVD are expected to be minimal. Some previous studies confirmed that the "NAFLD only" subjects did not require urgent diagnostic and therapeutic intervention due to a potentially favorable disease [30]. However, a recent brief report using NHANES III database noted that "NAFLD only" patients with severe fatty liver in ultrasound are likely to include significant liver injury and fibrosis and require additional attention in clinical practice [34]. Since the definition of non-MR NAFLD in this study excluded only "excessive alcohol consumption" and metabolic risk, the results are likely to be confounded by viral hepatitis or other causes of liver disease. In another study of 1,217 cases with liver biopsy, there was no significant difference in the degree of liver inflammation and fibrosis among MAFLD, NAFLD, and "NAFLD only" groups histologically. They suggested that MAFLD criteria overlook a subset of patients with steatohepatitis and significant fibrosis. However, since the majority of the cases were infected with HBV (93.26%), the confounding effect cannot be excluded [35]. Our study revealed that "NAFLD only" group had a lower risk of liver fibrosis and ASCVD compared with the "MAFLD only" group after the switch of disease name and diagnostic criteria from NAFLD to MAFLD.
Our study comprises of numerous strengths. First, it is a grand, population-based study from Taiwan bio-bank with simultaneous assessment of liver and ASCVD risk. Second, this is the first study to compare the risk of ASCVD between NAFLD and MAFLD patients using the data of carotid duplex ultrasound. Third, since the database includes detailed description of the type, amount, frequency, and duration of alcohol drinking, the amount of alcohol consumed in a day can be accurately calculated to obtain the correct judgment about the positive or negative diagnosis of NAFLD. However, some limitations should also be addressed. First, fatty liver was determined by an ultrasound without histology in our study. Moreover, liver biopsy is not well suited in population-based studies. Furthermore, ultrasound-based studies show poor diagnostic accuracy of fatty liver when hepatic steatosis is < 30%, for which ultrasounds tend to underestimate the true prevalence of NAFLD and MAFLD. Second, this was a cross-sectional study and unable to demonstrate causal relationships. Third, since the data of high-sensitivity C reactive protein and insulin resistance were not available in our study, the prevalence of MAFLD might be underestimated. Finally, the risk of cardiovascular complications mainly focusing on ASCVD, not including other CVD such as cardiomyopathy, rheumatic heart or congenital heart diseases is a major limitation in the study. Furthermore, the risk of ASCVD was indirectly assessed using the markers of atherosclerosis, not directly from the actual events of ASCVD, such as coronary artery disease, myocardial infarction, ischemic stroke, or CVD-related death. In addition, data of echocardiography and computed tomography were not available in this population study.
In summary, MAFLD is an "inclusive" diagnosis with the mention of underlying causes, which makes explanation and education easier between physicians and patients. In addition, this large-scale, population-based study confirmed the change of nomenclature and diagnostic criteria from NAFLD to MAFLD, which could identify more patients at risk of metabolic, liver, and CVD complications for early intervention, suggesting that MAFLD may be a more suitable nomenclature than NAFLD in clinical practice. However, since MAFLD patients with dual etiology such as viral hepatitis, alcohol or autoimmune liver diseases were new disease groups, the natural history and clinical outcomes of those patients need further investigations [36].