Introduction

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide and is a progressive chronic condition.1 The potential disease risk of NAFLD is a significant factor threatening public health.2 Recent evidence suggests that NAFLD is closely associated with various chronic diseases, including tumors, cardiovascular diseases, kidney diseases and sarcopenia, among others.3–7 Moreover, NAFLD is strongly linked to insulin resistance (IR) and type 2 diabetes mellitus (T2DM). Individuals with T2DM are five times more likely to develop NAFLD compared to those without T2DM.8 Therefore, timely identifying and screening of NAFLD in individuals with T2DM is crucial for the prevention and treatment of NAFLD-related complications.

A large body of research indicates that IR is closely linked to the pathogenesis of NAFLD through related molecular-biochemical and immune mechanisms.9 The gold standard for IR assessment is the hyperinsulinemic euglycemic clamp,10 but its high cost and invasiveness limit its clinical utility. Therefore, it is necessary to explore a convenient and practical method for assessing IR. Researchers from Mexico introduced the IR metabolic score (METS-IR) as a novel indicator for evaluating insulin sensitivity.11 It was calculated through simple anthropometric measurements and biochemical parameters including fasting plasma glucose (FPG), triglycerides (TG), body mass index (BMI) and high-density lipoprotein cholesterol (HDL-c). Studies showed that this score was consistent with the hyperinsulinemic-euglycemic clamp technique and was widely used in clinical practice to reflect IR sensitivity.12 In addition to metabolic syndrome, METS-IR could also early predict coronary artery disease, T2DM and hyperuricemia.13,14 Recently, a research report from South Korea showed that METS-IR could detect and prevent NAFLD early in the general population.15 However, the utility of METS-IR in predicting NAFLD in individuals with T2DM remains unclear.

Liver biopsy is regarded as the standard diagnostic method for NAFLD, but its invasive nature restricts its clinical usefulness. Currently, ultrasound is commonly used for diagnosing NAFLD in clinical practice. Thus, this study aims to analyze the correlation between METS-IR and based on ultrasound-diagnosed NAFLD in Chinese T2DM population, and to preliminarily understand the predictive value of METS-IR for NAFLD.

Methods Study Participants

Based on the research objective, the data were collected from inpatients in the Department of Endocrinology of the Linyi People’s Hospital, Shandong Province, China, between January 2020 and March 2023. The inclusion criteria for participants included: individuals with T2DM aged 18 and above. Exclusion criteria for participants included: (1) severe liver insufficiency (previously or currently diagnosed with liver failure) and severe renal insufficiency (eGFR < 30 mL/min/1.73 m²); (2) history of significant abnormalities in lipid levels and autoimmune, drug-induced liver diseases, liver malignancies or hepatic steatosis; history of viral hepatitis; (3) history of extensive alcohol consumption (more than 70 g/week for women or 140 g/week for men); (4) insufficient clinical information. In the end, a total of 1097 T2DM participants aged 18 and above were included in the study (Figure 1).

Figure 1 The flow chart of study participants selection.

Anthropometric and Biochemical Measurements

The collection of basic medical history and health habits of participants included age, gender, height, weight, diabetes duration, smoking habits and alcohol consumption. In a quiet environment, participants rested in a seated position for at least 5 minutes, then blood pressure was monitored using a standard electronic sphygmomanometer, recording systolic and diastolic blood pressure (S/DBP). Fasting venous blood samples were collected to measure total cholesterol (TC), low-density lipoprotein cholesterol (LDL-c), TG, high-density lipoprotein cholesterol (HDL-c), alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transferase (GGT), FPG, serum creatinine (Scr), uric acid (UA) and glycated hemoglobin (HbA1c, high-performance liquid chromatography), using a clinical chemistry analyzer (Cobas c 702, Roche, Germany). Fasting insulin (FINS, direct chemiluminescence method) was tested by the fully automated sample processing system (Aptio Automation, SIEMENS, USA). This study utilized the Omron DUALSCAN BIA machine (HDS-2000, Omron, Kyoto, Japan) to collect measurements of visceral fat area (VFA) and subcutaneous fat area (SFA), with units in cm².

Definitions

The diagnosis of NAFLD was based on liver ultrasound examination, in conjunction with the participant’s history of alcohol consumption and medication use, to exclude autoimmune, alcohol-induced and drug-induced fatty liver disease before confirming the diagnosis.

Parameter Calculations BMI = weight (kg)/height (m);2 TG/HDL-c ratio = TG (mmol/l)/HDL-c (mmol/l);16 METS-IR = ln [(2*FPG (mg/dl)) + TG (mg/dl)] *BMI)/(Ln [HDL-c (mg/dl)]);11 HOMA-IR = FPG (mmol/L) * FINS (lU/mL)/22.5;17 TyG = ln [TG (mg/dl) * FPG (mg/dl)]/2.18Statistical Analysis

Data statistical analysis was performed using SPSS 26.0 (SPSS Inc, Chicago, IL, USA). Mean ± SD was used to describe normally distributed data, while median (interquartile range) was used for skewed data. The independent sample T-test and Mann–Whitney U-test were employed to compare continuous variables between groups with normal and non-normal distributions, respectively. One-way analysis of variance (ANOVA) and Kruskal–Wallis H were used to compare the differences in continuous variables among the four METS-IR groups with normal distribution and abnormal distributions, respectively. Chi square test was used to compare categorical variables. Spearman correlation analyses was used to assess the NAFLD-related correlations. Binary logistic stepwise regression analysis was conducted to identify independent risk factors for NAFLD. Analysis of the area under the receiver operating characteristic (ROC) curve was used to evaluate the ability of METS-IR in predicting NAFLD identification, and further conducted a differential analysis. A two-tailed P value <0.05 was considered statistically significant.

Results Clinical Characteristics of the Study Subjects

The clinical characteristics of the participants are displayed in Table 1. Compared to the non-NAFLD group (n=687), the BMI, smoking, SBP, DBP, TC, LDL-c, TG, ALT, GGT, UA, FPG, FINS, HOMA-IR, TyG, VFA, SFA, TG/HDL-c ratio and METS-IR were higher, while the HDL-c was significantly lower in the NAFLD group (n=410) (all P < 0.05). There were no statistically significant difference in HbA1c, AST and Scr between the two groups (all P > 0.05).

Table 1 Comparison of Clinical and Biochemical Characteristics Between Non-NAFLD and NAFLD Groups

The study population was subsequently divided into four groups, including Q1 (22.71–35.41), Q2 (35.41–39.99), Q3 (39.99–45.41) and Q4 (45.41–68.72) based on the quartiles of the METS-IR (Table 2). As the quartiles of METS-IR increased, the smoking, BMI, SBP, DBP, TG, ALT, GGT, UA, FPG, FINS, HOMA-IR, TyG, VFA, SFA and TG/HDL-c ratio showed a gradual increased, while the HDL-c exhibited a gradual decreased (all P < 0.05). There was no statistically significant difference in diabetes duration, TC and AST among the four groups (all P > 0.05).

Table 2 Comparison of Variables According to the Categories of the METS-IR

Univariate Analysis

As indicated by the Spearman correlation analysis in Table 3, NAFLD exhibited positive correlations with smoking, BMI, SBP, DBP, TC, LDL-c, TG, HbA1c, ALT, GGT, UA, FPG, FINS, HOMA-IR, TyG, VFA, SFA, TG/HDL-c ratio and METS-IR, while showing inverse correlations with age, diabetes duration and HDL-c (all P < 0.05). Gender, AST and Scr showed no correlation with NAFLD (all P > 0.05).

Table 3 The Correlation NAFLD by Univariate Analysis

Logistic Regression Analysis

NAFLD was defined as the dependent variable, and the above-mentioned relevant indicators (Table 3) as independent variables. Following the adjustment for potential confounders including the BMI, SBP, DBP, LDL-c, TG, HbA1c, FPG, FINS, HOMA-IR, TyG, TG/HDL-c ratio, age and HDL-c, the binary logistic regression analysis (Table 4) revealed that the METS-IR (OR: 1.120, 95% CI 1.080–1.161), TC (OR: 1.343, 95% CI 1.129–1.599), DBP (OR: 1.024, 95% CI 1.004–1.044), VFA (OR: 1.011, 95% CI 1.005–1.018), UA (OR: 1.003, 95% CI 1.001–1.005) and diabetes duration (OR: 0.967, 95% CI 0.937–0.996) were independently associated with NAFLD.

Table 4 The Relative Risk for NAFLD by Logistic Regression Analysis

Areas Under the ROC Curve Analysis

Finally, we compared the area under the ROC curves of METS-IR with those of the variables that constitute METS-IR (BMI, TG, FPG and HDL-c), traditional indicators related to NAFLD (AST, ALT and GGT), and commonly used effective IR-related indicators (TG/HDL-c ratio, TyG and HOMA-IR), and indicators entering the regression model (VFA, DBP, TC, UA and diabetes duration) in Table 5. The findings indicated that the area under the ROC curve for METS-IR was 0.781 (95% CI 0.746–0.817, P < 0.001) (Figure 2). Moerover, differential analysis of ROC showed that METS-IR was higher than BMI, GGT, TG/HDL-c ratio, TG, TyG, UA, DBP, HOMA-IR, FPG, ALT, TC, AST, diabetes duration and HDL-c (all P < 0.05), whereas the difference between METS-IR and VFA was not statistically significant (P = 0.077).

Table 5 Analysis of Areas Under the ROC Curves for Predicting NAFLD

Figure 2 ROC analyses for NAFLD.

Discussion

Through this cross-sectional study, we found a close association between METS-IR and NAFLD, with the incidence of NAFLD gradually increasing as the quartiles of METS-IR increase. Following adjustment for confounding factors using logistic regression, multivariate analysis revealed a persistent independent correlation between METS-IR and NAFLD. Furthermore, areas of ROC curve analysis indicated that METS-IR was relatively higher than other evaluation variables.

Obesity, hypertriglyceridemia, impaired FPG, diabetes mellitus, IR and other metabolic diseases were the main risk factors for NAFLD.19 METS-IR was considered an independent predictive indicator of IR.20 IR has been confirmed as a key factor in the development of NAFLD in previous studies.21,22 Currently, some studies had investigated the relationship between METS-IR and NAFLD in different populations. Cai et al pointed out that METS-IR was independently associated with NAFLD risk in 10,730 non-obese Chinese adults.2 Li et al’s study confirmed that among 72225 adults aged 60 and above in China, compared to the other six IR substitutes, METS-IR might more reliably identify NAFLD.1 In addition, Lee et al pointed out that in a general population of 10,030 individuals aged between 40 and 69 years in South Korea, METS-IR outperformed other indicators in predicting the occurrence of NAFLD.15 However, there was no evidence to indicate the ability of METS-IR to predict NAFLD in T2DM population. Our research findings showed that METS-IR was an independent risk factor for NAFLD in patients with T2DM, further demonstrating the tight connection between these two variables in different populations.

IR is a key mechanism in the development of NAFLD. HOMA-IR is one of the most commonly used indicators to assess IR. Studies had shown that HOMA-IR was significantly associated with NAFLD.23 The TG/HDL-c ratio and TyG index are newly proposed indicators of IR in recent years, reflecting lipid and glucose metabolism. Research indicated that they are also closely related to NAFLD.24,25 We included above the IR-related indicators. The results showed that they did not enter the regression model, and had significantly smaller areas under the ROC curve compared to METS-IR.

The GGT, ALT and AST levels were considered as the common indicators for evaluating liver cell damage. Research has shown that GGT, ALT and AST were closely associated with NAFLD and could be used as alternative markers for evaluating the condition.26–28 In our study, Spearman correlation analysis showed that both ALT and GGT were associated with NAFLD. However, after adjusting confounding factors, the regression model showed that the ALT and GGT did not enter the model. Furthermore, an analysis of the area under the ROC curve for METS-IR showed that area under the ROC curve was greater than that of ALT, AST and GGT. Therefore, this suggested that single ALT, AST or GGT might not be superior to METS-IR in identifying NAFLD in T2DM.

Limitations

This study has several limitations. Firstly, it is a cross-sectional study, so we can only analyze the relationship between METS-IR and NAFLD, and cannot establish a significant causal relationship. Secondly, in this study, the diagnosis of NAFLD was established through abdominal ultrasound, which may have led to the omission of some NAFLD patients in comparison to the gold standard method of liver biopsy. Additionally, an insulin clamp test should be conducted to compare the specificity of identifying IR between METS-IR and other indicators. Thirdly, our results were based on data analysis from hospitalized patients only, further prospective and multicenter studies are needed to support our findings.

Conclusion

METS-IR is a new index for assessing IR. Our study showed that METS-IR was an independent risk factor for NAFLD in T2DM population, and it might outperform other commonly used variables in identifying NAFLD in Chinese T2DM population. Future research should pay more attention to the value of METS-IR in identifying NAFLD in T2DM patients.

Ethics Approval and Consent to Participate

The Human Ethics Committee of the People’s Hospital of Linyi has reviewed and approved this study. We conducted this research in accordance with the ethical principles regarding human subjects as outlined in the Helsinki Declaration. And informed consent was obtained from the patients.

Acknowledgments

Xuan Ma and Baolan Ji are co-first authors for this study. Guanqi Gao and Bo Ban are co-corresponding authors for this study. All of our authors would like to express gratitude to everyone involved in this research.

Funding

This study was funded by grants from the Postdoctoral Program of Affiliated Hospital of Jining Medical University (JYFY322152).

Disclosure

The authors declare that they have no competing interests in this study.

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