The triglyceride glucose-body mass index: a non-invasive index that identifies non-alcoholic fatty liver disease in the general Japanese population

A non-alcoholic fatty liver disease (NAFLD) is marked by hepatic steatosis without evidence of excessive alcohol use or other obvious factors that damage the liver [1]. In the 21st century, NAFLD remains an essential public health issue [2]. Globally, an estimated 20% of the general population is suffering from NAFLD, with a range from 6 to 35% based on multiple measurements [3]. There is a continuum of NAFLD, from simple steatosis to non-alcoholic steatohepatitis (NASH), with varying degrees of fibrosis that eventually progress to cirrhosis [4, 5]. NASH may cause cirrhosis and hepatocellular carcinoma, while simple steatosis presents as a benign condition with slow progression over many years [3, 6, 7]. The extrahepatic form of NAFLD is characterized by its ability to aggravate the cardiovascular disease, kidney disease, and diabetes, resulting in adverse health effects [8‐10]. Even though the prevalence of NAFLD is increasing and its adverse effects are seen throughout multiple systems in the body, there are no effective treatments except for lifestyle changes along with regular physical activity [11]. It is therefore extremely important to identify patients whose risk of NAFLD is high at an early stage.

As far as diagnosis of NAFLD is concerned, liver biopsy has always been the gold standard [12]. However, due to its invasiveness and high cost, it could not become a widely accepted diagnosis. It is also unreasonable to perform routine liver biopsies as a screening or risk assessment test for the general population. In addition, a liver biopsy analysis with poor inter-observer variability and modest intra-observer variability has suboptimal reliability for measuring a treatment effect in clinical trials [13]. Clinical practitioners use liver ultrasound as a valuable tool in their practice for detecting fatty liver in the early stages. Ultrasonography (US), nevertheless, depends on the operator's experience and technological sophistication [14]. Besides, Steatosis less than 20% [15] or steatosis in morbidly obese individuals could not be detected by ultrasound [16]. In addition, the accuracy of US for hepatic steatosis assessment is affected by the presence of severe fibrosis [17]. Moreover, there is a drawback in that dietary and pharmacological interventions are unable to be qualitatively evaluated [18]. With the development of ultrasonic transient elastography, the controlled attenuation parameters of the liver and the liver stiffness value can be used to assess the degree of hepatic steatosis and fibrosis quantitatively, but they are affected by the operator’s skill level [19]. In addition, based on proton magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI), which can accurately determine the amount of liver fat content and the degree of fibrosis, and play the role of similar liver biopsy, but their cost is high, and difficult to obtain, so it has not been widely used clinically [20]. In recent years, serum non-invasive diagnostic markers or models have attracted widespread clinical attention due to their advantages of non-invasiveness, low cost, simple operation, strong reproducibility, and low requirements for operators, especially for early screening and evaluation of NAFLD [20]. Therefore, effective noninvasive methods should be used in clinical practice to identify NAFLD, track disease processes, and monitor treatment effects [21].

Overweight, obesity, and insulin resistance strongly correlate with NAFLD due to excessive fat accumulation, especially triglycerides in hepatocytes [22]. NAFLD is characterized by oxidative stress and inflammation. An increase in reactive oxygen species (ROS) can lead to lipid peroxidation by damaging both membrane structure and function. In addition to oxidizing key proteins for cell metabolism and function, it may also cause the oxidation of nucleic acids [23]. Since the liver has a limited ability for triglyceride accumulation, lipid deposition under overfeeding conditions, as in the case of NAFLD, determines the accumulation of high levels of fatty acids, generally saturated ones, which are associated with cell dysfunction [24]. Indeed, the excess of fatty acids induces high rates of β-oxidation, increasing the production of ROS in the mitochondrial respiratory chain, which causes cellular damage and oxidative stress [25]. Oxidative damage markers rise in response to this circumstance, Kupffer cells become active, pro-inflammatory pathways activate, and circulating immune cells are drawn into the body [26, 27].

An insulin resistance (IR) is characterized by decreased peripheral tissue insulin sensitivity, which is at the core of the pathogenesis of NAFLD by impairing glucose uptake and oxidation [11, 28]. Triglyceride-glucose (TyG) is an index combining fasting blood glucose (FPG) with fasting triglyceride (TG) that could better reflect insulin resistance. It has been widely accepted and used in clinical applications due to its convenience and simple calculation [29‐31]. Using a combination of body mass index (BMI) and TyG index, Er et al. found that the information imparted by a multitude of critical clinical indicators could be simultaneously reflected, such as blood lipids, blood glucose, and BMI, and could better reflect IR than the TyG index alone [32]. Given the importance of IR in NAFLD pathogenesis [11, 28], TyG-BMI has been linked to an increased incidence of NAFLD, according to certain studies [33‐36]. As a result, we hypothesized that the TyG-BMI might be an effective marker in identifying NAFLD in the general population. However, as a non-invasive and simple model, applying TyG-BMI to identify and evaluate NAFLD still needs further research.

The objective of this study was to determine the diagnostic accuracy of the TyG-BMI in detecting NAFLD in the general Japanese population.

In the present study, 14,280 participants (52.1% men and 47.9% women) were eligible. Figure 1 depicted the subjects’ selection and grouping process. The mean age of all participants was 43.53 ± 8.89 years. A total of 2515 (17.6%) participants were diagnosed with NAFLD. The mean BMI was 22.07 ± 3.14 kg/m². The mean FPG and TG were 92.74 ± 7.42 and 79.03 ± 56.07 mg/dL, respectively. The mean TyG and TyG-BMI were 8.01 ± 0.64 and 177.74 ± 34.53, respectively.

This cross-sectional study aimed to develop and validate a non-invasive index that uses routinely measured and readily accessible clinical and laboratory variables to discriminate between the presence or absence of NAFLD. This index, called the “TyG-BMI”, accurately distinguished the populations with or without NAFLD. The absence or presence of NAFLD was diagnosed in 9996 (70%) of the 14,280 patients using values below or above the lower or upper cutoff points. Of these 9996 individuals, 9308 (93.1%) were diagnosed correctly. Only 4284 participants (30%) of the 14,280 participants with TyG-BMI in the range of 182.2–224 were considered “indeterminate”. According to this, 70 percent (9996 out of 14,280) of participants in the whole population were able to avoid ultrasonography by applying the TyG-BMI. Both internal and external validations demonstrated that TyG-BMI was highly accurate in diagnosing patients. In addition, the authors summarized the positive and negative predictive values of the two cutoff points using a wide range of prevalence of NAFLD, ranging from 5 to 50%. TyG-BMI’s clinical application was demonstrated by the decision curve analysis.

A number of non-invasive and simple models have been developed to detect and evaluate NAFLD [18, 46]. Due to their calculation based on anthropometric and biochemical parameters, these could be easily obtained in clinical practice. Hepatic steatosis has been identified and managed with these models because they are cost-effective, practical, and reliable [46]. Several studies have shown that the fatty liver index (FLI), which is derived from a population of fewer than 8000 individuals in an Italian municipality [47], is acceptable for detecting NAFLD [48]. Based on a survey of nearly 10,000 Korean patients, the hepatic steatosis index (HSI) has also been shown to be an accurate and simple method for predicting NAFLD [49]. A few other indicators may be used to determine central lipid accumulation, including lipid accumulation product (LAP) and visceral adiposity index (VAI) [50, 51]. The underlying cause of NAFLD is a complex combination of environmental factors, heredity, and dietary habits [52]. Several dietary habits contribute to the development of NAFLD, such as excessive calorie consumption, fructose consumption, and physical inactivity [53]. Moreover, Western and Asian countries differ significantly in genetic backgrounds, dietary habits, and lifestyles [54]. It is possible, however, that these indices may not be appropriate for Asian populations since they were most originally designed for western populations. Furthermore, most centers do not have external validation of these models, making it challenging to apply the proposed scoring system daily.

In a Japanese population, Wang et al. developed a novel model called the TyG-BMI index that could help predict NAFLD [34]. After adjusting for confounding variables, according to the study, NAFLD was positively associated with TyG-BMI (OR: 3.90 per SD increase; 95% CI 3.54 to 4.29). Analysis of ROC showed that the TyG-BMI was more effective at predicting NAFLD risk than other traditional indicators [TyG-BMI (AUC): 0.886; TyG (AUC): 0.808; TG (AUC): 0.797; BMI (AUC): 0.858; FPG (AUC): 0.711], especially among young and middle-aged individuals and individuals who aren't obese. With the AUROC of 0.886 (95% CI 0.876, 0.896) in general populations and 0.88–0.97 in young and middle-aged people, and 0.84 in non-obese people (Additional file 7: Table S4), the results of the present study were consistent with Wang et al. [34]. However, hepatic steatosis commonly occurs in obese individuals. We consider the reasons for this phenomenon as follows. After further analysis of the baseline information of subjects for BMI stratification, we found that there were more non-obese women than men in this study. Sex differences in non-obese NAFLD have also been noted in some previous studies [55, 56]. There is a general tendency for females to have more subcutaneous and visceral fat [57, 58], and the BMI alone does not provide a complete picture of this information [56]. According to recent studies, people with non-obese NAFLD are more likely to develop metabolic diseases [59].

Wang et al. [59] carried out a receiver operating characteristic analysis that showed that the TyG-BMI could better predict the risk of NAFLD than other traditional indicators and obtained the optimal threshold for TyG-BMI. However, the performance of TyG-BMI has not been validated in an external population in the study of Wang et al. [34]. In addition, they did not explore two cut-off values of TyG-BMI to identify or exclude NAFLD and the corresponding positive and negative predictive values. It is essential to point out that although the optimal threshold had the largest Youden index, it is not associated with the greatest positive or negative predictive value. Therefore, a diagnostic and exclusion model of a disease requires 2 cut-off values, and the optimal threshold is not the best choice. To address this question, the present study developed and validated a simple, non-invasive, and cost-effective tool, TyG-BMI, to accurately separate participants with and without NAFLD in the Japanese population. A total of 2 cut-off values of TyG-BMI were found in this study, one for excluding NALFD and the other for diagnosing NALFD. The cut-off was 182.2 for the sensitivity of 0.894 and 224.0 for the specificity of 0.966 in the derivation cohort, leading to a negative predictive value of 0.969, a positive predictive value of 0.707, and an area under the ROC curve of 0.888 (95% CI 0.876–0.896). The results demonstrated that, as a result of applying this model, 9996 (70%) of the 14,280 participants would not have undergone ultrasonography, with an accurate prediction of 9308 (93.1%). Thereby facilitating the more accurate identification and selection of candidates for clinical intervention and reducing the number of unnecessary ultrasonography. Therefore, this model has good clinical application prospects.

In 2019, Mohammad et al. [36] developed triglyceride glucose index and related parameters (triglyceride glucose-waist circumference and triglyceride glucose-body mass index) to identify NAFLD in individuals with overweight/obesity in Iran. They found that TyG-WC showed the largest AUC for detection of NAFLD [0.693, 95% confidence interval (CI) 0.617–0.769], followed by TyG-index [0.676, 95% CI 0.598–0.754] and TyG-BMI (0.675, 95% CI 0.598–0.752). Another study [33] focuses on the association between TyG-BMI and NAFLD in the non-obese Chinese population with normal blood lipid levels. In their study, TyG-BMI had a good prediction value (0.85 area under ROC; 95% CI 0.84–0.86) for NAFLD incidence. The AUROCs of the two studies were a bit smaller than the present study. Besides, the AUROC of TyG-BMI was less than TyG for detecting NAFLD in the study of Mohammad E. et al.[36]. Several factors might explain the difference: (1) the study populations differed. The present study was performed on the general Japanese, while the above two studies focused on Iranian with overweight/obesity or the non-obese Chinese population with normal blood lipid levels. (2) The diagnosis method of NAFLD was different. There was a difference between ultrasonography and transient elastography. (3) The prevalence of NAFLD varied significantly by gender, age, dietary habits, and ethnicity [54].

Japanese dietary pattern is also different from Chinese dietary pattern. In the Japanese diet, the total energy is lower, and essential fatty acids (e.g., N-3 fatty acids) are higher because of more seafood consumption compared to the Chinese diet. NAFLD is a multifactorial disease related to a complex living environment, heredity, and dietary habits [52]. In patients with NAFLD, dietary n-3 polyunsaturated fatty acids (PUFAs) can reduce hepatic inflammation, fibrosis, and steatosis, lower plasma TG levels, and improve hepatic fatty acid metabolism [60]. Different dietary habits affected the prevalence of NAFLD in Chinese and Japanese populations, and the prevalence could affect the effectiveness of TyG-BMI in diagnosing NAFLD. However, our results validated in the Chinese population suggest that the AUC was 0.874. The results indicate that TyG-BMI has an excellent ability to identify NAFLD in both Chinese and Japanese people.

Applying the TyG-BMI index, the results of the present study suggested that ultrasonography would only be needed to identify NAFLD in 30 percent of participants, i.e., those considered “indeterminate” (TyG-BMI in the range of 182.2–224). Most importantly, since most persons seen in clinical practice were not suffering from NAFLD [82.4% (11,765/14280) of the study cohort], the lower cutoff point was exceptionally accurate in ruling it out. In both estimation and validation, the NPV was 97% and 97%, respectively, and ranged from 75.6 to 98.3% for a prevalence of NAFLD of 5–50%. Among 14,280 patients, 8568 (60%) had a negative diagnosis of NAFLD through TyG-BMI (TyG-BMI below 182.2), and thus, using the TyG-BMI would have prevented the need for ultrasound. Of these 8568 participants diagnosed as not having NAFLD by TyG-BMI, 8304 (97%) were confirmed by ultrasound to have non-NAFLD indeed.

It should be pointed out that, clinically, US is the preferred imaging test for individuals with suspected NAFLD [61], with a typical appearance of a hyperechogenic liver. In a recent meta-analysis, ultrasound showed 85% sensitivity and 94% specificity in diagnosing moderate-to-severe steatosis compared to histology [14]. In contrast, US could not detect steatosis of less than 20% [15] or steatosis in individuals with morbid obesity [16]. Moreover, ultrasound cannot determine how severe NAFLD steatosis is [17]. Using computed-assisted US hepatic/renal ratios and US hepatic attenuation rates, it is possible to detect NAFLD early [17, 62]. Compared to the conventional US, both measurements are excellent in detecting hepatic steatosis, with a sensitivity of 95% and specificity of 100%. However, the NPV is still low (72% for US H/R ratio and 67% for US hepatic attenuation rate) [17, 63]. In addition, by standardizing it with a tissue-mimicking phantom, this quantitative US model can improve its reliability and reproducibility, while these findings are needed to verify in further studies [63]. Above all, it is still recommended by current guidelines that US be used to diagnose moderate and severe steatosis [64]. Vibration-controlled transient elastography (TE) is one of the available non-invasive assessment tools for NAFLD. By generating vibrations of low frequency and mild amplitude, elastic shear waves propagate through liver tissues and are used for measuring stiffness [65]. With newer models, liver fibrosis can now be measured by liver stiffness measurement (LSM), and liver steatosis can be measured by controlled attenuation parameter (CAP) [66]. There are several benefits of TE, including its low cost, fast procedure time, immediate result availability, good reproducibility, and ability to be performed in an outpatient setting [67]. Several cross-sectional studies have investigated how it helps diagnose NAFLD and assess its severity [63, 65, 68]. In conclusion, the quantitative US model and transient elastography will become a good non-invasive method for diagnosing NAFLD in the future, as an essential improvement of traditional ultrasonography.

Lipotoxicity in hepatocytes and immune-mediated inflammation play a crucial role in the development and progression of NAFLD. Hepatocellular injury caused by the lipotoxicity of accumulated lipids and free fatty acids (FFAs) is characterized by oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction, apoptosis, and subsequent expression of pro-inflammatory cytokines and inflammatory factors [69]. Apoptotic and immune pathways are activated as a consequence of cellular injury, which represents a distinctive feature of NASH pathophysiology. The lipotoxic lipids can activate both the intrinsic- and extrinsic-mediated (death receptor) apoptotic pathway in hepatocytes through the transcriptional up-regulation of proapoptotic and downregulation of antiapoptotic proteins [70]. It is believed that apoptosis or other forms of hepatocyte cell death play a crucial role in promoting immune responses associated with the progression of NAFLD to a more severe stage, e.g., fibrosis and cirrhosis development [71].

Since oxidative stress is a significant feature of NAFLD, antioxidant therapy is of great value for NAFLD [23]. The Mediterranean diet, Silymarin and berberine could play an antioxidant role, thereby protecting liver cells [72‐74]. In humans, only a small amount of oral antioxidants are absorbed because they are easily destroyed by acids and enzymes. Consequently, the development of effective methods for efficiently delivering antioxidants is urgently needed. Nano-antioxidants, created as a sponge-like polymer, act as a protective vehicle to prevent antioxidants from being degraded in the human gut and promote improved absorption in the digestive tract. A nano-capsule binds itself to the intestinal wall and releases antioxidants right into the intestinal cells, where they are absorbed directly into the bloodstream. Numerous antioxidant units are connected in a branched pattern to form nano-antioxidants. It could provide numerous possible sites to couple with an active species and have enhanced free radical scavenging potency [75].

The current study has some strengths, as follows: (1) the present study included a sizable sample size and diverse individuals, making it simple to publicize outside of the study. (2) TyG-BMI is determined using objective clinical and easily accessible lab variables routinely measured during health checkups, without requiring any other tests. (3) The authors explored two cutoff points used to identify or exclude NAFLD and used a wide range of prevalence of NAFLD varying from 5 to 50% to study the changes in positive and negative predictive values. (4) Using our decision curve analysis, TyG-BMI’s clinical effectiveness was demonstrated, and individuals with low-risk NAFLD would not require additional screening (such as ultrasonography). (5) The authors validated the results both internally and externally to make sure they were reliable.

Despite TyG-BMI’s good performance, the study still has some potential limitations. First, due to its imperfect sensitivity, ultrasonography is not a gold standard in diagnosing NALFD. The liver biopsy in asymptomatic people was typically not available in this considerable population-based investigation. Besides, patients with BMI on extreme ends of the spectrum may skew the ratio and can lead to decrease sensitivity and predictive value for the NAFLD. In the future, we could design our studies to diagnose NAFLD with more appropriate methods, such as the quantitative US model and transient elastography. We could also compare TyG-BMI against liver biopsy, a definitive test to establish the diagnosis of NAFLD. Second, the authors did not receive information about the severity of hepatic steatosis, so we could not evaluate the ability of TyG-BMI to quantify hepatic steatosis. Third, in this study, the development and validation of the diagnostic value of TyG-BMI for NAFLD were conducted in Asians. The diagnostic effect of NAFLD in non-Asian populations may be limited. However, some other western-derived indices for diagnosing NAFLD, such as FLI, Framingham steatosis index (FSI), and LAP, have been validated in Asians. And they were valuable indices for identifying the presence of NAFLD [76‐79]. So, we tend to believe that the TyG-BMI index could help predict NAFLD in populations other than Asians. Fourth, this was a cross-sectional study, and we could not explore the predictive value of TygBMI for the occurrence of NAFLD in the future.

TyG-BMI, constructed from routine clinical and laboratory variables, is able to accurately diagnose NAFLD, thus rendering ultrasonography unnecessary for the vast majority of populations. TyG-BMI, therefore, could be used to identify candidates for hepatic ultrasound and those who need lifestyle modifications.