This population-based cross-sectional study suggested an increased risk of elevated ALT levels with increasing TyG index in general Chinese population. To our best knowledge, this study is the first to demonstrate a significant association between TyG index and elevated ALT after adjustment for potential confounders. We also assessed the relationship between elevated ALT and per-unit increase in TyG index for both genders. In addition, the discriminatory power of TyG index in the prediction of elevated ALT was detected and the optimal cut-off point of TyG for the identification of elevated ALT was suggested. Thus, TyG index could be a simple and clinically effective surrogate marker to identify elevated ALT.
In the present study, the TyG index was positively associated with elevated ALT as anticipated. Elevated ALT is not only considered as a consequence of hepatocytes damage as a result of NAFLD, but also a cardiometabolic risk factor, associated with type 2 diabetes mellitus, metabolic syndrome [5 12 25]. Moreover, recent studies reported that elevated serum level of ALT was marker of inflammation and oxidative stress, which could lead to insulin resistance and further promote excessive accumulation of triglycerides in liver [25 26]. TG and FPG are key components of metabolic syndrome, which are overproduced by the fatty liver. Thus, we speculated that there existed an association between a novel index, the triglyceride glucose index and elevated ALT level.
TyG index, a biomarker related to IR, can be used in clinical practice and large-scale, population-based health screening because measuring TG and FPG is inexpensive and routine. Our data suggested that TyG was positively associated with elevated ALT. This result was not surprising because the TyG index, derived from TG and FPG, two crucial metabolic variables altered in fatty liver, and highly correlates with insulin resistance, which was the key pathogenesis of NAFLD.
Recently, TyG index has been used as excellent surrogate marker to identify IR and T2DM [13 27]. Moreover, a new published study demonstrated that the TyG index, a simple measure reflecting insulin resistance, might be useful to early identify individuals at a high risk of developing a cardiovascular event . The present study showed that higher level of TyG index had a positive relationship with elevated ALT level the general Chinese population. Importantly, Guerrero-Romero et al.  indicated that TyG index could reflect hepatic insulin resistance due to its correlation with composition of liver fat. In recent years, the TyG index has attracted more attention. In fact, previous studies showed that the TyG index had an independent association with liver steatosis in chronic hepatitis C patients and NAFLD patients [29 30]. Another study also indicated that the TyG index was effective to screen simple steatosis and was superior to other indices for NAFLD .
In this study, we determined in a large Chinese population that thresholds of TyG ≥ 8.69 and 8.96 for men and women was effective enough to identify elevated ALT individuals. The cut-off values were similar to that of 8.5 in another Chinese study which evaluated the TyG and NAFLD , but quite different from the finding of 4.58 by Simental-Mendia . Our study indicated that the TyG index was closely associated with elevated ALT. The reason for this may be explained that in subjects with higher level of TyG index, excessive accumulation of triglycerides in the liver, which may lead to hepatic insulin resistance. Then, as a result of hepatic insulin resistance, FPG and LDL-C were overproduced, which could make the fat composition in the liver, further led to ALT levels elevation and chronic liver diseases.
Study strengths and limitations: The strengths of this study are its population-based design, large sample size, and the first evaluation of the associations of the TyG index with elevated ALT level in general Chinese population. These data are particularly important in the case of rural Chinese population who has relatively poor living standard and health resources. Several limitations in this study need to be acknowledged. First, because of its cross-sectional design, we were unable to determine whether or not there was a causal association. Thus, the obtained associations in this study should be considered with caution. Second, there are known causes of elevated liver enzyme levels that were not tested in our study, such as chronic viral hepatitis and other illnesses, and the possibility still exists that unmeasured confounders may explain part of the association.