It is already established that dyslipidaemia played the important role in NAFLD. However, the diagnostic ability of current indexes based lipids was limited. Surprisingly, in our study, the TG/TC ratio had a high diagnostic ability for identifying NAFLD than before, especially in young females. Furthermore, TG/TC had a high NPV(> 90%) in women and aged 18–34 groups, which means it could be used to exclude subjects with NAFLD for these population in clinical settings. The characteristics of TG/TC made it become a strong surrogate for screening NAFLD particularly in Chinese young females.
Over the last decades, the prevalence has risen in young adults, which was often unrecognized. A recent cross-sectional analysis, which evaluated the prevalence of NAFLD in subjects aged 18–35 years, suggest that the prevalence in young adults has increased almost 2.5 times over three decades, ranging from 9.6% in 1988–1994 to 24% in 2005–2010, and over one half of morbidly obese young adults had NAFLD (57.4%). Besides, some risk factors, such as obesities, type 2 diabetes, smoking, and unhealthy diets, were increasingly prevalent among young adults, which led to a higher risk for NAFLD[24–27]. This trend of NAFLD prompted us to explore the diagnostic ability of TG/TC in different age groups. Interestingly, our study showed that the diagnostic accuracy of TG/TC was significantly better in young age groups, in which we found an AUC of 0.943 in 18–34 years group and 0.921 in 35–44 years group. Our results supported that TG/TC was a powerful index that could be applied to screen NAFLD, especially in young population.
Furthermore, the diagnostic ability of TG/TC was significantly higher than other indexes based on lipid parameters, with AUROC of 0.920 in women and 0.863 in men. Fan and his colleague analyzed the association between different serum lipids (TG, TC, LDL-C, HDL-C,) and NAFLD in a cross-sectional study, with AUROC of 0.84,0.65,0.65,0.77 respectively. Meanwhile, they found the AUC of TG/HDL-C in women and men was 0.85 (0.84–0.86) and 0.79 (0.78–0.80), respectively.. A perspective cohort study including 3374 Chinese adults investigated the association between nonHDL-C/HDL-C with NAFLD, with AUROC of 0.717 in women and 0.682 in men. The Jinchang Cohort study consisting of 32,121 subjects, evaluated the ability of TC/HDL-C for identifying NAFLD, and the results suggested the AUC of TC/HDL-C was 0.645. Therefore, TG/TC was more powerful for identifying NAFLD.
The underlying mechanism about the association between TG/TC and NAFLD has not been clarified. The association between TG/TC could be partly explained by elevated plasma triglycerides. Our study showed people with NAFLD had a higher triglycerides levels, which is consistent with previous study [8, 29]. Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. Steatosis develops when fatty acid (FA) input rate (uptake and synthesis and subsequent esterification to TG) was greater than fatty acid output rate (oxidation and secretion), steatosis develops . Besides, insulin resistance may play a role in the association between TG/TC and NAFLD. Insulin resistance accelerates NAFLD by inducing lipolysis of TG in adipose tissue and de novo synthesis of TG in the liver. Furthermore, IR could be accompanied by systemic inflammation, which played an important role in hepatic steatosis. Some inflammation cells activation, such as Kupfer cells, stellate cells  and circulating mononuclear cells that infiltrate the liver, could trigger TG accumulation in the liver. However, because of the lack data of serum insulin level, our study couldn’t investigate the relationship between TG/TC and insulin resistance.
The advantages of our study included that it was a large-scale cross-sectional study and first investigated the association between TG/TC and NAFLD. Furthermore, as a novel index for diagnosing NAFLD, TG/TC was radiation-free, simple, cheap, and easy to perform, compared with CT and MRI. However, some limitations of our study were existed. First, our study was cross-sectional, and the effects of TG/TC for identifying NAFLD should be test in prospective observational studies. Second, the diagnosis of NAFLD in our study was based on ultrasonic examination. The diagnosis of ultrasonography examination could also be influenced by interobserver variation, even if the radiologist were very experienced. However, the gold standard for diagnosis of NAFLD was liver biopsy, which was not appropriate as a screening tool for a population-based epidemiological study.