In this study, we found that serum TBA was significantly associated with NASH in NAFLD population. Firstly, NASH patients exhibited higher serum TBA level compared to NAFL patients. Secondly, the prevalence of NASH, DM, MetS and hypertriglyceridemia increased with the increasing of serum TBA level in a dose-dependent manner. Most importantly, multivariate logistic regression analysis showed that elevated serum TBA level was independent factor for NASH. Moreover, further stratification analysis showed that serum TBA was independently associated with NASH in non-diabetes patients, while not in diabetes patients.
In a recent large-scale study, which involved in 152,336 participants, 27.5% of the study population had NAFLD[17]. The level of serum TBA in NAFLD patients was significantly higher than health population (3.4 vs.3.0 umol/L, p<0.001), however serum TBA level was not independently associated with NAFLD in multivariate regression analysis. In our study, the potential association between serum TBA level and NASH was investigated in the NAFLD population. The level of serum TBA was higher in NASH patients than in NAFL patients, which was consistent with the previous study from small sample[9, 18], and our data further confirmed that the hyperbileacidemia was independently associated with NASH.
The protective or adverse role of bile acid on the development of NAFLD remained controversial. The increased serum bile acids level in the high-fat diet (HFD)-induced obese C57BL/6 mice was correlated with weight loss after vertical sleeve gastrectomy. The bile acids and FXR signalling, which regulate the glucose and lipid metabolism, was served as an important molecular underpinning for the beneficial effects of this weight-loss surgery[19]. It has recently been shown that bile acids modulate insulin signalling by activation of FXR and can improve insulin resistance in cell-based and animal studies[20]. However, the FXR signalling pathway was suppressed in NASH patients compared to NAFL or health control[21]. The primary bile acids were increased while second bile acids were decreased in NASH. Moreover, the ratio of total cholate to total chenodeoxycholate was decreased in NASH[11]. Another study observed the increased FXR antagonistic deoxycholic acid and decreased FXR agonistic chenodeoxycholate in percentage, by which inhibited the FXR signalling in the liver of NASH patients[21]. Several studies demonstrated that the level of bile acid was positively correlated with the fibrosis of NASH. In the present study, although the liver fibrosis was not detected in NASH patients, the prevalence of NASH, MetS, DM and hypertriglyceridemia were increased with the increase of serum BAs level, thus the results from our large population supported that the FXR signalling pathway was suppressed in NASH patients.
An interesting finding from our study was that elevated serum TBA was an independent factor for NASH in non-diabetes subjects, while not in diabetes population. Although the bile acid signalling can modulate glucose metabolism, the studies on the relationship between bile acid and diabetes were not consistent. Some studies showed no changes in fasting bile acid level[22, 23], while others studies reported increased fasting bile acid in diabetes patients compared to health control[24, 25]. The recent large-scale population study showed no association between serum total bile acid and NAFLD[17]. Differences in control populations and the degree of insulin resistance or T2DM severity are potential confounding factors between these studies. In the present study, the diagnosis criteria of NASH were based on abnormal ALT, AST and r-GT level. Hyperglycemia can induce elevation of ALT and AST in some diabetes patients, but without obvious alteration of liver histology[26]. These patients might be falsely classified into NASH group, which might introduce bias in logistic regression analysis. Nevertheless, several noninvasive models for predicting NASH or advanced liver fibrosis of NAFLD showed lower performance in diabetes patients than in non-diabetes patients[27, 28]. Thus, the contribution of bile acid to the NASH development in diabetes patients need to further clarify.
There are some limitations to the present study. Firstly, in the present study, NASH was diagnosed based on the levels of serum ALT, AST and r-GT, which might exclude some NASH patients with normal liver function. Liver biopsy is “gold standard” for diagnosis and assessment of NASH, but it is not feasible in the general population for medical examination. Secondly, the composition of serum BAs was not detected in the present study. The alteration of bile acid composition may be more relevant to metabolic homeostasis but the serum TBA was more widely used in the clinic, some of which or its combination might provide a more valuable noninvasive diagnosis approach.
In conclusion, the serum TBA level was higher in NASH than in NAFL patients, and the elevated serum TBA was independently associated with NASH in non-diabetes population. These results need further validation in biopsy-proven NAFLD subjects. Furthermore, measuring levels of specific serum BAs might provide more information for identifying NASH patients, and deserve further research.