Association between serum uric acid and nonalcoholic fatty liver disease in community patients with type 2 diabetes mellitus

Introduction

Nonalcoholic fatty liver disease (NAFLD) is a major cause of chronic liver disease worldwide (Kitade et al., 2017). The global prevalence of NAFLD was recently estimated to be 24%, with the highest rates reported in South America and the Middle East followed by Asia, the USA and Europe (Younossi et al., 2018). NALFD has a wide spectrum of manifestations ranging from simple steatosis to non-alcoholic steatohepatitis, hepatic fibrosis, liver cirrhosis and even hepatocellular carcinoma (Kitade et al., 2017; Maiuolo et al., 2016; Mikolasevic et al., 2016; Neuschwander-Tetri, 2017). Previous research has indicated that NAFLD is associated with factors related to metabolic syndrome, including obesity, dyslipidemia, impaired insulin resistance and type-2 diabetes mellitus (T2DM) (Kitade et al., 2017; Maiuolo et al., 2016; Mikolasevic et al., 2016; Neuschwander-Tetri, 2017).

Serum uric acid (SUA) is an end product generated from the metabolism of purine nucleotides (Maiuolo et al., 2016). Previous investigations have reported positive associations of SUA with obesity (Duan et al., 2015), dyslipidemia (Wang et al., 2015), insulin resistance (De Miranda et al., 2015), prediabetes (Van der Schaft et al., 2017) and T2DM (Xue et al., 2015). Furthermore, SUA was found to be an independent predictor of T2DM, metabolic syndrome (or its components) and cardiovascular mortality (Choi et al., 2016; Fu et al., 2017; Lv et al., 2013; Wang et al., 2016; Xue et al., 2015; Zheng et al., 2017). This raises the possibility that elevated SUA may be involved in the development or/and progression of these disorders.

In recent years, several published studies have highlighted a relation between SUA and NAFLD (Darmawan, Hamijoyo & Hasan, 2017; Jaruvongvanich et al., 2017; Zhou, Wei & Fan, 2016). The level of SUA is elevated in people with NAFLD (Cai et al., 2014; Lin et al., 2015; Liu et al., 2016; Moon, 2013). Moreover, an increased SUA level was reported to be associated with a higher prevalence of NAFLD in the general population (Cai et al., 2014; Cai et al., 2013; Lee et al., 2010a; Lee et al., 2010b; Li et al., 2009; Shih et al., 2015; Sirota et al., 2013; Yamada et al., 2010). Longitudinal studies have shown that an increase in the level of SUA is associated with a higher risk of new-onset NAFLD (Bai et al., 2018; Lu et al., 2016). SUA concentration is also related to greater NAFLD severity (Liu et al., 2017; Petta et al., 2011; Sirota et al., 2013). Interestingly, gender differences in the relation between SUA and NAFLD have been reported, with some studies suggesting a stronger association in men (Fan et al., 2016; Yu et al., 2017) and others a stronger association in women (Wu et al., 2015).

A small number of previous studies have indicated that SUA level is independently associated with NAFLD in patients with prediabetes (Hossain et al., 2016) or T2DM (Fan et al., 2016; Li et al., 2015). Furthermore, urine uric acid excretion is also related to NAFLD in patients with T2DM (Zhao et al., 2016). Only one previous investigation has explored whether the relation between SUA and NAFLD differs between men and women with T2DM, with the results indicating an association only in male patients (Fan et al., 2016). Nevertheless, data are very limited regarding gender-specific associations between SUA and NAFLD in people with T2DM. We hypothesized that SUA would be independently associated with NAFLD in patients with T2DM and that this association would show differences between men and women. Therefore, we investigated whether SUA was independently associated with NAFLD in patients with T2DM, and we performed a stratified analysis based on gender.

Patients and Methods

Results

Comparison of the clinical characteristics between T2DM patients with and without NAFLD

The clinical characteristics of the 597 study participants, stratified according to gender and presence/absence of NAFLD, are presented in Table 2. When compared with patients without NAFLD, those with NAFLD were younger (P < 0.001) and had a shorter T2DM disease course (P < 0.001), higher BMI (P < 0.001), larger waist circumference (P < 0.001), greater WHR (P < 0.001), lower HDL-C (P < 0.001), and higher DBP (P = 0.031), TC (P < 0.010), TG (P < 0.001), 2hPG (P = 0.032), FINS (P < 0.001), HOMA-IR (P < 0.001), ALT (P < 0.001), GGT (P < 0.001), AST (P < 0.001) and eGFR (P = 0.021). In comparison to men without NAFLD, men with NAFLD were younger (P < 0.001) and had a shorter T2DM disease course (P < 0.001), higher BMI (P < 0.001), larger waist circumference (P < 0.001), greater WHR (P < 0.001), lower HDL-C (P < 0.001), and higher TC (P = 0.019), TG (P < 0.001), 2hPG (P = 0.007), FINS (P < 0.001), HOMA-IR (P < 0.001), ALT (P < 0.001), GGT (P < 0.001), AST (P < 0.001) and eGFR (P = 0.021). Compared to women without NAFLD, women with NAFLD had a shorter T2DM disease course (P = 0.001), higher BMI (P < 0.001), larger waist circumference (P < 0.001), lower HDL-C (P < 0.001), and higher TG (P < 0.001), FINS (P < 0.001), HOMA-IR (P < 0.001), ALT (P < 0.001), GGT (P < 0.001) and AST (P = 0.023). Notably, the SUA level was significantly higher in the NAFLD group than in the non-NAFLD group (338 ± 86 vs 300 ± 86 µmol/L, P < 0.001), and this was also the case for women (351.6 ± 88.1 vs 316.1 ± 76.4 µmol/L, P < 0.001) and men (321.1 ± 80.1 vs 283.1 ± 92.8 µmol/L, P < 0.001).

Discussion

An important finding of this cross-sectional study of patients with T2DM in the community was that SUA was higher in the NAFLD group than in the non-NAFLD group for both women and men. Furthermore, SUA correlated positively with BMI and TG and negatively with eGFR in both genders. Logistic regression analysis with adjustment for age, duration of T2DM, BMI, SBP, alcohol intake and eGFR showed that SUA tertiles 2 and 3 in women and SUA tertile 3 in men were associated with significantly increased odds of NAFLD (as compared with SUA tertile 1), with a trend toward increased odds for tertile 2 in men. After rigorous adjustment for multiple clinical parameters, a significant increase in NAFLD prevalence was only observed for SUA tertile 3 in women. However, there were clear trends suggesting an increased prevalence of NAFLD for tertile 2 in women (P = 0.122) and tertiles 2 and 3 in men (P = 0.074 and 0.118, respectively) as compared with tertile 1. Taken together, our data suggest that although there seems to be an association between SUA and NAFLD that is stronger in women than in men, this association may largely be mediated by other metabolic factors.

NAFLD is the most common cause of chronic liver disease globally, and the positive association between NAFLD and T2DM is well established (Kitade et al., 2017; Mikolasevic et al., 2016; Neuschwander-Tetri, 2017). The prevalence of NAFLD in the 597 patients with T2DM in our study was 59.0%, consistent with a previous meta-analysis of 24 studies that yielded a pooled prevalence of 59.7% (95%CI [54.3–64.9]%) (Dai et al., 2017). This illustrates the high prevalence of NAFLD in patients with T2DM, highlighting the importance of identifying factors predictive of incidental NAFLD.

Many clinical investigations have reported that an elevated level of SUA is associated with a higher prevalence of NAFLD (Cai et al., 2014; Cai et al., 2013; Lee et al., 2010a; Lee et al., 2010b; Li et al., 2009; Lin et al., 2015; Liu et al., 2016; Moon, 2013; Shih et al., 2015; Sirota et al., 2013; Yamada et al., 2010). Moreover, an increased SUA level is related to a higher risk of new-onset NAFLD (Bai et al., 2018; Zhou, Wei & Fan, 2016) and more severe NAFLD (Liu et al., 2017; Petta et al., 2011; Sirota et al., 2013). This previous research not only suggests that measurement of SUA could be used to predict the presence or future occurrence of NAFLD but also implicates SUA as possibly contributing to the development of NAFLD. Some preclinical studies have suggested that uric acid might directly influence fat accumulation and hepatic steatosis by inhibiting insulin signaling to cause insulin resistance (Zhu et al., 2014), inducing mitochondrial oxidative stress (Lanaspa et al., 2012) or generating endoplasmic reticulum stress (Choi et al., 2014). It has even been proposed that suppression of SUA levels might be a potential new therapy for NAFLD (Sun et al., 2016). Nonetheless, the mechanisms linking elevated SUA levels and NAFLD have not yet been fully characterized in the clinical setting, and it is possible that the effects of SUA are not direct but are mediated via other risk factors for NAFLD. In this study, Pearson correlation analysis revealed that SUA level was positively correlated with BMI, TG and HOMA-IR and negatively correlated with eGFR in both men and women. Furthermore, SUA was also negatively correlated with HDL-C in women, with a trend toward a negative correlation in men (P = 0.082). Our findings are broadly consistent with a previous study of patients with T2DM in China, which also suggested that SUA was positively correlated with BMI and TG and negatively correlated with eGFR and HDL-C (Fan et al., 2016). Higher BMI, elevated TG and impaired insulin resistance are factors known to be associated with an increased risk of NAFLD (Kitade et al., 2017; Mikolasevic et al., 2016; Neuschwander-Tetri, 2017), raising the possibility that the relation between SUA and NAFLD is mediated, at least in part, by these other factors.

Comparisons of NAFLD prevalence between SUA tertiles suggested that the association between elevated SUA level and increased prevalence of NAFLD was stronger in women with T2DM than in men with T2DM (Fig. 1), and this possibility was supported by the results of the univariate analysis (Table 1). Previous studies of general populations have reported conflicting results regarding sex differences in the association between SUA and NAFLD. A large-scale study of people attending medical centers in China found that the association between elevated SUA and increased prevalence of NAFLD was greater in females than in males (Wu et al., 2015), in broad agreement with our findings. However, another study in China concluded that the relationship between elevated SUA and increased prevalence of NAFLD was stronger in males than in females (Yu et al., 2017), which would not be consistent with our current data. Furthermore, a previous investigation of patients with T2DM in China determined that the association between elevated SUA and increased prevalence of NAFLD was stronger in men than in women (Fan et al., 2016). In fact, this latter study found no significant differences in NAFLD prevalence between tertiles of SUA in women (despite a trend toward increasing NAFLD prevalence with increasing SUA tertiles), whereas significant differences were detected for men (Fan et al., 2016). Furthermore, univariate logistic regression analysis revealed a significant association between increasing SUA tertile and higher NAFLD prevalence for men but not for women (Fan et al., 2016). Interestingly, after adjustment for age, duration of T2DM, SBP, DBP and BMI, SUA tertiles showed significant associations with NAFLD in men but not women, and this gender difference was maintained after further adjustment for T2DM therapy, FPG, HbA1C, TG, TC, LDL-C, HDL-C, eGFR and HOMA-IR (Fan et al., 2016). By contrast, the present study found no significant independent associations of SUA tertiles with NAFLD in men after adjustment for age, duration of T2DM, BMI, SBP, alcohol intake, eGFR, TG, HbA1c, HOMA-IR, statin use, anti-hypertensive drug use and history of hyperuricemia, while a significant association in women was observed only for tertile 3 (i.e., increased odds of NAFLD in tertile 3 vs. tertile 1). Since both our study and that of Fan et al. (2016) used comparable criteria to enroll Chinese patients with T2DM, it is perhaps unlikely that the differences in results were due to notable dissimilarities in population characteristics. Instead, the apparent discrepancies between our results and those of Fan et al. (2016) may reflect differences in the analytical methods used. For example, Fan et al. (2016) adjusted for fewer factors in the multivariate analysis and defined the SUA tertiles using different ranges (SUA tertiles 1, 2 and 3 were defined as ≤265.6, 265.6–338.4 and >338.4 µmol/L for men and ≤232.0, 232.0–299.4 and >299.4 µmol/L for women). An additional potential limitation of the study by Fan et al. (2016) is that they did not adjust for ALT and AST. Elevated levels of liver enzymes are known to be associated with an increased presence and severity of NAFLD (Bi et al., 2014; Chien-Min & Cheng-Chuan, 2017; Tasneem, Luck & Majid, 2018). Furthermore, a study of patients with T2DM found that those with NAFLD had significantly higher levels of ALT, AST and GGT (consistent with our data), and SUA concentration was positively correlated with the levels of all three liver enzymes (Chien-Min & Cheng-Chuan, 2017). Therefore, it is also possible that the levels of these liver enzymes may mediate part of any association between SUA tertiles and NAFLD. However, in our analysis, additional adjustment for AST and ALT did not alter the finding that tertile 3 was associated with significantly increased odds of NAFLD (vs. tertile 1) in women.

Overall, our findings suggest that the association between elevated SUA and increased prevalence of NAFLD is at best weak after rigorous adjustment for relevant metabolic factors. Nonetheless, it should be noted that we did observe a clear trend toward an association between higher SUA tertile and higher prevalence of NAFLD in both men and women even after adjustment for numerous metabolic factors. Thus, it cannot be completely excluded that our study was underpowered to detect real independent associations of SUA tertiles with NAFLD prevalence.

Previous research has indicated that hyperuricemia is common in patients with T2DM. For example, the prevalence of hyperuricemia in patients with T2DM was reported to be 45.5% in Indonesia (Sunjaya & Sunjaya, 2018) and 32.6% in China (Wang et al., 2013). This contrasts with our finding that the prevalence of hyperuricemia was only 1.5% (9 of 597 patients). The likely reason for this apparent discrepancy is that our study did not enroll patients who were taking medications that might affect SUA, and this would have excluded patients with hyperuricemia who were receiving therapy with uric acid-lowering drugs.

This study has some limitations. First, this was a single-center study so the generalizability of the findings remains unknown. Second, the sample size was small; hence it is possible that the study was underpowered to detect some real differences between groups. Third, this was a cross-sectional study, preventing any conclusions being drawn regarding cause and effect. Fourth, the diagnosis of NAFLD was made using ultrasonography, but this technique may miss cases of mild liver steatosis detectable by liver biopsy. Fifth, due to the diagnostic method used, it was not possible to examine the relationship between SUA and NAFLD severity.

Conclusion

Our study provides evidence of a positive association between SUA level and NAFLD prevalence that may be stronger in women than in men. This association may be largely mediated by other metabolic factors since SUA tertiles were not independently associated with NAFLD in multivariate analyses, except for tertile 3 in women, after adjustment for these metabolic factors. Nonetheless, considering the trends toward associations for tertiles 2 and 3 in men and tertile 2 in women, it is possible that our study was underpowered to detect a real independent association between elevated SUA level and increased NAFLD prevalence.

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