The gold standard for the diagnosis of NAFLD is usually considered to be liver puncture biopsy, but the feasibility of implementation in large populations is low, so it is important to find an index for the evaluation of nonalcoholic fatty liver disease. Studies suggest that FLI has a high sensitivity and specificity for the evaluation of nonalcoholic fatty liver disease, and in our population, FLI also has a high sensitivity and specificity. It can be seen that this index of FLI is also applicable to our population to some extent. In this study, the population was divided into two groups according to FLI, which to some extent can distinguish whether the study subjects have non-alcoholic fatty liver disease status, and the results showed that the elevated FLI was significantly associated with an increased probability of abnormal glomerular filtration rate estimates, which is consistent with many international studies reported. With the westernization of the dietary structure of our population and the prevalence of a more sedentary lifestyle, nonalcoholic fatty liver disease has now replaced hepatitis B as the number one chronic liver disease in China and is often the primary cause of abnormal liver enzymes in health checks. A meta-analysis showed that NAFLD affects nearly a quarter of the world's population, with prevalence rates of 27.37% in Asia, 23.71% in Europe, and 24.13% in North America(7). Data also show that the prevalence of NAFLD in Asia continues to increase(8). NASH not only leads to cirrhosis, hepatocellular carcinoma, and liver failure through nonalcoholic steatohepatitis (NASH), but also significantly increases the incidence of type 2 diabetes, metabolic syndrome, and cardiovascular disease. Lin et al. also concluded(9) that cardiovascular and cerebrovascular disease topped the list of causes of death in patients with NAFLD (35.6%), while renal disease was not negligible (7.5%).
NAFLD and chronic kidney disease(CKD) share many common risk factors, and NAFLD and CKD often co-exist, which makes the correlation between them a cause for consideration. In recent years, more and more clinical studies have suggested that NAFLD may be an independent risk factor for the development of CKD. A number of domestic and international studies have shown (10, 11) that NAFLD is significantly associated with CKD in the past. Data from Korea revealed(10) that individuals with NAFLD combined with CKD had a significant decrease in annual GFR compared to individuals without NAFLD, and the mean difference in the percentage decrease in annual GFR between individuals with NAFLD and non-NAFLD individuals remained significant in multivariate adjusted models. Mantovani et al. also found(12) that NAFLD (especially in patients with advanced NAFLD) elevates the risk of CKD by approximately two-fifths in Asian populations. Data from the united network of organ sharing (UNOS) from 2002 to 2011 showed that NASH-related cirrhosis has become a common indication for combined liver and kidney transplantation(13). Allen et al. found that cirrhosis due to NAFLD was a potential risk factor for the development of CKD compared to other causes of cirrhosis(14). Three prospective studies(15-17) showed NAFLD as a risk factor for high prevalence of KD, including Targher G (17), who followed 1760 diabetic patients for 6.5 years and found that fatty liver was significantly associated with increased incidence of CKD after correcting for a large number of confounding factors. Another prospective study also showed that NAFLD was a risk factor for a high prevalence of CKD, and after correcting for a large number of confounding factors, fatty liver was found to be significantly associated with an increased incidence of CKD (18). Foreign studies (19) showed that NAFLD was associated with the incidence of CKD (HR=1.58, 95% CI:1.51∼1.66). It indicates that NAFLD is closely associated with CKD. In a study of 1412 adult Han Chinese, Li(20) found that NAFLD diagnosed by ultrasound, independently of blood glucose levels, was closely associated with impaired renal function and that the prevalence of mild renal impairment was significantly higher in patients with NAFLD. It has been shown that the prevalence of CKD in patients with NAFLD is 21-54% compared to 3.7%-24.2% in patients without NAFLD, and that the risk of CKD in NAFLD diagnosed by ultrasound is 1.5-2 times higher than in NAFLD without NAFLD after correction for metabolic syndrome components. Another Meta-analysis also showed that NAFLD was associated with an increased prevalence and risk of CKD, and both studies showed that the severity of NAFLD was positively associated with the prevalence of CKD(2, 21).
Understanding the complex and interrelated mechanisms between NAFLD and CKD is important because of the serious public health burden of these two diseases, and the common mechanism of liver and kidney injury may be a research target for the prevention and treatment of CKD. NAFLD and CKD are characterized by ectopic toxic lipid accumulation due to disorders of hepatic and renal lipid metabolism, and trigger lipid peroxidative stress, apoptosis, inflammation and fibrosis(22). However, the pathophysiological mechanisms underlying NAFLD and CKD are not well defined at present. a review by Targher G (23) et al. provides a more comprehensive summary of the possible underlying mechanisms: (a) increased visceral adiposity and inflammation (b) insulin resistance (c) fatty liver formation (d) in the presence of increased free fatty acid (FFA) in-flow and chronic inflammation, the liver can cause an amplification of the chronic inflammatory response leading to a systemic inflammatory response, while the liver will be the target of the systemic inflammatory response (e) cytokine imbalance may lead to the development of CKD and its related diseases through a variety of deleterious pleiotropic effects, such as activation of various pro-inflammatory pathways, upregulating adhesion molecules, inducing endothelial dysfunction and oxidative stress, and decreasing lipocalin expression, thus contributing to the pathogenesis of CKD and its related diseases. (f) Decreased concentrations of plasma lipocalin(an adipose-secreting cytokine with antidiabetic and anti-inflammatory effects)may be another potential mechanism linking NAFLD and CKD.
As mentioned earlier, many studies have confirmed that NAFLD is an independent risk factor for CKD, but early CKD is also associated with high morbidity and mortality, and earlier detection of abnormal glomerular filtration rate (eGFR) would be of great significance. Machado et al (24) found in their study that patients with NAFLD diagnosed by liver biopsy were more likely to have low eGFR levels. Although many studies have confirmed that NAFLD increases the incidence and prevalence of CKD, whether NAFLD has an effect on eGFR in people with eGFR >60 mL/(min-1.73m2) has yet to be investigated, and there are few studies in this area. Therefore, the purpose of this study is to investigate the effect of NAFLD on early eGFR by studying the correlation between FLI and eGFR in the general medical examination population. It is hoped that we can intervene the related risk factors in the pre-CKD stage to reduce the occurrence of CKD and provide research targets for the prevention and treatment of CKD. At the same time, due to the asymptomatic nature of NAFLD and the lack of specific treatment methods, people do not know enough about NAFLD and do not pay enough attention to it. It is hoped that this study can enhance people's knowledge about NAFLD, and pay enough attention to NAFLD and intervene in it as early as possible.
Overall, this study found that eGFR decreases as FLI rises in an elderly population in the Yangzhou.