Adipocyte-specific fatty acid-binding protein (A-FABP, FABP4, or aP2) is a cytoplasmic protein expressed in macrophages and adipocytes. It has been reported that A-FABP is a serum marker involved in metabolic disorders such as metabolic syndrome, type 2 diabetes and atherosclerosis, although its functional mechanisms are not fully understood [1-3].

There are some reports on the association between A-FABP and nonalcoholic fatty liver disease (NAFLD) in humans [4]. A liver biopsy is the definitive method for diagnosing NAFLD, but this technique is an invasive procedure and is not completely precise due to sampling variability. Thus some proxy markers of NAFLD have been reported. Recently, some experts have introduced new noninvasive diagnostic tools for NAFLD, the fatty liver index (FLI) [5] and the NAFLD fatty liver score [6,7]. FLI uses an equation with γ-glutamyltransferase (GGT), triglycerides, body mass index (BMI), and waist circumference. The score ranges from 0 and 100 and estimates the percentage chance of having fatty liver disease. According to the study by Bedogni et al. [5], FLI <30 rules out fatty liver as detected by ultrasonography. In a study of a population with previous gestational diabetes mellitus, Bozkurt et al. [8] found a significant correlation between FLI and hepatocellular lipid content as measured by ¹H-magnetic resonance spectroscopy (r=0.70; P<0.001; R²=0.47). In our study, we divided the study subjects into two groups using an FLI of 30 as the cutoff value on the basis of Bedogni's criterion.

To our knowledge, there have been few studies that have assessed whether FLI is related to A-FABP or other metabolic markers. The aim of this study was to investigate the relationship between serum A-FABP and FLI as indicators of NAFLD in apparently healthy Korean subjects. We also investigated the associations of FLI with other metabolic markers.

The subjects included 158 men (66.4%) and 80 women (33.6%), and the mean age was 42.8 years. Previously published studies suggested cutoff values for the FLI (i.e., FLI <30 as a rule-out criterion for fatty liver) [5]. Our subjects were divided into two groups according to FLI in 2007 using a cutoff score of 30, with 152 subjects <30 and 86 subjects ≥30. The baseline characteristics of the subjects who were subsequently diagnosed with NAFLD by FLI ≥30 and the characteristics of subjects with FLI <30 are shown in Table 1. As shown in Table 1, the mean ages of the group with FLI <30 (suggestive of non-NAFLD, i.e., normal) and the group with FLI ≥30 (suggestive of NAFLD) were not significantly different in 2003; however, the group with FLI ≥30 included a significantly higher proportion of male subjects (P<0.001).

The parameters indicating metabolic syndrome such as systolic blood pressure, diastolic blood pressure, fasting blood glucose, and triglycerides at baseline (in 2003) were significantly higher in the group with FLI ≥30 than the group with FLI <30 (P<0.001). In contrast, HDL-C levels were significantly lower in the group with FLI ≥30 (P<0.001). In addition, LDL-C, total cholesterol, fasting insulin, AST level, and ALT level were significantly higher in the group with FLI ≥30 (Table 1).

We also investigated the correlations between FLI at the 4-year follow-up and various baseline parameters, including A-FABP. In an unadjusted model, all parameters of metabolic markers had significant correlations with FLI. A-FABP also had a positive correlation with FLI. The baseline serum A-FABP was significantly associated with FLI after adjusting for age and sex (P<0.001). After adjusting for age and sex, we found that AST, ALT, fasting insulin, total cholesterol, and HDL-C were significantly correlated with FLI (Table 2). The association between FLI and ultrasonographic findings of NAFLD was significant. The median value of FLI for NAFLD on ultrasonography was higher than the normal finding on ultrasonography (26.05 [11.24 to 50.21] and 14.64 [4.79 to 37.61] respectively; P=0.006; values expressed as median [25th to 75th percentiles]).

When subjects were grouped into A-FABP tertiles, rising tertiles of A-FABP had significantly higher values of FLI even after adjusting for age and sex (P for trend <0.001) (Fig. 1). In the multiple logistic regression analysis with development of NAFLD defined by FLI greater than or equal to 30 as the dependent variable, the highest tertile of A-FABP had an increased likelihood of FLI ≥30 compared with the lowest tertile (odds ratio [OR], 1.97; P<0.05). After adjusting for age and sex, the OR for the risk of FLI ≥30 in the highest tertile of A-FABP maintained significance (OR, 2.68; P=0.012) (Table 3).

In this study, we found that serum A-FABP levels in healthy subjects are significantly associated with FLI and that there is a trend of increasing FLI score in proportion to serum A-FABP tertile. Subjects in the highest tertile of A-FABP were approximately three times more likely to have NAFLD compared with those in the lowest tertile in multiple logistic regression analysis, with development of NAFLD defined by FLI greater than or equal to 30.

Many studies have reported that A-FABP binds fatty acid ligands and functions in the regulation of lipid metabolism and gene expression [11-13]. Cao and colleagues [14] demonstrated that ob/ob-aP2-mal1-deficient mice were protected against fatty liver disease. Another study found that fatty infiltration of the liver was attenuated and total liver triglyceride content was reduced in aP2-inhibitor treated ob/ob mice [15]. In humans, there was one report about an association between A-FABP and fatty liver disease. Milner et al. [4] reported that increased serum A-FABP levels were a predictive factor for intrahepatic inflammation and fibrosis in patients with histologically confirmed NAFLD with abnormal liver function. They also reported an association between high serum A-FABP levels and progression of the disease, which was consistent with our results showing an association between A-FABP and FLI as a noninvasive marker of fatty liver disease.

Liver biopsy is the definitive method for diagnosing NAFLD, but this technique is an invasive procedure and is not completely precise due to sampling variability. Thus, some proxy markers such as the ratio of liver enzymes or ultrasonography are preferably used in epidemiology [16]. The FLI uses an equation with GGT, triglycerides, BMI, and waist circumference. It varies between 0 and 100 and is known to have significant accuracy in detecting NAFLD [17,18]. In addition, according to a study by Bedogni et al. [5], an FLI <30 rules out fatty liver. In this study, we divided the study subjects into two groups using an FLI of 30 as the cutoff value.

There are several limitations to the current study. First, the subjects in this study were selected from participants in a health check-up and are not representative of the general population. Second, there was a relatively large proportion of male subjects (66.4%). Third, we did not consider lifestyle risk factors such as exercise habits or diet. Despite with these limitations, we demonstrated a significant relationship between A-FABP levels and the development of NAFLD in healthy subjects at 4-year follow-up using the noninvasive marker FLI. Many studies on the association of A-FABP with type 2 diabetes or NAFLD as seen on ultrasonography have been reported; however, there is no study about the association between serum A-FABP levels and FLI in healthy Korean subjects.

In conclusion, this study shows that higher serum A-FABP levels at baseline were associated with FLI as a predictive indicator of NAFLD. In other words, high A-FABP levels were associated with a future risk of developing NAFLD. Further studies are needed to confirm the role of FLI as a predictive marker for NAFLD and its association with A-FABP.