Epicardial fat thickness and nonalcoholic fatty liver disease in obese subjects

Authors


  • Disclosure: The authors declared no conflict of interest.

Abstract

Objective

Ectopic fat accumulation within the heart and the liver are linked to an increased cardiovascular risk. Ultrasound-measured cardiac and liver steatosis are easily accessible markers of intra-organ ectopic fat accumulation. The hypothesis that echocardiographic epicardial fat thickness is independently associated with nonalcoholic fatty liver disease (NAFLD) in obese subjects is tested.

Design and Methods

Sixty-two obese (BMI > 30 kg m−2) subjects with ultrasonographic evidence of NAFLD and 62 control obese subjects without history or signs of NAFLD underwent echocardiographic epicardial fat thickness measurement.

Results

Epicardial fat thickness was significantly higher (P < 0.01) in obese subjects with NAFLD when compared to those without NAFLD. Epicardial fat thickness was significantly higher (9.7 ± 0.2 vs. 8 ± 0.7 mm, P < 0.01) in subjects with severe (ultrasound score 3) than those with moderate (score 2) liver steatosis. Among waist circumference and BMI, epicardial fat thickness resulted in the best independent correlate of liver steatosis (R2 = 0.77, P < 0.001).

Conclusions

Our study suggests that epicardial fat is a good predictor of liver steatosis in obese subjects. Echocardiographic epicardial fat predicts ultrasound-measured fatty liver better than BMI or waist circumferences does. Patients with severe fatty liver infiltration presented with the highest amount of cardiac fat accumulation.

Introduction

Ectopic fat accumulation within and around key organs such as the heart and the liver are linked to an increased cardiovascular risk [1-6]. Liver and cardiac steatosis may coexist and interplay as we and others previously described [7-9].

Nonalcoholic fatty liver disease (NAFLD) is one of the most common manifestations of excessive fat accumulation in the liver [4]. NAFLD can be accurately evaluated by ultrasonography [10]. However, whilst NAFLD is a traditional and established model of organ-specific fat accumulation, only more recently the research and clinical interest was focused to the fatty infiltration of the heart [1, 2]. Fatty heart is often and best represented by the epicardial fat, the true visceral fat of the heart [1, 2]. Epicardial fat displays peculiar biomolecular and anatomic characteristics, as its direct contiguity to myocardium, and nicely reflects the intramyocardial fat accumulation [11]. Epicardial fat thickness is easily and accurately measured by ultrasonography, as we first developed [12]. Both ultrasound-measured cardiac and liver steatosis are easily accessible markers of intra-organ ectopic fat accumulation.

Nevertheless, whether epicardial fat and fatty liver are related is unknown. Hence, our goal is to test the hypothesis that echocardiographic epicardial fat thickness is independently associated with NAFLD in obese subjects.

Subjects

We studied 62 consecutive obese (BMI > 30 kg m−2) subjects with ultrasonographic evidence of NAFLD and 62 control obese subjects without history or ultrasonographic signs of NAFLD. Both groups of obese subjects were recruited from an outpatient population who was referred for routine evaluation. None of the obese subjects had history, clinical signs or symptoms of coronary artery disease, cerebral vascular diseases, renal or endocrine diseases.

Study design

This was an observational, correlative study. Each subject, from both NAFLD and no NAFLD group, underwent echocardiography to measure epicardial fat thickness and liver ultrasound to confirm or to exclude NAFLD. Anthropometrics and blood tests were also obtained from both groups.

Inclusion criteria

Obese subjects with NAFLD met the following inclusion criteria: normal serum liver enzymes, no history of current or past excessive alcohol drinking as defined by an average daily consumption of alcohol < 30 g die−1 in men and < 20 g die−1 in women; negative tests for the presence of hepatitis B surface antigen and antibody to hepatitis C virus; absence of history and findings consistent with cirrhosis and other chronic liver diseases.

The occurrence of metabolic syndrome was identified by the presence of three or more of the following parameters: waist circumference >88 cm in women and >102 cm in men, fasting glucose ≥110 mg dL−1, blood pressure ≥130/85 mm Hg, high-density lipoprotein cholesterol (HDL-C) < 40 mg dL−1 in men and <50 mg dL−1 in women, triglycerides ≥150 mg dL−1, as defined by the National Cholesterol Education Program's Adult Treatment Panel III report (NCEP-ATP III).

Methods

Epicardial fat thickness

Transthoracic 2D guided M-mode echocardiogram was performed using commercially available equipment (Philips iE33 2006 (USA). Standard parasternal and apical views were obtained in the left lateral decubitus position. All echocardiograms were recorded and analyzed offline for epicardial fat thickness quantification, according to the method previously described and validated by Iacobellis [12]. Epicardial fat was identified as the echo-free space between the outer wall of the myocardium and the visceral layer of pericardium. Epicardial fat thickness was measured perpendicularly on the free wall of the right ventricle at end-systole in three cardiac cycles. Maximum epicardial fat thickness was measured at the point on the free wall of the right ventricle along the midline of the ultrasound beam, perpendicular to the aortic annulus, used as anatomical landmark for this view. For the midventricular parasternal short-axis assessment, maximum epicardial fat thickness was measured on the right ventricular free wall along the midline of the ultrasound beam, perpendicular to the ventricular septum at mid-chordal and tip of the papillary muscles level, as anatomic landmark. The average value of three cardiac cycles from each echocardiographic view was considered. Concordance of long and short-axis epicardial fat measurement was excellent.

NAFLD

Liver ultrasound was performed with an Esaote Medica apparatus equipped with a convex 3.5 MHz probe. NAFLD was defined by ultrasonographic (US) detection of hepatic steatosis in the absence of other liver diseases according to the criteria defined by Saverymuttu et al. [13]. Ultrasound scans were performed by the same operator who was unaware of the aims of the study and blinded to laboratory values. Liver steatosis was scored semiquantitatively on a scale of 0-3; 0, absent; 1, mild; 2, moderate; 3, severe on the basis of abnormally intense, high level echoes arising from the hepatic parenchyma, liver–kidney difference in echo amplitude, echo penetration into deep portion of the liver and clarity of liver blood vessel structure, according to the methodology previously described [13].

Anthropometric measurements

Weight and height were measured while the subjects were fasting and wearing only their undergarments. BMI was calculated as body weight divided by square of the height. Minimum waist circumference (in centimeters; minimum circumference between the lower rib margin and the iliac crest, midwaist) was measured while the subjects were standing with their heels together.

Blood tests

Blood samples for serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST), glucose, insulin, total cholesterol, HDL-C, low-density lipoprotein cholesterol (LDL-C), triglycerides were obtained after a 12-h overnight fasting. Blood tests were analyzed according to standardized procedures as previously described [8].

Statistical analysis

Data in the text and in the tables are expressed as mean and SD. Comparisons for variables between the two groups of patients was performed using t test. ANOVA test was used to compare the three groups of subjects (no NAFLD, moderate NAFLD and severe NAFLD), as data were normally distributed. Linear regression analysis was performed to evaluate the relationship among variables in all subjects. Multiple linear regression models were used to identify the best independent correlates of liver steatosis. Two-tailed P < 0.05 indicated statistical significance. Analysis was performed using Stata 5.0 (Stata, College Station, TX).

Results

The main characteristics of study subjects are summarized in Table 1. There was no significant difference in age, sex distribution, BMI, waist circumference, prevalence of metabolic syndrome (75% of vs. 72%), diabetes, dyslipidemia and hypertension between obese subjects with NAFLD and obese individuals without NAFLD. Also, there were no statistically significant differences in glucose-, lipid-, and blood pressure-lowering medications between the two groups.

Table 1. Clinical features of study subjects
VariablesNAFLD; obese (n = 62)No NAFLD; obese (n = 62)P
  1. BMI (body mass index); Waist (waist circumference); alanine aminotransferase (ALT); aspartate aminotransferase (AST); high-density lipoprotein cholesterol (HDL-C); low-density lipoprotein cholesterol (LDL-C).

Age (years)43.9 ± 9.344 ± 8.5ns
Female/male20/4217/45ns
BMI (kg m−2)36.2 ± 2.735 ± 3ns
Waist (cm)110 ± 4110 ± 3ns
ALT (U I−1)36.6 ± 334.5 ± 3ns
AST (U I−1)36 ± 2.435.8 ± 2.7ns
Fasting glucose (mg dL−1)116 ± 11110 ± 10ns
Fasting insulin (μU mL−1)29.2 ± 427.5 ± 3ns
Total cholesterol (mg dL−1)239 ± 13230 ± 19ns
HDL-C (mg dL−1)39.9 ± 539 ± 7ns
LDL-C (mg dL−1)148 ± 14145 ± 10ns
Triglycerides (mg dL−1)251 ± 29238 ± 35ns
Epicardial fat (mm)8.7 ± 17.5 ± 1.50.01

NAFLD

Ultrasound-measured liver steatosis was scored as severe [3] in 28 [45%] and moderate [2] in 34 [55%] obese subjects with NAFLD, according to the method previously described [13].

Epicardial fat thickness

Epicardial fat thickness ranged from 6.5 to 10 mm in NAFLD obese subjects. Epicardial fat thickness was significantly higher (P < 0.01, Table 1) in obese subjects with NAFLD when compared to those without NAFLD. When NAFLD subjects were stratified according to the degree of liver steatosis, epicardial fat thickness was significantly higher (9.7 ± 0.2 vs. 8 ± 0.7 vs. 7.5 ± 1.5 mm, P < 0.01) in subjects with severe than those with moderate and no liver steatosis (Figure 1).

Figure 1.

Epicardial fat thickness in subjects with severe, moderate and no liver steatosis. NAFLD subjects were stratified according to the ultrasound degree of liver steatosis. Epicardial fat thickness was significantly higher (9.7 ± 0.2 vs. 8 ± 0.7 vs. 7.5 ± 1.5 mm, P < 0.01) in subjects with severe (severe NAFLD) than those with moderate (moderate NAFLD) and no liver steatosis (no NAFLD).

Simple regression analysis

Simple regression analysis showed that epicardial fat thickness and liver steatosis were very well-correlated r = 0.85, P < 0.01.

Multiple regression analysis

Epicardial fat thickness, waist circumference and BMI, were entered in a model of multiple regression analysis to predict ultrasound-measured liver steatosis, the dependent variable. Epicardial fat thickness was the best independent correlate of liver steatosis (R2 = 0.77, P < 0.001, β = 6.70) (Table 2).

Table 2. Multivariate analysis of ultrasound-measured liver steatosis
Independent variablesβP
  1. Epicardial fat thickness, waist circumference and BMI, were entered in a multiple regression analysis to predict ultrasound-measured liver steatosis, the dependent variable. Epicardial fat thickness was the best independent correlate of liver steatosis.

Epicardial fat thickness6.7<0.001
BMI (kg m−2)1.20.12
Waist (cm)1.00.28

Discussion

Ectopic fat accumulation is an emerging cardio-metabolic risk factor. Given this concept, the need of easily accessible markers of ectopic fat accumulation was compelling. Ultrasound procedures measuring liver and cardiac fat have been therefore developed and validated [12, 13].

Nevertheless, whether ultrasound-measurements of ectopic cardiac and liver fat may be related was unclear.

Our study suggests that epicardial fat is a good predictor of liver steatosis in obese subjects. Echocardiographic epicardial fat predicts ultrasound-measured fatty liver better than BMI or waist circumferences does. Interestingly, when patients were stratified by the ultrasound score those with severe fatty liver infiltration presented with the highest amount of cardiac fat accumulation. We believe that our findings are novel and with potential immediate clinical application.

NAFLD is caused by the excessive accumulation of fat in the liver and commonly associated with obesity and metabolic syndrome. NAFLD can be considered the hepatic expression of the metabolic syndrome [14, 15].

Epicardial adipose tissue is a “novel” visceral fat depot with peculiar anatomic and biochemical features [1, 2]. While liver steatosis was traditionally considered, both liver and epicardial fat accumulation have been now related to an increased cardiovascular risk [16-18].

The relation of epicardial fat with liver steatosis was previously evaluated. Our group was the first to emphasize the relationship between ultrasound-measured epicardial fat, as a cardiac steatosis index, and serum transaminases, as markers of hepatic steatosis in subjects with increased visceral adiposity [8]. This correlation was independent of obesity and rather related to the excessive visceral fat. Lai et al reported graded increases in fasting glucose, insulin resistance, and alanine transaminase levels across higher tertiles of epicardial fat thickness [19]. Kankaanpää et al showed that epicardial fat, as measured with magnetic resonance imaging, and the degree of hepatic steatosis were correlated with abdominal adiposity and hypertriglyceridemia [20]. Perseghin et al. described that subjects newly diagnosed with fatty liver had higher mediastinal visceral fat and abnormal left ventricle energy metabolism [21]. Recently, a reduced coronary flow reserve has been reported and attributed to a complex interplay between epicardial fat, serum vaspin levels, and fatty liver in subjects with NAFLD [22]. Hyperuricemia has been recently suggested as an independent risk factor for NAFLD [23, 24]. However, a relation between epicardial fat and uric acid in subjects with NAFLD is still unexplored.

As recently highlighted, obesity leads not only to increased fat depots in classical adipose tissue locations, but also to significant lipid infiltration within internal organs. Fatty infiltration may also occur within the heart and because cardiomyocytes have a very limited capacity to store excess fat, they may undergo steatosis [1]. We were the first to show an independent and significant relation of ultrasound-measured epicardial fat and intra-myocardial lipid content [11]. Consistently with this notion, not only the epicardial, but also the intra-myocardial fat has been related to the liver steatosis [25]. However, if compared to our results, this study found that intra-myocardial was a weaker predictor of the liver steatosis, although statistically significant [25]. In addition, bariatric surgery-induced weight loss reduced epicardial [26] and liver, but not intra-cardiac fat [27]. Given its peculiar anatomical and functional properties, it is plausible that epicardial fat can rapidly decrease, probably more rapidly than an intra-myocellular lipid accumulation, during weight loss interventions, as we previously demonstrated [28]. Ultrasound can easily monitor changes in epicardial fat thickness during these interventions. Others described a reduction in liver fat, but no or opposite effect on intra-myocardial or pericardial fat during treatment with glitazones [29, 30]. We previously underlined that pericardial and epicardial fat are substantially different [31] and therefore they may differently respond to pharmacological interventions. Interestingly, epicardial fat inflammatory secretome improved with pioglitazone treatment [32].

Although this study was not aimed to elucidate cause-effect pathways, some possible mechanisms could be evoked to explain, at least partially, the relationship of epicardial and liver fat. First, both are organ-specific fat depots and markers of visceral adiposity. Certainly epicardial fat and fatty liver share similar biochemical properties with the intra-abdominal visceral fat [1]. Cardiac and hepatic fat are associated with insulin resistance and lipotoxicity [20]. The accumulation of triglyceride around the myocardium and liver is related to free fatty acids exposure. It is interesting to note that, among other visceral fat depots, epicardial fat is the highest source of free fatty acid levels in humans [1]. Albeit the reasons are probably more complex, we believe that the poor accuracy of the current anthropometric markers in reflecting organ-specific adiposity may explain why some obese subjects showed NAFLD and greater epicardial fat whereas some others did not, despite a similar BMI and waist circumference.

Different imaging methods are available to measure epicardial fat [12]. In this study we chose the echocardiographic method that we first developed and validated. Whilst computed tomography or magnetic resonance can certainly provide a volumetric assessment of the epicardial fat, echocardiography has the great advantage of being readily accessible, no invasive and at lower cost. Ultrasound still provides accuracy of the measurement and ability to clearly distinguish epicardial from the pericardial fat thickness [12].

We would like to emphasize the clinical implication of our data. Our reader may argument the reason of measuring epicardial fat as marker of liver steratosis instead of detecting it directly with a liver ultrasound. Obviously, we are not suggesting of replacing the ultrasound assessment of liver fat with echocardiography. Our input is to consider that ultrasound measurement of epicardial fat may serve as additional marker of ectopic fat accumulation and nicely reflect fatty liver infiltration, too. All of this may come together with the advantage of obtaining important information on cardiac morphology and function from the standard echocardiography.

Hence, our data suggested that echocardiography-measured epicardial fat thickness is independently associated with fatty liver disease. It measurement can be clinically feasible and represent an additional tool for the stratification of the cardiometabolic risk in obese subjects.

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