Both the prevalence and the degree of NAFLD are substantially associated with obesity. It is widely recognized that abdominal obesity, particularly high visceral adipose tissue, plays a significant role in the development of metabolic disorders and NAFLD. NAFLD can proceed to steatohepatitis, liver cirrhosis, and hepatocellular failure. It is also linked to metabolic syndrome, type 2 diabetes, insulin resistance, cardiovascular disorders, and hepatoma [15,16,17].
Although the pathophysiology of NAFLD is complicated, obesity is a major risk factor for its occurrence. The adipocytes' ability to release free fatty acids into the portal venous system is activated by the high levels of abdominal fat. This may subject the liver to a significant amount of fat, resulting in NAFLD [18,19,20].
Only a few studies have examined the effect of the amount of body fat and its distribution in the development of NAFLD. We postulated that body fat distribution shown by the visceral-to-subcutaneous fat (V/S) ratio would indicate the risk for the development of NAFLD and its severity.
In the current study, we aimed to assess the relationship between the severity of fatty liver in NAFLD and the amount of visceral fat, muscle mass, and liver volume. Also, to evaluate the correlations among the increase in the liver enzymes in NAFLD, visceral and subcutaneous fat, muscle mass, and liver volume.
Based on the radiological assessment of our patients, we found that 60 (46.2%) had grade I fatty liver, 42 (32.3%) had grade II fatty liver, and 28 (21.5%) patients had grade III fatty liver. We used abdominal ultrasonography to grade fatty liver, a simple widespread noninvasive modality. It is now replacing liver biopsy in NAFLD grading because it avoids any hazards that can occur during or after liver biopsy [5].
We found that the patients with grade III fatty liver had significantly higher total fat volume, visceral fat volume, subcutaneous fat volume, visceral-to-subcutaneous fat rate, fat rate in the body, and visceral fat volume rate in comparison with those with grade I and grade II fatty liver.
Our results are based on the fat volume rather than the fat areas used in the previous studies. For more accurate results, we calculated the fat rate in the body by dividing the sum of subcutaneous and visceral fat volumes by the volume of the entire body, the V/S rate by dividing VFV by SFV, and the visceral fat volume rate by dividing VFV by abdominal cavity volume.
In line with our findings, some previous studies found that the amount of visceral fat was significantly correlated with the degree of fatty liver, the degree of liver inflammation, as well as hepatic fibrosis [5, 19,20,21,22,23,24]. Additionally, the amount of visceral and subcutaneous fat and muscle was significantly higher in patients with grade II and III compared to those with grade I fatty liver [25].
These results were confirmed histologically by Eguchi et al. who reported that the amount of visceral fat was correlated with the progress of NASH, as evaluated by the histological grade. Therefore, from the onset of fat deposition in hepatocytes to the emergence of NASH, visceral fat accumulation continually affects the histological abnormalities in NAFLD [23].
Few previous papers studied fat distribution in the body as a risk factor in the development of NASH. One of these studies reported that the visceral-to-subcutaneous fat (V/S) ratio is more important than the visceral fat area as a metabolic risk factor in the progress of NASH and liver fibrosis [26]. However, the contribution of subcutaneous fat in NAFLD is debatable because some research found no association between SFV and NAFLD [27], while others found an opposite association [28].
In our study, we found that the amount of visceral fat is more important than the amount of subcutaneous fat in the detection of the severity of NAFLD which is similar to the results of the previous research. We found the V/S rate the most significant parameter (P = 0.02) followed by VFV, VFV rate, total fat volume, fat rate in the body (P = 0.03, each), and lastly SFV (P = 0.04).
The current research also studied the correlation between the increase in liver transaminases in NAFLD and the amount of visceral and subcutaneous fat. Recent epidemiological studies have demonstrated that the rise of liver enzymes is closely linked to liver fat content, especially in patients with a very high incidence of obesity and NAFLD [29,30,31].
In this study, we found that both transaminases had a significant positive correlation with total fat volume and visceral fat volume rate. Similar to our results, some authors found a correlation between visceral fat deposition and moderate elevation of the liver transaminases [29, 32]. On the other hand, a previous study revealed no significant correlation between liver transaminase and visceral fat area [25]. However, to our knowledge, no previous work handled the correlation between liver enzymes and visceral fat volume rate in which we used the fat volume rather than the fat area.
Regarding the psoas muscle mass, we found that patients with grade III fatty liver had a significantly higher psoas muscle volume and psoas muscle ratio than those with grade I and grade II. We also found that liver transaminases significantly correlated with psoas muscle volume and psoas muscle ratio. In this study, we used psoas muscle volume which is more accurate than the psoas muscle area used in the previous studies. Our results are consistent with Chen et al. [33] and Nachit et al. [34] who found that NAFLD is strongly correlated with muscle fat rather than muscle mass. Although other studies reported that sarcopenia (decreased appendicular muscle mass) is associated with NAFLD [35, 36], they did not use muscle mass in grading NAFLD.
Regarding the liver volume, we found a strong positive correction between it and the liver transaminases but the difference in the liver volume among the different grades of NAFLD was insignificant. Only recent research studied the liver volume obtained by magnetic resonance imaging in NAFLD and reported that there is a strong correlation between liver volume, transaminases, and high risk of mortality [37].
The main limitations of this study are the lack of histopathological correlation, a single-center study, and lastly no long-term follow-up of those patients. Yet this is the first study in our locality that discussed the relation between NAFLD and visceral and subcutaneous fat and its correlations with liver transaminases.