Non-alcoholic fatty liver disease (NAFLD), characterized with more than 5% intrahepatic fat accumulation in the absence of classical causes of steatogenesis, is considered as a growing epidemic with a global prevalence estimated around 25%. NAFLD usually manifests as simple steatosis, however in about 15-30% cases it can progress to its inflammatory type of NASH, which may in turn lead to fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Given that patients typically have no or few symptoms, it is fundamental to detect and treat NAFLD at the earliest stage to decrease liver-related morbidity and mortality.

NAFLD is known to be the hepatic manifestation of the metabolic syndrome (MetS), a cluster of metabolic disorders including abdominal obesity, type 2 diabetes mellitus (T2DM), and cardiovascular diseases (CVD). It has been reported that NAFLD risk increases with obesity, diabetes mellitus, and dyslipidemia. In line with this, the highest prevalence of NAFLD is reported from South America and the Middle East, where MetS is also widespread, making it a major public health issue in these regions. Similarly, the clinical significance of NAFLD has progressively increased in Turkey lately. With an obesity rate of 32% according to latest WHO reports, and T2DM rate of 15%, Turkey can be considered a risky region in terms of NAFLD burden. Despite the increasing burden of NAFLD in Turkey, few studies have been undertaken to identify its prevalence showing conflicting results due to excessive heterogeneity in design. 8 studies that were undertaken in the past 15 years reported a prevalence between 10,6% and 60%. One study that included subjects with MetS reported as high as 94.3%. It is critically important to develop new evidence regarding NAFLD prevalence among this patient group in Turkey and the association between MetS comorbidities and NAFLD severity so as to establish NAFLD risk groups and develop effective population screening and public health policies targeting this patient group.

The optimal screening method to detect hepatic steatosis in the general population remains to be established. The reference standard is liver biopsy; however, it is problematic to conduct studies that involve liver biopsy due to its invasiveness, high cost and patient discomfort. Therefore, ultrasonographic imaging (USG) is usually favored as the first-line modality in most clinical practices. However, USG has limited sensitivity and does not reliably detect steatosis when \<20% or in individuals with BMI\>40 kg/m2. Controlled attenuation parameter (CAP) measured with transient elastography (TE), on the other hands, has recently emerged as a reliable imaging tool for the screening and diagnosis of NAFLD on a population scale. It is suggested to have high sensitivity in identifying mild steatosis and yields a good correlation with steatosis grades. Therefore, CAP can enable early and noninvasive detection of NAFLD at the subclinical level. It is important to detect NAFLD at the earliest possible stage because once it is established hepatic insulin resistance increases, which, in turn, may trigger, in 30-40% of cases, NAFLD progression to its inflammatory phenotype Non-alcoholic Steatohepatitis (NASH) and eventually, hepatocarcinoma, cirrhosis, and liver failure.

As of today, treatment of NAFLD is only possible through lifestyle modifications such as following a healthy balanced diet and doing regular physical activity, given that no pharmacological treatment has been approved yet for NAFLD. Weight loss of 7-10% BW seems to have proven efficacy especially in overweight subjects. Different dietetic approaches have been proposed to reach the weight loss goal; however, no consensus has been reached yet on the type of medical nutrition therapy to be offered to NAFLD patients. Having said that, in the recent EASL-EASD-EASO Clinical Practice Guidelines, Mediterranean diet (MED DIET) was recommended as the diet of choice for the treatment of NAFLD together with weight loss initiatives. Indeed, a growing body of evidence suggest that MED DIET exerts health benefits even without accompanying weight reduction, which is the main obstacle in lifestyle interventions.

Higher adherence to the MED DIET has shown to have beneficial effects on the progression of hepatic steatosis in observational studies and clinical trials (n\<90 followed up for\<6 months) in patients with existing hepatic steatosis. These findings are confirmed in The Swiss CoLaus and UK Fenland cohorts which reported lower risk of hepatic steatosis with higher adherence to MED DIET.

Not only the dietary pattern, but also certain micronutrient deficiencies might be related to NAFLD. New evidence suggests that also Vitamin D deficiency, which is known to associate with obesity and sedentary lifestyle, may cause NAFLD. Hypovitaminosis D is proposed to have a causative relationship with the severity and incidence of NAFLD. The association between Hypovitaminosis D and NAFLD has been examined previously. A lower serum 25(OH)D level was shown to be an independent risk factor for NAFLD. Various studies has shown a correlation between Vitamin D level and disease severity. A cross-sectional study of 6567 men concluded that participants in the lowest tertile of serum 25(OH)D levels had a significantly increased risk for NAFLD compared to those in the highest tertile. Another study found similar results. CAP was used as a tool to define hepatic steatosis in a study that showed a significant association between serum 25(OH)D levels and NAFLD.9 However, some other studies reported quite contradictory results. Collectively, the data from the published studies indicate that NAFLD subjects are more likely to be vitamin D deficient compared to controls. However, definite directionality of the results cannot be ascertained due to the limitations which include the variability in the method of diagnosis of NAFLD, clinical heterogeneity among the study groups and variability in defining vitamin D deficiency. Demonstration of a causal role of hypovitaminosis D in NAFLD progress could have important therapeutic implications. Vitamin D supplementation is easy to accomplish on a population level, hence, it can be an efficient tool to prevent NAFLD. Cochrane performed a systematic review of randomized clinical trials of Vitamin D supplementation in chronic liver diseases, 11 of which included NAFLD patients. Given the high heterogeneity of studies included, they failed to assess the beneficial and harmful effects of vitamin D supplementation in adults with chronic liver diseases. Well-designed future studies are needed to investigate this potential relation.