Vitamin D Status and Bone Turnover Markers in Patients With Non Alcoholic Fatty Liver Disease

Non Alcohlic Fatty Liver (NAFLD) is a spectrum of diseases that ranges from accumulation of fat in the liver (Hepatosteatosis) that may be accompanied by inflammation (Steatohepatitis) to necrosis, fibrosis and even cirrhosis resembling alcoholic hepatitis in the absence of alcoholic abuse (Pardee et al., 2012).

It has been estimated that the global prevalence of NAFLD is as high as one billion. In the United States, NAFLD is estimated to be the most common cause of chronic liver disease, affecting between 80 and 100 million individuals, among whom nearly 25% progress to NASH (Loomba et al., 2013).

In general, the prevalence of NAFLD has increased over the last 20 years. The Middle East and South America have the highest NAFLD prevalence at 31% and 32% respectively with the lowest prevalence in Africa at 13.5% (Younossi et al., 2016).

Liver biopsy (LB) is still the standard test of NAFLD diagnosis and the presence of early liver fibrosis. However, histologic lesions are not evenly distributed throughout the liver. A sampling error is the biggest limitation in the diagnosis of NAFLD by LB with inflammatory lesions and ballooning degeneration potentially resulting in misdiagnoses and staging inaccuracies (Lee et al., 2016).

To overcome these limitations, several non-invasive markers have been used instead of liver biopsy. These methods are either laboratory markers or imaging modalities. Controlled attenuation parameter (CAP) is a new technology based on the principle of the ultrasonic attenuation of transient elastography depending on the viscosity [fat] of the medium [liver] and the distance of propagation of the ultrasonic signals into the liver, providing a useful method for the quantitative detection of liver fat content and is considered a better assessment method for hepatic steatosis. Compared with ultrasound, this technology improves the sensitivity and specificity for the diagnosis of fatty liver and can be used for universal screening, diagnosis, and follow-up in NAFLD patients (Sasso et al., 2016).

NAFLD is known to be closely associated with metabolic conditions, including insulin resistance, abdominal obesity, dyslipidaemia and type 2 diabetes, and is thus regarded as the hepatic manifestation of the metabolic syndrome (Ballestri., 2016). In recent epidemiological studies, NAFLD was shown to be connected with diseases that are usually not dependent on obesity, such as sarcopenia and osteoporosis (Poggiogalle et al., 2017).

Osteoporosis is becoming a public health problem all over the world. Disability resulting from low-energy fractures, e.g: hip or vertebral fractures, is the major concern for early detection and treatment. It is estimated that osteoporosis affects 200 million women worldwide (Kanis et al., 2007).

Liver is the source of many proteins and is the regulator of several pathways involving bone metabolism; one of the most well-known of all is vitamin D metabolism pathway. Considering the role of liver in bone metabolism, the association between NAFLD and bone abnormalities is not surprising especially with substantial supporting evidences in recent years (Eshraghian et al., 2017).

Besides its role in the calcium and bone metabolism, vitamin D may also exert pleiotropic effects in many tissues. NAFLD patients were reported to have a marked reduction in serum 25(OH) vitamin D when compared with controls (Yilmaz et al., 2011).

In adults, bone is constantly being remodeled, first being broken down (bone resorption) and then being rebuilt (bone formation). The resorption and reformation of bone is important for repair of microfractures and to allow modification of structure in response to stress and other biomechanical forces. Bone formation is normally tightly coupled to bone resorption, so that bone mass does not change. Bone diseases occur when formation and resorption are uncoupled. Several assays are available that measure bone turnover markers (BTMs). These assays measure collagen breakdown products and other molecules released from osteoclasts and osteoblasts during the process of bone resorption and formation. Markers that are specific to bone formation include bone-specific alkaline phosphatase (BSAP), osteocalcin, and N-terminal propeptide of type I procollagen (PINP); markers specific to bone resorption include N-terminal telopeptide of type I collagen (NTX), C-terminal telopeptide of type I collagen (CTX), and pyridinoline cross-links (Rosen et al., 2019).