Non-Alcoholic Fatty Liver Disease (NAFLD) describes a wide range of hepatic pathological conditions beginning with accumulation of lipids, especially triglycerides, in the hepatocytes in the absence of significant alcohol intakeReference Sanyal1. NAFLD shows a strong association with obesity, type 2 diabetes (T2D), insulin resistance (IR), hyperlipidaemia and arterial hypertension. Hence it is referred to as the hepatic manifestation of the metabolic syndromeReference Loomba2. Studies have shown significant inverse associations between 25-hydroxyvitamin D (25(OH)D) concentration and other diseases including diabetes, hyperlipidaemia, hypertension and peripheral vascular disease, suggesting a possible role for vitamin D in the pathogenesis of NAFLDReference Anderson3, Reference Li4.The aim of this study was to systematically review the association between NAFLD and vitamin D using data from both randomised control trials (RCT), cross-sectional and case-control studies. This was to quantify differences in 25(OH)D status between individuals with and without NAFLD, as well as the effect of vitamin D supplementation in NAFLD patients on metabolic function.
The PUBMED database was electronically searched for relevant studies in adolescents and adults from inception up until April 2017. A total of 129 relevant studies were identified and 24 of these were suitable for inclusion in the systematic review.
Meta-analysis of cross-sectional, cohort and case control studies suggested a statistically significantly lower serum 25 (OH) in those with NAFLD, compared with those without NAFLD (Mean Difference (random)= -16.80 nmol/L [-24.38, -9.21] I2=96 % P <0.0001). In addition, meta-analysis of 4 randomised control trials (RCTs) suggested a statistically significant effect of vitamin D supplementation on serum 25 (OH) D levels (Mean Difference (random)= 53.08nmol/L [28.22,77.93] I2=96 % P < 0.0001), Furthermore, meta-analysis of HOMA-IR suggested no statistically significant effect of vitamin D supplementation (Mean Difference (random) = −0.26 [-2.02, 1.51] I2= 99 % P = 0.78).
In conclusion, 25(OH)D status was lower in individuals with NAFLD than in those without NAFLD, suggesting that serum 25(OH)D levels may be a factor in the development of NAFLD. However, it may also simply be a result of persons with NAFLD having higher adiposity than those without NAFLD, as increased adiposity is associated with reduced 25(OH)D concentration. This is the first study to report a meta-analysis of vitamin D supplementation in NAFLD patients. No effect was found of vitamin D supplementation on HOMA-IR. Therefore this review does not support the use of vitamin D supplementation for NAFLD patients in terms of improving metabolic function, although it could have other health benefits, such as for immune and musculoskeletal health.
