Interest in the role of 25-hydroxyvitamin D3 [25(OH)D3] in the pathogenesis of metabolic disturbances (i.e., insulin resistance, type 2 diabetes, cardiovascular disease, and liver abnormalities of different etiologies) has been growing in recent years.1-4
The study by Petta et al.1 might suggest the importance of 25(OH)D3 as a common marker of both metabolic abnormalities and hepatic damage in a sample of individuals with chronic hepatitis C. Patients suffering from chronic hepatitis C presented with low levels of 25(OH)D3. Concentrations of vitamin D were associated with characteristics of metabolic syndrome (i.e., a high waist circumference, ferritin, and low high-density lipoprotein cholesterol levels) and with severity of inflammation and fibrosis. A relative vitamin deficiency was associated with reduced expression of a cytochrome P450 isoform (cytochrome P450 27A1). Importantly, the authors proposed low levels of 25(OH)D3 as serum markers of fibrosis to be validated in external populations.
Targher et al.3 found that circulating 25(OH)D3 was reduced in adults with nonalcoholic fatty liver disease (NAFLD) in comparison with controls. The association with histological severity was independent of any metabolic abnormality.
We ruled out associations of 25(OH)D3 (measured by high-performance liquid chromatography; Bio-Rad, Munich, Germany) with metabolic characteristics and histological features in 64 children with biopsy-proven NAFLD (46 males, age = 12.6 ± 2.7 years, body mass index z score = 1.94 ± 0.34 SDS). The mean insulin sensitivity, as assessed by the insulin sensitivity index, was 3.4 ± 2.3; the alanine aminotransferase level was 55 ± 30 IU/L, and the aspartate aminotransferase level was 51 ± 27 IU/L. The NAFLD activity score (NAS) was 3.7 ± 1.5, and 31 patients were diagnosed with an NAS ≥ 5. In the whole sample, the mean level of 25(OH)D3 was 21.9 ± 10.2 ng/mL. Thirty-five patients had levels of 25(OH)D3 below 20 ng/mL. Low levels of 25(OH)D3 were associated with an increased likelihood of fibrosis (10.6, 95% confidence interval = 6.2-15.0, P < 0.0001) and necroinflammation (6.168, 95% confidence interval = 0.823-11.5, P = 0.024). Specifically, 35 patients with fibrosis (16 with grade 1) presented lower levels of 25(OH)D3 in comparison with patients without fibrosis (17.1 ± 7.4 versus 27.7 ± 10.3 ng/mL, P < 0.0001). Low 25(OH)D3 levels predicted fibrosis by receiver operating characteristic analysis (area under the curve = 0.81) with a sensitivity of 79% and a specificity of 63%. Conversely, the prediction of necroinflammation was poor, even though levels of 25(OH)D3 differed significantly in children with and without necroinflammation (19.9 ± 9.8 versus 26.1 ± 10 ng/mL, P = 0.16). Concentrations of 25(OH)D3 correlated significantly but poorly with body weight (ro = −0.248, P = 0.048), age (ro = −0.248, P = 0.048), and NAS (ro = −0.587, P < 0.0001). In agreement with observations in adults by Targher et al.,3 low levels of 25(OH)D3 in NAFLD correlated with histological severity independently of metabolic characteristics.
Adiposity seems to be an important confounder as concentrations of 25(OH)D3 decrease by approximately 4.8 nM for each kilogram of fat mass.4 Obesity can lead to vitamin D insufficiency, which, in turn, favors progression from NAFLD to necroinflammation and fibrosis. Indeed, vitamin D protects directly against inflammation and fibrosis by binding its hepatic receptors. The main mechanisms were reported by Petta et al.1 Alternatively and more intriguingly, low levels of vitamin D can promote endotoxemia and thus activation of the innate immune system.5