The direct mechanism between 25(OH)D deficiency and liver fibrosis has not yet been clearly revealed by scientific research. However, we can explore the relationship between the two in terms of their respective effects and possible interactions. In the ANOVA, there was a significant difference in 25(OH)D levels between the different groups. In correlation analysis, we have found a significant negative correlation between 25(OH)D and FIB-4 and FLI. Serum 25(OH)D was an influential factor for NAFLD in multivariate logistic regression analysis. 25(OH)D activation exerts anti-inflammatory and anti-fibrotic effects in the liver by modulating the immune system, inhibiting the expression of pro-fibrotic inflammatory mediators such as platelet-derived growth factor (PDGF) and transforming growth factor β (TGF-β), and promoting the expression of collagen, α-smooth muscle actin, and tissue inhibitor of metalloproteinase-1 [18, 19]. However, once cirrhosis develops, vitamin D loses its antifibrotic protective effect [20].
Some studies suggest that 25(OH)D levels may decline as the severity of NAFLD increases [21]. This decline may be related to impaired liver function, as the liver is an important organ for 25(OH)D synthesis and metabolism [22].Liangpunsakul S et al found that serum vitamin D levels were significantly lower in those with unexplained elevations in ALT than in controls. In this article, we observed significant differences in ALT between subgroups with different degrees of NAFLD, besides, in a multiple logistic regression analysis, ALT was also found to have a significant effect on the severity of NAFLD [23]. However, in a meta-analysis based on six cross-sectional biopsy studies, Saberi et al. did not find a significant difference between 25(OH)D levels and the degree of liver fibrosis in patients with nonalcoholic fatty liver disease (NAFLD) [24].
In this study, FLI and FIB-4 were statistically different between groups determined by ultrasound NAFLD severity. Therefore, we concluded that FLI and FIB-4 scores were significantly associated with ultrasound NAFLD stratification. We used both of these indicators to score the degree of hepatic fibrosis in T2DM patients. We found that 25(OH)D was negatively correlated with FIB4 and FLI, and, moreover, FLI and FIB-4 levels were higher in the low 25 hydroxyvitamin D level group, all of which suggest that 25(OH)D deficiency may promote the progression of hepatic fibrosis in T2DM patients with NAFLD [25].
Our study showed that the was lumbar spine bone density significantly associated with ultrasound NAFLD stratification. The results of this study suggest a correlation between NAFLD and osteoporosis. However, our findings do not support a causal role of osteoporosis on NAFLD. This finding suggests that there may be bone-liver interactions, which are worthy of further study[26].
There was a statistically significant difference in BMI between groups. BMI showed a significant association with NAFLD in both univariate and multivariate analyses. The higher the BMI, the greater the risk of NAFLD (OR 1.12 in univariate analysis and 1.04 in multivariate analysis). This suggests that obesity is a significant risk factor for NAFLD. The increased prevalence of NAFLD is associated with a rising trend in obesity, especially among morbidly obese patients [27–29].
Among the glycemic-related indices, the levels of C-peptide and fasting glucose (FPG) varied with increasing grades of NAFLD. C-peptide levels were significantly higher in grades 2 and 3, whereas FPG levels were relatively high in grades 1 and 3. These changes may be related to insulin resistance due to hepatic steatosis. Alfadda et al. investigated the prevalence of NAFLD in patients with T2DM using transient elastography and found that 80.8% of T2DM patients had steatosis, of which 82.3% had severe steatosis and 17.6% had mild to moderate steatosis[30]. 25(OH)D deficiency promotes the development and progression of NAFLD by causing insulin resistance, increasing hepatic resistin gene expression, and upregulating hepatic steatosis and oxidative stress gene expression[31].
Overall, further studies are needed to clarify the apparent differences in 25(OH)D levels between different degrees of NAFLD at this time. Future studies could conduct larger surveys and clinical trials in patients with different degrees of NAFLD to more accurately assess the relationship between 25(OH)D levels and NALD severity. There is also a need to further explore the mechanism of 25(OH)D’s role in the pathogenesis and treatment of NAFLD and how to improve the condition of patients with NAFLD by adjusting 25(OH)D levels. This will help provide new ideas and methods for the prevention and treatment of NAFLD.