Abstract

Background/Objectives: As obesity continues to increase globally, it has become a major contributor to the growing burden of metabolic diseases, including type 2 diabetes mellitus, cardiovascular disease, and nonalcoholic fatty liver disease (NAFLD). Hepatic lipid accumulation during obesity contributes to oxidative stress and alterations in mitochondrial homeostasis, key features of NAFLD progression. This study investigated whether vitamin D insufficiency aggravates obesity-related hepatic steatosis in association with changes in hepatic sirtuin 1 (SIRT1) activity and nuclear factor erythroid 2–related factor 2 (Nrf2)-related antioxidant gene expression. Methods: Male C57BL/6J mice were maintained on one of three dietary regimens for 16 weeks: a normal diet (NOR; 10% fat containing 1000 IU vitamin D/kg), a high-fat diet (HF; 60% fat containing 1000 IU vitamin D/kg), or a vitamin D-deficient high-fat diet (HF + NVD; 60% fat with no added vitamin D). Results: Vitamin D insufficiency in HF-fed mice (HF + NVD) was associated with significantly higher hepatic triglyceride accumulation and lipid peroxidation than those observed in the HF group. A concomitant reduction in the expression of Nrf2-dependent antioxidant genes was observed in the HF + NVD group. Furthermore, hepatic mitochondrial DNA content as well as hepatic SIRT1 mRNA level and activity, the NAD+/NADH ratio, and CPT1α mRNA expression were significantly decreased in the HF + NVD group. Conclusions: An inadequate vitamin D status was associated with greater hepatic lipid deposition, enhanced oxidative stress, and reduced hepatic SIRT1 activity, and lower expression of Nrf2-related antioxidant genes. These findings support that maintaining adequate vitamin D status might help preserve hepatic metabolic homeostasis during obesity.

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