Vitamin D attenuates metabolic dysfunction-associated steatotic liver disease (MASLD) and downregulates hepatic gluconeogenesis in obesity.

Metabolic dysfunction-associated steatotic liver disease (MASLD) is often linked to vitamin D deficiency. Insulin resistance (IR) plays a central role in MASLD and is tied to an abnormal activation of hepatic gluconeogenesis, contributing to the hyperglycemia found in this condition. This study evaluated whether vitamin D supplementation improves MASLD and reduces hepatic glucose production in cafeteria diet-induced obese rats. Inflammation, oxidative stress, and IR were additionally assessed both systemically and in the liver. Gluconeogenic and glycogenolytic fluxes were measured in perfused livers. Wistar rats received a cafeteria or standard diet for 60 d and remained on these diets for another 5 weeks; during this period, a subgroup in each condition received weekly vitamin D (5,600 IU/kg) by oral gavage. The cafeteria diet promoted obesity, MASLD, IR, oxidative stress, and inflammation. Obese rats displayed elevated hepatic gluconeogenesis from lactate and intensified glycogenolysis and glycolysis. Vitamin D supplementation reduced food intake and body weight gain and improved IR and glucose tolerance, changes accompanied by lower hepatic steatosis. The treatment decreased hepatic mRNA expression of nuclear factor-kappa B (NF-κB), tumor necrosis factor alpha (TNFα), and interleukin 6 (IL-6), and increased nuclear factor erythroid 2-related factor 2 (Nrf2) expression. It also upregulated vitamin D receptor, sirtuin-1, fibroblast growth factor 21 (FGF21)/β-klotho axis, and IRS-2 expression, consistent with improved hepatic insulin signaling and antioxidant defense. Vitamin D reduced the elevated gluconeogenesis and glycogenolysis in obese rats, as determined by direct metabolic flux measurements, likely due to improvements in IR and hepatic steatosis. These findings support the therapeutic potential of vitamin D for obesity-associated MASLD, with additional benefits for glycemic control. Importantly, these findings were obtained in a cafeteria diet-induced obesity model that closely resembles obesogenic human dietary patterns.