BACKGROUND:The impact of vitamin D3 (VD3) on obesity has been reported in the past. Our study was aimed at investigating the possible mechanisms by which VD3 affects obesity induced by a high fat diet. METHODS:Eight-week-old C57BL/6 J male mice were fed a normal- or high-fat diet for 9 weeks and were treated with a gavage of vehicle (corn oil) or cholecalciferol (50 μg/kg, daily). Body weight, white adipose tissue weight, blood lipid and glucose levels were measured. In addition, we investigated the expression of 1,25(OH)2D3 (calcitriol)/VDR-regulated genes involved in energy and lipid metabolism, such as of uncoupling protein 3 (UCP3), by using qRT-PCR in the liver, adipose tissue, skeletal muscle and C2C12, L6, and H-EMC-SS cells. We also measured UCP3 promoter transcription in the same cell lines using a Dual Luciferase Assay. Furthermore, we analyzed the binding site consensus sequences of VDR on the UCP3 promoter. RESULTS:Mice consuming a high-fat diet treated with cholecalciferol had lower body weight and adipose tissue weight and higher expression of UCP3 compared to the other treatment groups. Changes in the expression of genes correlated with calcitriol/VDR. Luciferase activity was dose-dependently associated with calcitriol/VDR levels. We confirmed the functional VDR binding site consensus sequences at -2200, -1561, -634, and +314 bp in the UCP3 promoter region. CONCLUSION:We suggest that VD3/VDR inhibits weight gain by activating UCP3 in the muscles.