Increased adiposity is a feature of aging in both mice and humans, but the molecular mechanisms underlying age-related changes in adipose tissue stores remain unclear. In previous studies, we noted that 18 month old normocalcemic VDR knockout (VDRKO) mice exhibited atrophy of the mammary adipose compartment relative to wild type (WT) littermates, suggesting a role for VDR in adiposity. Here we have monitored body fat depots, food intake, metabolic factors and gene expression in WT and VDRKO mice on the C57Bl6 and CD1 genetic backgrounds. Regardless of genetic background, both subcutaneous and visceral white adipose tissue depots were smaller in VDRKO mice than WT mice. The lean phenotype of VDRKO mice was associated with reduced serum leptin and compensatory increased food intake. Similar effects on adipose tissue, leptin and food intake were observed in mice lacking Cyp27b1, the 1-hydroxylase enzyme that generates 1,25-dihydroxyvitamin D₃, the VDR ligand. Although VDR ablation did not alter expression of PPAR or fatty acid synthase, PCR array screening identified several differentially expressed genes in white adipose tissue from WT and VDRKO mice. Uncoupling protein-1 (ucp-1), which mediates dissociation of cellular respiration from energy production, was >25fold elevated in VDRKO white adipose tissue. Consistent with elevation in ucp-1, VDRKO mice were resistant to high fat diet induced weight gain. Collectively, these studies identify a novel role for 1,25-dihydroxyvitamin D₃ and the VDR in the control of adipocyte metabolism and lipid storage in vivo.