Dairy foods and dietary calcium (Ca) are potential regulators of body weight and insulin sensitivity. The specific components of dairy responsible for these actions are not known but may include leucine. Our objective was to determine the effect of dietary protein (casein, skim milk or leucine) and Ca level [low, 0.67% (LC) or high, 2.4% (HC)] on adiposity and insulin sensitivity. Obesity was induced in Sprague-Dawley rats with a 6-week period of high-fat/high-sucrose (HFHS) diet intake. Rats were randomly assigned to one of six HFHS diets for 8 weeks where dietary protein was provided as casein, skim milk or casein enriched with leucine, and contained either LC or HC. Body composition via dual-energy x-ray absorptiometry and insulin sensitivity via euglycemic-hyperinsulinemic clamp were measured. Microarray was used to assess gene expression in liver and skeletal muscle. Rats fed leucine had greater insulin sensitivity than those fed casein or skim milk (P<.05). Dietary protein differentially regulated hepatic and skeletal muscle genes associated with insulin, peroxisome proliferator-activated receptor and mammalian target of rapamycin pathways. Specifically, two key genes responsible for insulin sensitivity, hepatic insulin receptor substrate (IRS) and protein kinase B (Akt), were altered in hepatic tissue in response to leucine. Rats fed skim milk and leucine diets had lower body weight compared to those fed casein (P<.05). HC reduced fat mass compared to LC (P<.05). While skim milk and leucine both reduced fat mass, only leucine improved insulin sensitivity compared to casein. Differential expression of genes such as IRS and Akt may be responsible for changes in insulin sensitivity in obese rats.