The role of dietary fat and fiber in energy restriction for the management of obesity was examined. Twelve male castrated dogs were energy restricted for 7 weeks by feeding 60% of their calculated maintenance energy requirements (MER = 1500 kcal/m2/d) for ideal body weight. Six dogs were restricted on a high-fat (35.4 kcal% from fat), low-fiber (2.9% dry matter basis [DMB]) diet while the other six dogs were restricted on a low-fat (24.5 kcal% from fat), high-fiber (27% DMB) diet. Compared with the high-fat, low-fiber diet, energy restriction on the low-fat, high-fiber diet resulted in significantly greater decreases in body fat (1472 +/- 166 vs. 853 +/- 176 g; p < 0.05) and total serum cholesterol concentrations (108.7 +/- 11.3 vs. 51.5 +/- 13.9 mg/dL; p < 0.005). Reductions in body weight (2.86 +/- 0.3 vs. 2.14 +/- 0.3 kg; p < 0.09), and mean arterial blood pressure (17.4 +/- 6.1 vs. 6.7 +/- 2.9 mmHg; p < 0.12) were also greater on the low-fat diet; however, these diet effects did not reach statistical significance. These data suggest that the fat and fiber content of the diet during energy restriction are important factors in the management of obesity.