The current American Diabetes Association guidelines for nutrition recommend a moderate increase in monounsaturated fats and a reduced intake of carbohydrate in patients with diabetes in whom high-carbohydrate diets deteriorate glycemic control and lipoprotein levels. High-fat diets, however, are believed to promote obesity, and some investigators may have reservations recommending such diets. This review thus investigates the role of diet composition in promoting obesity or achieving weight loss and its implications in patients with diabetes. Epidemiological studies show some evidence that fat intake is more importantly related to body weight than carbohydrate intake, but conclusions are weak because confounding variables, such as physical activity, smoking, and energy intake, were generally not controlled for. Metabolic studies under isoenergic conditions report no change in energy balance when fat intake is increased, but report a negative fat balance with substantial increase carbohydrate intake. During overfeeding, excess fat intake is stored as fat, whereas excess carbohydrate is mostly oxidized in the short term but can lead to substantial gain in fat stores because of reduced fat oxidation and considerable de novo lipogenesis in the long term. Spontaneous energy intake, however, is higher on an unrestricted high-fat diet compared with a high-carbohydrate diet, but the long-term effects are not known. Weight-loss intervention studies show that a hypocaloric high-carbohydrate diet is not associated with more weight loss than a high-fat hypocaloric diet. In conclusion, a high-monounsaturated fat diet to control glycemic control and lipoprotein levels in patients with diabetes should not affect weight loss or maintenance, provided that energy intake is carefully controlled.