Metabolic diseases such as diabetes mellitus or impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) are often associated with overweight and obesity. People with diabetes or IFG or IGT have in addition an increased cardiovascular risk. Nutritional therapy and weight loss in overweight or obese subjects with metabolic diseases is the main therapeutic target. In subjects with IFG or IGT who are overweight or obese, weight loss has been shown to be effective in preventing or delaying type 2 diabetes mellitus and improving main cardiometabolic risk factors (such blood pressure, lipids, inflammation, waist circumference). In type 2 diabetes mellitus a modest weight loss improves glycemia and the main cardiovascular risk factors. A greater weight loss could improve long-term cardiovascular outcomes and mortality. To improve glycemic outcomes most adults with type 1 diabetes should receive education on how to match mealtime insulin doses to carbohydrate intake and, additionally, to fat and protein intake.