Tree nuts such as pecans, pistachios and walnuts should be included in a healthy diet for weight management because they offer fiber, protein and heart-healthy fats that help people feel full, registered dietitian Rachel Begun writes. Portions are an issue, however, and Begun writes that because nuts are calorie dense, they should be substituted for other foods, such as those high in saturated fat, and kept to 1 to 2 ounces per day.