The best diet for breast-feeding is very similar to the diet recommended for pregnancy. Protein recommendations are the same, vitamin B12 recommendations are higher, and the recommendations for iron and calories are lower than during pregnancy.
If you eat too little while breast-feeding, you may not produce as much milk. Although the recommended calorie intake is 330 calories above your usual intake for the first six months (1), you still may lose weight because of a loss of calories in breast milk. It is safe to lose about 1/2 to 1 pound a week while breast-feeding but more rigorous dieting is not recommended. As in pregnancy, small frequent meals are the best way to be sure that you are getting enough calories. Since you do need extra fluid while breast-feeding, use nutritious beverages like juices, soy milk, soups, and vegan smoothies to provide calories.
The recommendation for protein is the same as in pregnancy (1) and can be obtained easily from the extra food you are eating. You should still eat good quality food because you are providing all nutrients to your infant. You will need to be careful to get enough vitamin B12, vitamin D, iodine, and DHA in order to be sure that these nutrients are present in your milk in adequate amounts. See the Pregnancy section (pages 176-185) for more information on sources of these nutrients. Requirements for most other nutrients are similar to those in pregnancy and should be obtained from a varied, healthy vegan diet. The Vegan Food Guide (page 198) includes suggestions for food choices for women who are breast-feeding.
Reference
1. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: National Academy Press, 2002.