K.N.
You're not suppose to intentionally diet/lose weight while breastfeeding since it causes your body to release a chemical (into the milk) which is not good for the baby to ingest.
Also, nutritionally speaking, your milk is as good as your own diet... Hence, if you eat poorly, your milk will not contain the essential fat, protein and calories that the baby needs.
My advice is not to diet but to eat better... Meaning no pre-made or processed food; no foods containing high fructose corn syrup; no foods containing nitrites (preserved meats, most lunch meats, hot dogs). Limit sweets and sugar. Instead eat *lots* of fruits, vegetables, broiled/baked meat, whole wheats, nuts, yogurt, cheese. Cut out calories from fast food, junk food and processed food and feed your body clean calories; you WILL lose weight and keep your milk supply.