S.W.
I have drunk a protein powder shake almost every morning for almost 20 years, including when I was pregnant and nursing. I have not done this to try to lose weight, but to keep from being excessively hungry. Protein is what our bodies need to make milk, to build muscle, and it keeps us from being hungry longer than any form of carbs. I started this habit when I was training in karate and was working out for many hours each week. Breastfeeding, to me, felt like a physical workout and the protein shake helped me to keep up my energy.
This is the one I drink, mixed with milk: http://www.spiru-tein.com/