The topic of protein drinks pops up from time to time. Many question which products are SBD friendly, and if they can be used as a meal replacement. The consensus seems to be that they aren't suggested as a meal replacement because there is a high possibility that they won't satisfy one's appetite over a period of time.
I, however, am one who has a protein/greens smoothie that I drink every morning of my life as a meal replacement. I starting this practice over a year before starting SBD and had it reviewed by a nutritionist to make sure it had everything in it to "accomplish" what I wanted. I wanted it to be fast, tasty, nutritious AND carry me through the morning. It does that and so I stick with it to this day. I stuck with it on P1 because it fit into the P1 requirements. Even when I prepare breakfast for the family I like it so much I stick with my drink.
Since WaistingTime asked on another thread, I'll post it here. It's a combination of 2 products, "Supreme Meal", by Peaceful Planet and "Green Vibrance" by Vibrant Health. It was a little boring during P1, but normally I add 1/2 apple or fresh blueberries. Taken along with my vitamins, it carries me through to lunch, and that's without a snack break.
As we follow SB we also need to key into what is right for us individually. If I had not already had this habit for a year prior to SBD, I would have definitely not experimented with it. Since it was already my breakfast of choice, I stuck with it.
With regards to nursing, I have no idea, I have no kids. I would leave that decision up to a health care professional.