Dehydration would come before any nutrient deficiency. I was never so thirsty as when I was breastfeeding. Since you are going to eliminate some of that water anyway, you'll run out pretty fast.
As to the mechanics, I hate to tell you but several full term pregnancies and extended breastfeeding will take care of that.
Which reminds me...
I am a strong believer in the benefits of breastfeeding for both mother and child. I believe women should be encouraged to breastfeed and supported in their effort. I do not, however, believe they should be bullied or guilted into it, nor should they be lied to. During my years of breastfeeding, I saw quite a few brain-hurty statements on various forums.
1. One woman questioned all the mothers at her Church, asking them if they breastfed, and if not, what their excuse was. And she complained on the forum that not one of them gave her a proper excuse. Attempts to explain that those mothers were not accountable to her about their parenting decisions never got through to her.
2. I have a friend who is a lovely woman, but she is, err, opinionated. She affirms that breastfeeding will not cause sagging. She says that the weight gain from pregnancy does. Yes, it does, but that does not mean that breastfeeding does not contribute too. If your breast become significantly bigger and heavier for several more months than just the pregnancy, how could it not cause sagging?