IMHO, your baby is getting all of his/her nutrients from you. You are already burning large amount of calories feeding your baby, and your baby needs every bite you put in your mouth, every day. This is a good time to eat healthy well balanced meals so both you and your baby gets a constant source of nutrition. Before you know it your baby will be happily on solids and you can absorb the 2 day severe calorie reduction two days a week without pulling too much nutrition away from your baby or yourself. Like I said, just my two cents.

I didn't fast when breastfeeding but I had two very healthy days sticking to 1200 calories and ate normally the rest of the week and started normal 5:2 again when I stopped feeding completely at 10months (he got teeth, haha)