I'm currently breastfeeding DD2 who is eight weeks and have done two weeks of 5:2, so four fast days so far. I'm finding it great but the day after fast day two I did notice DD was fussing during her first feed of the day so wondered whether my supply had been effected.
DD has a bottle of formula a few times a week to get her used to it (DD1 was a bottle refuser so don't want that situation again for various reasons!), so I do one FD at 1000 calories and one at 750. I spoke to my GP at the six week check and he is a 5:2er for the health benefits do very supportive. It's a huge help to me that DH also fasts, no weight to lose but he finds it energising!