I did this with no problems.
A quick recap of my experience from a couple of years ago (I trawled through some old posts of mine for accuracy), it is all copied and pasted:
Written at the end of March 2015 (roughly 11 weeks after giving birth, still EBFeeding)
I have some baby weight to lose as I was a human dustbin while pregnant last year and managed to put on 24kg which is roughly 4st, more than a third of my normal weight! I was quite appalled when DD came out a dinky 7lbs4oz and, having weighed myself a week after giving birth, I found that I only lost a stone, BMI 24.5. Another stone came off by itself soon afterwards, but my weight plateaued at 11.5st, leaving me with another two stone to lose. I was hoping it would come off effortlessly, but my carby pregnancy habits and a life-long love of sweets got in the way. I started 5:2 within a month of giving birth and initially I was losing 2lbs a week, which then became 1lb a week.
At that point, I reassessed what I was doing since I felt as though I was fasting an awful lot without seeing particularly quick results, especially in terms of reduced cravings. I guess I don't have a lot of patience and find not being able to fit into any of my own clothes quite depressing. I decided to give 4:3 a try and have been sticking to it for a couple of weeks now. I also picked up tips from this board about using MFP for keeping track of my NFD calories and found that I was eating back some of them. Yesterday I weighed 10.9st, BMI 22. I want to get back to my pre-pregnancy weight of 9.7st, and I have now realistically accepted it will take months rather than weeks. I'd love some company along the way.
As for exercise, I have a nearly 4 year old DS, who keeps me reasonably active, and used to practice yoga most days a week before I became pregnant again. I am now slowly getting back to it. DD is 11 weeks and EBF, but unfortunately I don't find that I am losing an awful lot of weight just through feeding. 5:2 or 4:3 has had absolutely no effect on my supply as far as I can tell and DD is gaining weight along her percentile.
I tend to fast on Mondays, Wednesdays and Fridays or Saturdays. Getting back into yoga makes avoiding food a lot easier as it is always a rush trying to get the kids to the gym crèche before the class and I am then not around snacks.
As for FD kcals, I tend to aim for 500-600. I am still hungry on a 1000kcals, and I find I don't lose as much weight to keep myself motivated long term, but I decreased the calories gradually as I did not want to affect my milk supply or my mood too much.
My top tips for bfeeding fasters - lots of tea and frequent naps. Easier said than done, I know!
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A month later, I reached my target weight and then stopped doing 5:2, but now I am back on ADF as I started putting the weight back on because chocolate.
Best of luck with the fasting.