Somer
Your question is not very clear. Do you propose to break a fast with whey, or to fast on whey?
How long an "extended fast" do you plan?
I have done extended fasts, 60 days last summer. Muscle loss was never an issue. I exercise every day I fast.
Rather than one long fast this summer, I did a series of ten dayers (four this summer, just finishing one this fall and will do one more) with ten days cleansing/rebuilding diet between, including quality protein. This was much easier on the body, and far easier to break than one long fast.
Whey powder would be the wrong stuff to fast on. You need to give your kidneys/liver/gallbladder/digestive system and the body in general a rest from constant digestion. As far as breaking, I cannot imagine why you could not add protein powder to say, a morning smoothie, after breaking normally for 1-2 days, depending on the length of the fast.
Unless you are planning a SERIOUSLY long fast, muscle loss will not be an issue with juices. Drink plenty of fresh fruit juices in the morning and vegetable juices in the afternoon, and you will get everything you need.
If you plan to fast long term, I highly recommend you find a sympathetic health professional to supervise. I could NOT have done it without the support of my excellent naturopath, whom I called upon more than once in crisis situations.
Best to you,
Somer