Dear Superman,
Hi. I don't know all the details, but I have read that kidney donors should be careful about taking in too much protein, as it may be a strain for one kidney to handle a lot. I am not clear about whether that is just for animal protein or for vegetable protein as well, nor do I know what the muscle-building protein powder contains. So I would suggest you play it safe until you know for sure. Perhaps consult a nephrologist or dietician? Better to be safe than sorry in the meantime.
Best wishes.
Fr. Pat (donor, '02)