It was so extreme, apparently, that it wound up altering his thinking, as he notes in this quote via ET Canada, and that brought him to a place where he was able to enter the proper mindset to play Arthur Fleck in Joker.
Joaquin Phoenix has never exactly been known for being a “big” guy, so taking more than 50 pounds off of his frame is pretty extreme.
That’s something that a doctor would have to determine, but it certainly wouldn’t be surprising to learn as a diagnosis.