Many people fail to correct candida overgrowth or serious diseases on any possible diet and supplements. I was one of those. When the digestive system is too far gone, you can't just throw food at it, even the healthiest, and expect to digest it and get well. In that case, vegetable juice fasting is the way to go and will bring quick results if combined with other beneficial protocols (enemas, liver flushing, herbs, castor oil packs, skin brushing, hot/cold therapy...).
Many assume they're not made to fast because they feel miserable on a vegetable juice fast. Well, it IS one of the most intense cleansing program one can do, and if you're not healthy to start with, chances are you will feel miserable at first. Also, don't be scared to waste away even if you're underweight to start with. I was extremly underweight despite the huge amount of calories I was ingesting, and my weight immediatly started to stabilize on the fast.