If you're concerned about muscle loss, then I would consider doing a water fast instead of a juice fast. A water fast allows you to go into ketosis which gives your body a chance to burn fat rather than muscle tissue for energy. Also digestion (even on juice) puts a lot of stress on the body. Water fasting gives your body a chance to devote a lot of energy into repairs and expelling toxins. I've done quite a bit of reading into juice fasting vs water fasting and I think the latter is healthier. I have done 3 day of water fasting and felt great by the end of it.

Serre