Book 1's clinic did fruit juice fasting, and used OJ for a perfect record.
Another grand author is Prof. Arnold Erhet. If I'm not mistaken, he stated that grapes were the strongest detox ... I tried it in the early days, TOO MUCH FOR ME!
Fruit eaten in mono-meal style needs very little digestion. When you take away the the pulp, it's simply absorbed, giving digestive energy the freedom to do house-cleaning/detox/weeding.
I'm not sure if MH does his fast in Jan because his favorite orange is just coming into season or if he does OJ because it's in season when he chooses to fast. Maybe he'll tell us here in a bit.