The small amount of natural grapefruit juice just for the time it takes to do a Liver Flush has been fine for me, and I never drink juices.
Here's some info on the type of diet that has been really helpful to me:

http://www.successfulhealing.com/Eating-for-health.htm

The other replies are very helpful and I agree with their opinion - grapefruit juice used in the Liver Flush should not put an overwhelming toll on you and the benefits of Liver Flush far outweigh it. However, I used only a small amount of grapefruit juice (1/4cup) and chase the mixture with 1/2 cup lemon juice (I do this not because of Sugar content in grapefruit juice but because I find it far more effective).

Fasting with confirmed adrenal fatigue is typically not recommended as this can exarcebate symptoms; the adrenals need to be rebuilt so you should stop wearing them by cutting refined sugars from diet (and any processed food) and rebuild them with good nutrition (fast oxidizer diet is appropriate if you have the metabolic type for it , or something close to Dr Weston Price diet and lifestyle)

Many people get good results following Master-Cleanse because it allows their digestive system to rest and rebuild while eliminating large amounts of toxins, but a lot of people might not be able to go through it because they are deficient in nutrients (like aminoacids and EFAs) or have malabsorption of these nutrients.

Mike