For the best fasting results, I would cleanse my liver prior to fasting.
1/2 cup of freshly squeezed grapefruit juice or orange juice mixed with 1/2 cup of organic Extra-Virgin Olive-Oil , I drink it on an empty stomach in the evening or in the morning.
No food after that for 6-12 hours.
This is the most simple liver flush, there are far more complex recipes.