Hi, from my experience, and others I have discussed this with, I would suggest a gentle warm Sea Salt cleansing enema every day during your fast. Not only does the enema prevent you from becoming constipated, but it helps insure that toxins released into your colon as a result of fasting don't hang around long enough to cause malaise.
An added benefit I find, especially during those first few difficult days at the beginning of a fast, is that after I take an enema it seems to diminish my urge to eat.
Whatever you decide on, best wishes for a successful fast!