But protein drinks have a lot of calories, sugar, ect. So how could you fast on that? Your body will hold the calories, since there are so many! I'd say the best way to fast, and not feel as tired is to drink herbal teas, and homemeade juice! Tea will give you energy in the morning since it does have caffine and the jucie will give you nutrients. If you planning to do a fast for a month or so I would take multi-vitamins. It will help you, trust me.