First of all you are not fasting. You are juicing or juice fasting. Either way teas and coffees are enervating during a fast, but are also not healthy when you are not fasting. The reason? All foods with caffeine are diuretic and rob your body of water. When you cleanse your body you need water to clear out junk. So you can juice without tea and coffee. Caffeine is also hard on your kidneys.
Also make sure you are using organic fruits and veggies to juice. Consume fruit and veggie juices separately at least two hours, they digest at a different pace.