I'm no expert, but here's my thought: fasting is a way of allowing your body to heal itself. By drinking only water (or other natural liquids, like juice), you're giving your digestive system a rest. It doesn't have to work out any gunk from processed foods. The enzymes in your body can work on repairing the wear & tear of the average American lifestyle, because they aren't having to focus on getting new toxins out of your body every time you eat (or drink) something unnatural. So coffee, being a processed drink, would probably waste energy that could be better used to heal your body. ALSO, it's got an addictive substance (caffeine), so you want to avoid all of those! And caffeine also dehydrates you, which is very bad already, but even worse when you're not eating & trying to heal your body. It kind of defeats the purpose. Which is why juice fasting should only be done with organic vegetables. Kind of the same concept there. I think tea is okay because you can get it naturally w/out caffeine & it's also all-natural, containing antioxidants in some instances as well. I don't know much about tea though, but it seems a much better alternative to coffee.