Hmmm...caffeine doesn't seem to affect me in any negative ways...I do drink a couple of cups of coffee in the morning, sometimes as much as 3 cups, but I also drink iced tea and regular soda, so I get caffeine from them, too. I literally can drink a cup of Starbucks coffee and go to sleep without any trouble. I apparently have a high tolerance for it. I have occasionally had days where I didn't have any coffee, but I have never experienced the headaches, or other withdrawal symptoms. It does pick me up in the morning, and actually helps me with regularity.
If I were fasting, I wouldn't drink anything with caffeine, and I think there are several different types of fasts, some which are just partial, some like "juice fasts", or abstaining from certain types of food. If God leads you to fast, it is for a specific reason, usually, and I think it's important to let Him lead you and guide you as to how he wants you to do so. Self-initiated fasts can turn into a works-like activity if one is not careful. Also, one can fast just for health reasons. It is thought that occasional fasts for health reasons can help to balance and correct disharmony in the body's systems. Given the exposure to chemicals, pesticides, and other contaminants in much of our food supply, fasting may give the body a chance to "catch up" in its processing of such foreign substances.
Personally, I prefer naturally grown, organic, as-God-intended foods, as close to fresh as possible. If you can grow your own, so much the better, because you can control how and where they're grown, and what is put on them. Not everyone can do that. My health is very good, I do take whole food supplements from reputable sources, and try to get enough sleep, to keep me healthy. At 60, I'm feeling more like I'm 30.
I realize I strayed a little from the subject, and for that I apologize.