The best options to keep you going during fasts would be :a) Switch to green tea/herbal tea. Consume fresh and dried fruits particularly apricots, figs and raisins during the day. You could also consume fruit juices. All these have an alkaline effect on the system.b) Although fruits have natural acids, these are weak compared to the pH of the stomach. Less acidic fruits include papaya, apple, peach, guavas, bananas etc. Feel free to consume a variety of fruits of your choice.c) Natural acids like vinegar and apple cider work very well. It might sound paradoxical but it is very effective. A spoon full in a glass of water alongwith meals is an effective natural therapy.d) It may be better to use psyllium husk (isabgol) to prevent acidity during fasts. However, over the counter antacids also work to give quick relief.