With a multi=vitamin, some components are fat soluble and others are water soluble. If you're swallowing a multi-vitamin on an empty stomach while you're fasting, I'm not surprised you're experiencing GERD and stomach upset. It's throwing a lot of stuff down the gullet without the natural buffers you'd have when you're eating. When I fast, I stop supplements with the exception of magnesium and salt. I don't consume anything except for water and sometimes tea. I take salt about 3 times a day in the morning, afternoon, and evening. It makes a world of difference. While Fung recommends vitamin supplementation for longer fasts, I don't do that other than what I've mentioned. Once you're truly fat adapted and have fasted a few times, you become very acclimated to it and can do it easily for any length of time without getting hungry. My recommendation? Stay away from that multi-vitamin, as it is reacting in a state where it is causing harm based on your reactions. You're not going to experience any nutritional deficits no matter how long you're fasting.