I started a water fast of 22 days while thin. I lost around 20 pounds but I was still OK to fast for many more days. I just missed eating food. Some thin people, like Gandhi, were great at fasting. I don't think Moses and Jesus were fat. If your body does not have enough for more fasting, it is supposed to let you know with true hunger. At that point you will be starving, not fasting, if you continue. Many juice fasters fast for 120 days. See this site on Fasting with quotes from 18 MDs.