I just ended a 16 day fast today. Any caloric intake was limited to miso soup broth (< 35 cals) and a pickle every few days. Lots of coffee, black tea, some diet sports drinks and water (with electrolytes added). Was surprisingly easy, physically, but I also have a relatively high BMI. I ended because 1) it gets very boring, 2) I was constantly consumed with the idea that I was in a fast -- it could basically crowd out any other thoughts and my work suffered a bit.