In the early Dr Fung graphics about insulin and obesity, he would show fiber and vinegar though he doesn't mention it as much lately. Then recently Megan Ramos said on Fasting Talk that she uses ACV to help the "gut bugs", and that got me re-started. That old trick of eating something sour if craving something sweet works (for me) when fasting too...if hungry try 2T ACV in water, sometimes I add acacia fiber as well. Have no idea if this is beneficial or not for my "microbiome" but I haven't gotten sick, my digestive track is fine and it seems to stop hunger for me...or the taste is just so gross

In the Complete Guide of Fasting the list of fasting fluids does include adding vinegar, lemon, limes, and salt to water. He also lists six herbal teas, to which can be added coconut oil, cinnamon, lemon, butter, HWC, and more.