All herbivores have saliva to digest part of the food in their mouth. Humans are also built that way, digestion begins with our mouth and saliva. Anything that mimics food triggers the digestive system, during a fast you do not want to keep your digestive system active when there is no food coming in. Also toothpaste had very toxic chemicals that get sucked by all capillaries in your gum. I think one should not use toothpaste even when not fasting, there are very good herbal poweders and toothpastes that have no chemicals.