Fonty
First why would you want to take it? Fasting just works you don't need any help.
Second the effects of the herb could change while fasting, and it will be going through at least part of the digestive system while that part of our body is getting some much needed rest and probably maintenance. It could be irritating to linings being replaced for example.
Third it will take energy from the body to assimilate and will add elements to the body that it is less likely to allow for while it is keeping a good balance within all our systems.
Fourth even stimulating the tastebuds can interfere with a fast by sending a message to the brain that the body needs to begin the digestive cycle, thus again diverting energy from the fasting process to useless digestion.
Fifth just accept the bodies abilities and it's limitations. It is very able to fast on water only incredibly successfully. We don't know enough about adding artificial ingredients to our body while eating or fasting so the more natural you can stay at any time the better.
Andrew.