I just do it to get through the hardest 3 days. I'm on day 4 and no desire for the rubbish drinks but I am glad they were there to get me this far. Now I go water only or water with lemon. I might be wrong but my taste changes through a fast and whilst I love Pepsi Max on the 1st 3 days by now I feel that it tastes rank and seriously believe that someone has swapped my tins over overnight as they are truly rank.
Coffee again the same thing.
But if your fast is for weightloss - do what you need to do to get the unhealthy weight off.