So water and coffee or tea (without milk, of course) would be fine. You're trying to avoid an insulin response and firing up the entire machinery of digestion.

I think it's non-controversial to say that this starves the cancer cells.

Perhaps less certainly we can also say that digestion demands a lot of resources and is an interrupt for a lot of other processes. When your body has nothing to do for 24-36 hours eventually lower priority tasks get attended to ... like garbage collection.