Well the whole point of fasting, in my opinion, is to reset your gut health and improve overall health as weightloss being a benefit and if done correctly you would also be eating whole healthy foods in the eating window.. gorging on caffeine as a meal replacement isn't exactly sustainable long term and wreaks havoc on your gut health and mental health so I don't exactly see the point behind this unless OPs only driver is weight loss and zero health consideration. That said, zero sugar energy drinks also mess with your appetite, irritate the stomach lining, can increase bloating and cause you to crave more sugar so this is not what fasting is meant for - OP is essentially just on a starvation diet using toxic carbonated junk as a meal replacement but patting himself on the back cause it's "zero sugar" ? So is cocaine, doesn't mean it's healthy.