My daughter got into that habit for a while, at that level of consumption, and ended up with daily massive debilitating headaches. I did the Sherlock Holmes thing, concluded it was the diet Coke, and decided to do a cold-turkey withdrawal test before we went trooping off to the doc. Guess what disappeared, never to be felt again...
I have a mantra that I have spouted for years... "If I eat right, I feel right. If I feel right, I exercise right. If I exercise right, I think right. If I think right, I eat right..." Phil-SC