Heavy coke drinkers here. The diet stuff (especially zero) makes me feel really unwell and gives me bad headaches. Plus I don't trust artificial sweetners. I have 2 glasses a day, only with meals. DH has a lot more. Funnily, when I was pregnant I went off coke completely. Couldn't stand it, weird huh. You have to be careful with diet drinks as artificial sweeteners can actually increase your appetite (apparently for sweet things) also, diet drinks have huge amounts of sodium compared with regular drinks - not good for fluid retention as well as making you drink more.