Some good some bad - too much and you can get addicted (I think I'm suffering a slight caffine overload, went through a 2L bottle of Pepsi Max in one day recently!) and can affect you really badly.
Some studies say that it can inhibit fat burn and if you have caffine and sugar together then it's even worse for fat loss. And some studies say that caffine before exercise improves performance stamina and energy burn. So the jury's still out with me on the caffine front!
I personally try to limit my caffine intake to 3 or 4 drinks a week. I used to drink a lot of diet coke and have cut way back, but sometimes it's a no calorie treat that fills you up (with gas at least!).
I would say 1 iced tea a day would probably be a moderate consumption and I personally wouldn't worry too much, but some people prefer to cut it out completely. Let's see what everyone else says
