To my knowledge Aspartame is a sweetening chemical which is bacteria feces. As it is the main sweetener for 'diet' drinks they say as the sweetener is in the stomach it tricks the body into thinking that sugar is on the way and as the sugar isnt actually there your body creates insulin which creates more fat than regular coke. So you may aswel have regular coke but then again. Just give up fizzy drinks altogether, i have done since the 1st of this month, ive been tempted but don't give in. Sugar is the most addictive drug in the world.
Best thing you can do is drink water, the body craves it, your body can work at its best with pure H2O, forget about cartons of fruit juice, they are just full of sugar. Just turn pro, drink water.