For starters the artificial sweeteners in these drinks increase your insulin production and put your body into a fat storage mode. This is only part of the problem while your body is producing and filling fat cells, it is filling them with substances that are unrecognizable to the body. This makes it harder to get rid of the fat you just stored.
Now let’s look at the stats, people who drink diet sodas:
On average have a larger waist circumference
Increase risk of Type to Diabetes (36%)
30% more likely to be depressed
Lower bone density
Increase risk of heart attacks.
I am not telling you to not switch out those full power sodas, just switch them to water. Look at it this way: the more calories you drink, the more you have to burn. A Coke and a Smile is ok every now and then and should not put you over the edge after you are done with maintenance, but sodas should always be looked at as a treat and not an everyday occurrence.