Doesn't caffeine already increase dopamine transmission? (Thus indicating its very addictive side)

I'm doubtful that the artificial sweetener in diet sodas can really contribute all that much to mental disorders. Everything you do alters neurotransmitter production/activity in the brain. Food especially is very mind altering -- such as the surge of serotonin you receive after eating a lot of carbs.

I think that caffeine and sodium would be the worst ingredients to look out for. Sodium is linked to bipolar disorder and blood pressure. I've heard that MSG -- a flavoring found in a lot of highly processed foods -- can also act like a neurotransmitter in the brain. MSG is just another form of sodium.

I've gotten to be pretty bad about drinking diet soda these past few months -- I used to never be a soda drinker. Luckily I've been switching it out with crystal lite lately. It's still an artificial sweetener, but no caffeine, no sodium, and all hydration!