Dunno. One of my profs drinks a lot of diet soda. He specializes in GI physiology, with a focus on incretins. I'm sure he'd know more about whether diet soda is badder than regular soda (not that either are good, but you choose the lesser of the evils).
Or maybe he just doesn't care.
Fake edit: Well, there's aspartame (which is just a dimer of phenylalanine, I think). Unless you can't digest phenylalanine (for some people), I dunno if we know all that much more about aspartame's purported effects.
Fake fake edit: Agreed. Diet and artisan are absolutely silly labels. Along with gluten-free (though some recent study "suggests" that you don't have to have celiac disease to avoid gluten or something; or it could be research backed by food companies).
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Also, it's fine to drink sugar cola. No problem (key is moderation). When you drink a lot of it (like me), you want less immediately harmful alternatives.