It the aspartame
In another study, aspartame, acesulfame potassium, and saccharin were all associated with heightened motivation to eat and more items selected on a food preference list [35]. Aspartame had the most pronounced effect, possibly because it does not have a bitter aftertaste.
drink diet = crave more= eat more = thirst more = weight gain.
Aspartame breaks down into three components in the body: aspartic acid, methanol, and phenylalanine. Phenylalanine is an amino acid that’s a precursor to tyrosine, which is used to produce dopamine and norepinephrine. In theory, this could indirectly influence mood-related hormones, but the amount of phenylalanine from typical aspartame consumption (e.g., a diet soda) is tiny compared to what you get from regular foods like meat, eggs, or dairy. So, it’s unlikely to have a noticeable effect on feel-good hormones.