Getty Images

Diet soda is falling out of favor. According to new research from Wells Fargo, low-calorie and zero-calorie soda sales slipped about 7% over the past year. Regular soda has fallen just over 2%.

The reason remains a mystery, but perhaps folks are realizing that the benefits of drinking diet soda are just not there. At least, if you’re trying to shed pounds.

Several studies in the past couple of years have torn holes in the theory that zero-calorie sodas mean zero weight gain. Indeed, research presented at the American Diabetes Association’s (ADA) Scientific Sessions in 2011 showed the opposite is true—diet sodas can actually contribute to weight gain.

(MORE: Studies: Why Diet Sodas Are No Benefit to Dieters)

How? There are a few theories. First, scientists speculate that artificial sweeteners fool more than just your palate; they also fool your brain. When you taste something sweet, your body naturally expects a calorie-load that diet beverages don’t deliver. As a reaction, the metabolic system may start converting the sugar that’s already circulating in the blood into fat, on the assumption that more has just come in that can be used as energy. In the alternative, the body may go in the other direction, burning though the circulating sugar so that the incoming soda doesn’t leave you with too much. But since the soda has no sugar at all, you wind up with a net loss—which may lead to a craving for candy or some other high-sugar snack.

It’s also possible that the lack of calories causes diet-soda drinkers to overeat later for psychological reasons. They either feel unsatisfied and eat more to make up for it, or they think they saved on calories earlier by opting for diet soda—a handy justification for eating more.

The plunging sales numbers may suggest that consumers are catching wise to all of this. The solution: it’s better to kick the soda habit and stick to water.