Could diet drinks without calories cause weight gain? A new systematic review investigates all prior studies, and the results are still inconclusive.
The limited results from randomized controlled trials show no weight benefit from consuming artificial sweeteners, nor any clear negative effects. But in the observational data, there's a clear correlation with increasing weight.
So why could non-caloric sweeteners potentially cause weight gain? This is still speculative, as the Washington Post writes:
The reason could be in people's heads, as consuming a steady stream of artificially sweet foods might make people more likely to choose calorie-packed sweets the rest of the time. Or the mechanism could be a function of the gut, Azad said. She specializes in gut bacteria and theorized that diet drinks could affect the makeup of the small organisms in the digestive tract, which can affect digestion and overall health.
The causality could go in the other direction, too - people who are gaining weight for other reasons may seek out more artificially sweetened foods. Or, as other research has shown, people who go on diets (and who may be more likely to drink diet sodas) often lose weight but then gain more afterward.
The bottom line is that people who drink non-caloric sweeteners - and thus presumably focus on calories - on average end up gaining weight. There are many possible reasons why that may be the case.
Personally, I always recommend people who want to lose weight to avoid unnaturally sweet foods, for many reasons. And certainly don't focus on calories, focus on diet quality instead.