If you’re watching your waistline, then replacing sugary, high-calorie sodas with diet varieties might seem like a no-brainer. But two new studies show diet soda may not be any better for you. Why? Because drinking these low- or no-cal pops might actually lead to weight gain and higher disease risk.

Researchers at the University of Texas Health Science Center in San Antonio found that diet soda drinkers gained substantially more abdominal fat during a near 10-year period than people who drank regular soda. (Abdominal fat is a risk factor for illnesses such as cancer and cardiovascular disease.)

But it’s not just belly fat that worries researchers. Another University of Texas study showed that aspartame—the sugar substitute often found in diet sodas—raised blood-sugar levels of diabetes-prone mice, which can indicate diabetes or pre-diabetes. What scientists don’t yet know is whether aspartame has the same effect on humans. So, while tests continue, perhaps a moderate approach is best.

Only drink diet sodas occasionally; don’t guzzle them down each and every day.