Researchers from the University of Texas Health Science Center monitored 475 adults for 10 years.
They found those who drank diet soda had a 70 percent increase in waist circumference over the 10-year study, compared to those who didn't drink any soda.
Those who drank more than two diet sodas per day saw a 500 percent waist expansion.
In a separate study, conducted on mice, the same researchers suggested it was aspartame, which raised blood glucose levels, that caused the weight gain.
When your liver encounters too much glucose, the excess is converted to body fat.