Choosing diet soda in an effort to take off a few pounds can have the opposite effect, a study has found; it can lead to increased amounts of belly fat, especially in older people.

Over a period of nine years covered by the study, people who regularly drank diet soda gained almost triple the amount of fat in the abdominal area as those who didn't consume it, researchers at the University of Texas Health Science Center at San Antonio say.

Their increase in waist circumference was three times that of people who didn't drink diet sodas, and even people who consumed diet soda only occasionally had double the increase over people who drank no diet soda, they reported in the Journal of the American Geriatrics Society.

"The more people drank diet sodas, the more their waistlines expanded," says Sharon Fowler, one of the study authors. That's a concern, she says, because belly fat has been linked to increased risks of cardiovascular disease and developing type 2 diabetes.

The researchers say the mechanism through which diet sodas lead to weight gain isn't fully understood, but they have some theories.

One is based on the knowledge that diet sodas are sweetened by substances having sweetness levels that are as much as 200 to 600 times that of sugar.

Sugar intake triggers satiety, the feeling of being full and satisfied.

"Your body is used to knowing that a sweet taste means you are ingesting energy in the form of calories that, if you don't burn them off, is going to convert to fat," says study senior author Helen Hazuda.

However, she notes, artificial sweeteners don't have the same effect and weaken the link our brains make between sweetness and calories, which can lead to weight gains and a craving for ever more and sweeter treats.

The Calorie Control Council, an association representing the low- and reduced-calorie food and beverage industry, said it disagreed with the researchers' findings, noting that the study could not show a cause and effect, only an indication of a possible association between diet soda and fat increase.

"The use of low-calorie sweeteners (LCSs) in weight management has been shown to be beneficial," the council said in a statement. "While approaches to treat obesity in older individuals is controversial, diet modifications can be a successful part of a weight-management program for older adults."

Fowler, for her part, suggested people should still consider reducing their intake of diet soda or quit it altogether.

"The more people can try to duplicate some of the things they love about diet sodas with something else that is really a whole food, the better," she says.