It could be, they suggest, that even no-calorie sweet drinks increase the craving for more sweets, and that people who indulge in sodas probably have less healthy diets overall.
The study's senior author, Dr. Vasan Ramachandran, emphasized the findings don't show diet sodas are a cause of increased heart disease risks. But he said they show a surprising link that must be studied.
"It's intriguing and it begs an explanation by people who are qualified to do studies to understand this better," said Vasan, of Boston University School of Medicine.
However, a nutrition expert dismissed the study's findings on diet soda drinkers.
"There's too much contradictory evidence that shows that diet beverages are healthier for you in terms of losing weight that I would not put any credence to the result on the diet (drinks)," said Barry Popkin, of the University of North Carolina in Chapel Hill, who has called for cigarette-style surgeon general warnings about the negative health effects of soda.
Susan Neely, president of the American Beverage Association, said the notion that diet drinks are associated with bulging waistlines defies common sense.
"How can something with zero calories that's 99 percent water with a little flavoring in it ... cause weight gain?" she said.
The research comes from a massive, multi-generational heart study following residents of Framingham, Mass., a town about 25 miles west of Boston. The new study of 9,000 observations of middle-aged men and women was published Monday online in the journal Circulation.
The researchers found those who drank one or more sodas a day _ diet or regular _ had an increased risk of metabolic syndrome, compared to those who drank sodas infrequently. Metabolic syndrome is a cluster of symptoms that increase the risk for heart disease including large waistlines and higher levels of blood pressure, blood sugar, cholesterol and blood fats called triglycerides.
At the start of the study, those who reported drinking one or more soft drinks a day had a 48 percent increased prevalence of metabolic syndrome compared to those who drank less soda.
Of participants who initially showed no signs of metabolic syndrome, those who drank one or more sodas a day were at 44 percent higher risk of developing it four years later, they reported.
Researchers expected the results to differ when regular soda and diet soda drinkers were compared, and were surprised when they did not, Vasan said.
But Popkin said that result isn't that surprising. He said much of the market for diet sodas are people who have unhealthy lifestyles and know they need to lose weight _ with the other portion being thin people who want to stay that way. That means many people drinking diet sodas have unhealthy habits that could lead to increased heart disease risks, whether they drink diet soda or not.
In studies in which some users were randomly given diet sodas and others were given regular soda, diet soda drinkers lost weight and regular soda drinkers gained weight, Popkin said.
In a statement, the American Heart Association said it supports dietary patterns that include low-calorie beverages.
"Diet soda can be a good option to replace caloric beverages that do not contain important vitamins and minerals," the association said, adding further study is needed before any association between diet soda and heart risk factors would lead to public recommendations.
Vasan also said poor overall health habits may be one reason diet soda drinkers did not show lower heart disease risks in the Framingham study, but there hasn't been enough research to say for sure.
Another possible reason is a controversial theory called "dietary compensation," which holds that if someone drinks a large amount of liquids at a meal, they aren't satisfied and will tend to eat more at the next meal, Vasan said.
Other theories, Vasan said, are that people who drink a large amount of sweetened drinks are prone to develop a taste for sweeter foods, or that the substance that gives soda its caramel color promotes resistance to insulin, which is needed to process calories.
Without a more definitive explanation, Vasan offers only this advice to diet soda drinkers: "Consume in moderation and stayed tuned for more research."