BOSTON — People who drank one or more diet sodas each day developed the same risks for heart disease as those who downed sugary regular soda, a large but inconclusive study found.

The results surprised the researchers who expected to see a difference between regular and diet soda drinkers. 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 of Boston University School of Medicine, emphasized the findings don't show diet sodas are a cause of increased heart disease risks. But he said they show a surprising link. 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.

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. 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.