Sodas -- even diet ones -- may be linked with increased risk factors for heart disease and diabetes, US researchers say.

They found adults who drink one or more sodas a day had about a 50 percent higher risk of metabolic syndrome -- a cluster of risk factors such as excessive fat around the waist, low levels of "good" cholesterol, high blood pressure and other symptoms.

"When you have metabolic syndrome, your risk of developing heart disease or stroke doubles. You also have a risk of developing diabetes," said Dr Ramachandran Vasan of Boston University School of Medicine, whose work appears in the journal Circulation.

Prior studies have linked consumption of sugar-laden sodas with multiple risk factors for heart disease, but Vasan and colleagues also found the link extends to diet sodas.

The study included about 6,000 middle-aged men and women who were observed over four years. Those who drank one or more soft drinks a day had a 31 percent greater risk of becoming obese.  They had a 30 percent increased risk of developing increased waist circumference -- which has been shown to predict heart disease risk better than weight alone.

They also had a 25 percent increased risk of developing high blood triglycerides as well as high blood sugar, and a 32 percent higher risk of having low high-density lipoprotein or "good" cholesterol levels.

The researchers then analysed a smaller sample of participants on whom data on regular and diet soft drink consumption was available. Those who drank one or more diet or regular sodas per day had a 50 to 60 percent increased risk for developing metabolic syndrome.

"The part about diet soda is more intriguing," Vasan said. He said people who drink soda, whether diet or sugar-sweetened, tend to have similar dietary patterns.

"On average, soda drinkers tend to eat more calories, consume more saturated fat and trans fat, eat less fiber, exercise less and be more sedentary," Vasan said in a telephone interview.

The researchers adjusted for those factors and still observed a significant link between soft drink consumption and the risk of developing metabolic syndrome.

Vasan said there are several theories about how diet sodas could increase a person's metabolic risk.

"One possibility is that diet soda is sweet. Maybe drinking something sweet conditions you in such a way that you develop a preference for sweet things," he said.

"Also, diet soda is a liquid. When you take liquids at a meal, they don't satiate you as much (as solids)," he said. The caramel coloring of some sodas also may play a role. He said caramel coloring in animal experiments was associated with tissue inflammation.

 "These are all theories which we have not studied," Vasan said.

 He said while the study showed an association between soda consumption and having a higher risk of metabolic syndrome, it does not prove soda was the cause. "Before people change their habits, we would like to see these data replicated in other studies, he said.

"We'd also like nutrition scientists to conduct additional research to help us understand why diet soda is associated with metabolic risk."

The American Heart Association, which publishes Circulation, said people should understand that the study did not demonstrate that diet sodas cause heart disease and said it can be better to have a diet drink than a full-calorie soda.

"The American Heart Association supports dietary patterns that include low-calorie beverages like water, diet soft drinks, and fat-free or low-fat milk as better choices than full calorie soft drinks," the group said.

The American Beverage Association said in a statement e-mailed to Reuters it appreciated the heart group made clear "the report ... does not show that soft drinks cause an increased risk of heart disease and it recognizes that diet soft drinks are a good option