Drinking as little as one can of soda per day - either regular or diet - is associated with nearly 50 percent increased risk of a cluster of factors considered to be key predecessors of heart disease and diabetes, researchers reported yesterday.
Scientists previously had found that drinking regular soft drinks contributed to the risk of "metabolic syndrome," but this is the first major study implicating diet sodas, according to results published online in Circulation: Journal of the American Heart Association.
The researchers were uncertain why diet soda also made such a difference.
Ramachandran S. Vasan of Boston University School of Medicine, the lead author of the study, said it was unlikely that an ingredient in diet soda caused the effect.
More likely, he said, is that soft-drink consumption changes dietary patterns - perhaps people who drink any kind of soda, for example, become accustomed to eating more sweets - or that soda consumption simply accompanies generally poor eating habits.
Meir Stampfer of Harvard Medical School, who was not involved in the study, said the findings were not unexpected, although he added, "I'm surprised by the magnitude of the association."
Stampfer previously had reported that diet sodas increase the risk of obesity and high blood pressure.
Metabolic syndrome, the association reported yesterday, is broader - a cluster of symptoms that includes excessive abdominal fat, high blood-glucose levels, high blood pressure, high blood triglycerides, and low levels of HDL ("good") cholesterol.
People with three or more of these symptoms have double the normal risk of heart disease and diabetes.
In the study, sponsored by the National Heart, Lung and Blood Institute, Vasan and his colleagues looked at more than 2,400 middle-aged, white residents of Framingham, Mass., a group that has been followed for other significant medical research over many decades.
At the beginning of this study, those who had consumed more than one soda per day - either regular or diet - had a 48 percent higher risk of having metabolic syndrome.
The team then focused on the more than 1,600 people who did not have metabolic syndrome at the start of the study and followed them for at least four years. Those who drank at least one soda per day had a 44 percent higher risk of developing metabolic syndrome during the four years of the study.
Those who drank at least one soda per day also had:
The percentages were the same whether a subject drank regular or diet soda.
Soda-makers rejected the study.
"The assertions defy the existing body of scientific evidence, as well as common sense," said Susan K. Neely, president and chief executive of the American Beverage Association. "It is scientifically implausible to suggest that diet soft drinks - a beverage that is 99 percent water - cause weight gain or elevated blood pressure."
Vasan said other research had shown that people who drink sodas also tended to have a diet that is higher in calories, higher in saturated fats and trans fats, and lower in fiber. They also are more sedentary.
The authors tried to control for all those factors in the diet, but "even after all that, we still found an increased risk," he said. "Maybe it is very difficult to adjust for lifestyle."
Another possibility is that drinking soda with a meal reduces the feeling of satiety, so that the person eats more at the next meal, he said.
Alternatively, drinking sweet sodas might get people used to a sweet taste and "into the snacking mode," Stampfer said. "It's not the artificial sweetener but what goes along with it."
None of those theories has been confirmed.
"We do not claim that this is a causal link," Vasan said. "It is up to scientists to help us understand this better."