Researchers knew that drinking regular sodas contributed to the risk of metabolic syndrome, but this is the first finding implicating diet sodas, according to results published online in Circulation: Journal of the American Heart Assn.
The researchers were uncertain why diet soda seemed to have such a large effect.
The study's lead author, Dr. Ramachandran S. Vasan of the Boston University School of Medicine, said it was unlikely that an ingredient in soda caused the effect. More likely is that consuming sweet sodas changes dietary patterns or that soda was simply a marker for participants' poor eating habits, he said.
Dr. Meir Stampfer of the Harvard School of Public Health, 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 has previously reported that diet sodas increase the risk of obesity and high blood pressure.
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 Assn.
She continued, "It is scientifically implausible to suggest that diet soft drinks -- a beverage that is 99% water -- cause weight gain or elevated blood pressure."
Metabolic syndrome is a cluster of symptoms including excessive abdominal fat, high blood-glucose levels, high blood pressure, high blood triglycerides and low levels of high-density lipoprotein, the so-called 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 studied more than 2,400 middle-age white residents of Framingham, Mass.
At the beginning of the study, those who had consumed more than one soda per day -- either regular or diet -- had a 48% 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 a day had a 44% higher risk of developing metabolic syndrome during the four years of the study.
Those who drank at least one soda per day also had:
* a 31% greater risk of becoming obese;
* a 30% higher risk of having a larger waist line;
* a 25% higher risk of developing high blood triglycerides or high blood sugar;
* a 32% greater risk of having low levels of good cholesterol;
* a trend toward an increased risk of high blood pressure.
The percentages were the same whether subjects drank regular or diet soda.
Vasan said a great deal of research had shown that people who drank sodas also tended to have diets higher in calories, saturated fats and trans fats and lower in fiber. They are also 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 may 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, however, has been confirmed by experiments.
"Our task is to report associations," Vasan said. "We do not claim that this is a causal link. It is up to scientists to help us understand this better."
In a statement on Monday, the American Heart Assn. said that diet soda remained "a good option to replace caloric beverages that do not contain important vitamins and minerals."
Water, diet soft drinks and fat-free or low-fat milk remain better choices than full-calorie soft drinks, the group said.
From the July 30, 2007, Prepared Foods e-Flash