"But this is the first study to show the association in a prospective fashion and in a large population," Vasan said.
That population consisted of more than 6,000 participants in the Framingham Heart Study, which has been following residents of a Massachusetts town since 1948. When the soda portion of the study began, all participants were free of metabolic syndrome, a collection of risk factors including high blood pressure, elevated levels of the blood fats called triglycerides, low levels of the artery-protecting HDL cholesterol, high fasting blood sugar levels and excessive waist circumference. Metabolic syndrome is the presence of three or more of these risk factors.
Over the four years of the study, people who consumed more than one soft drink of any kind a day were 44 percent more likely to develop metabolic syndrome than those who didn't drink a soda a day.
The findings are published in the July 24 issue of the journal Circulation.
A variety of explanations, none proven, have been proposed for the link between diet soft drink consumption and metabolic syndrome, Vasan said. That association was evident even when the researchers accounted for other factors, such as levels of saturated fat and fiber in the diet, total calorie intake, smoking and physical activity.
One theory is that the high sweetness of all soft drinks makes a person more prone to eat sugary, fattening foods. Another is that the caramel content of soft drinks promotes metabolic changes that lead to insulin resistance. "These are hotly debated by nutritional experts," Vasan said.
Vasan, who noted that he is not a nutritional expert, said he leans toward the theory that "this is a marker of dietary behavior" -- that people who like to drink sweet soda also like to eat the kind of foods that cardiac nutritionists warn against.
"But we cannot infer causality," Vasan said, meaning there is no proof that soda itself is the villain. "We have an association. Maybe it is a causal one or maybe it is a marker of something else."
Carefully controlled animal studies might resolve the cause-and-effect issue, he said.
Dr. Elizabeth G. Nabel, director of the U.S. National Heart, Lung, and Blood Institute, which funds the Framingham Heart Study, said in a prepared statement: "Other studies have shown that the extra calories and sugar in soft drinks contribute to weight gain, and therefore heart disease risk. This study echoes those findings by extending the link to all soft drinks and the metabolic syndrome."
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