Writing in the journal Circulation: Journal of the American Heart Association, lead author Ramachandran Vasan, from Boston University School of Medicine, states that it made no difference if the drinks were low-calorie or regular.
"We were struck by the fact that it didn't matter whether it was a diet or regular soda that participants consumed, the association with increased risk was present," said Vasan. "In those who drink one or more soft drinks daily, there was an association of an increased risk of developing the metabolic syndrome."
However, industry experts and trade associations have quickly reacted to the study, with Roger Clemens, spokesperson for the Institute of Food Technologists, calling the findings "oversimplified."
"There are many attributes associated with the development of metabolic syndrome," said Clemens. "Some of which are part of lifestyle choices, such as eating too many calories."
"It's way too soon to say stop drinking diet soda," he added. "Diet soda, in moderation, can be part of a healthy lifestyle."
The research investigated the link between soft drink consumption and metabolic syndrome in people taking part in the Framingham Heart Study (6039 person, 3470 women, average age 52.9). None of the subjects had metabolic syndrome (MetS) at the start of the study.
Metabolic syndrome (MetS) is a condition characterised by central obesity, hypertension, and disturbed glucose and insulin metabolism. The syndrome has been linked to increased risks of both type-2 diabetes and CVD.
Fifteen per cent of adult Europeans are estimated to be affected by MetS, while the US statistic is estimated to be a whopping 32 per cent.
In a cross-sectional study - a "snapshot of time" - the researchers report that consumption of one soft drink per day was associated with a 48 per cent increase in the metabolic syndrome risk.
In a longitudinal study over an average of four years, 765 participants (18.7 per cent) developed MetS in the 4095 participants consuming less than one soft drink per day, compared to 474 (22.6 per cent) of the 2059 persons consuming one soft drink per day.
Vasan and co-workers calculated that consumption of one soft drink per day was associated a 44 per cent increased risk of developing MetS, a 31 per cent increased risk of being obese, and a 32 per cent increased risk of having low levels of HDL ('good') cholesterol.
"Results also don't appear to be driven by the dietary pattern of soft drink users, i.e, by other food items that are typically consumed along with soft drinks," said Vasan. "We adjusted in our analyses for saturated fat and trans fat intake, dietary fiber consumption, total caloric intake, smoking and physical activity, and still observed a significant association of soft drink consumption and risk of developing the metabolic syndrome and multiple metabolic risk factors."
Various explanations for the observations included less dietary compensation when drinking lots of fluid, or the sweetness of the beverages promoting the consumption of other sweet foods.
"These are all theories, and experts debate their importance," said co-author Ravi Dhingra from Harvard Medical School. "Our study was observational, and so right now all we demonstrate is an association. We have not proven causality."
The AHA released a statement to stress this same point.
"Since this is an observational study, it is important to note that the study does not show that soft drinks cause risk factors for heart disease. It does show that the people studied who drank soft drinks were more likely to develop risk factors for heart disease," said the association.
"However, it is possible that other factors could explain this relationship. Often people who drink soft drinks also eat and drink more calories, saturated fat and trans fat and less fibre and dairy products. Also, these people tend to be less physically active. This was true among the subjects in this study," it added.
The AHA statement was welcomed by Susan Neely, president and CEO of the American Beverage Association.
"The AHA acknowledges that the report published in Circulation does not show that soft drinks cause an increased risk of heart disease and it recognizes that diet soft drinks are a good option for those looking to cut calories in their beverages," she said. "We appreciate the AHA clearing up any confusion surrounding this report."
"It defied common sense and the existing body of scientific evidence to assert a link between soft drinks and increased risk of heart disease," said Neely. "Even the researchers themselves admit their study can't support a link. Further, it is scientifically implausible to suggest that diet soft drinks - a beverage that is 99 per cent water - cause weight gain or elevated blood pressure."
Published on-line ahead of print, doi:10.1161/CIRCULATIONAHA.107.689935
"Soft Drink Consumption and Risk of Developing Cardiometabolic Risk Factors and the Metabolic Syndrome in Middle-Aged Adults in the Community"
Authors: R. Dhingra, L. Sullivan, P.F. Jacques, T.J. Wang, C.S. Fox, J.B. Meigs, R.B. D'Agostino, J.M. Gaziano, and R.S. Vasan