July 24, 2007 - A new U.S. study published in the early online edition of Circulation, the journal of the American Heart Association, found that drinking more than one soft drink a day, whether regular or diet, may be linked to an increased risk of developing heart disease, due to an increase in metabolic syndrome.
The findings are part of the Framingham Heart Study (FHS), which was started in 1948 and now in its third generation of participants, grandchildren of the original cohort. The FHS looks at common factors or characteristics that contribute to cardiovascular disease (CVD) by following its development over a long period of time in a large group of people who joined before they had any overt symptoms of CVD or heart attack or stroke.
The FHS was started under the direction of the National Heart Institute (now known as the National Heart, Lung and Blood Institute or NHLBI).
"Moderation in anything is the key. If you are drinking one or more soft drinks a day, you may be increasing your risk of developing metabolic risk factors for heart disease," said Ravi Dhingra, M.D., an instructor in medicine at Harvard Medical School, and lead author of the study.
The study included 9,000 "person observations" taken at three different times over a four year period from participants enrolled in the FHS, all middle aged men and women. At the start of the four year observation period ("baseline"), the scientists established that participants who drank one or more soft drinks a day had a 48 percent higher chance of having metabolic syndrome than those who drank less.
Over the four year follow up, a longitudinal study of those participants who did not have metabolic syndrome at baseline (6,039 person observations) showed that drinking one or more soft drinks a day was linked to a 44 percent higher risk of getting metabolic syndrome for the first time.
Researchers found that participants who drank one or more soft drinks a day, when compared to those that drank less, had a number of increased risk factors for metabolic syndrome, including:
* 31 percent greater risk of developing new-onset obesity (defined as a body mass index or BMI of 30 kilograms per meter squared or higher).
* 30 percent higher risk of developing an increased waist circumference.
* 25 percent increased risk of developing high blood triglycerides or high fasting blood glucose.
* 32 percent increased risk of having low HDL ("good" cholesterol).
* A trend towards an increased risk of developing high blood pressure, but this was not statistically significant.
They also took data from a smaller separate group of participants who had filled in food questionnaires about their soft drink consumption and found that those participants who drank one or more diet or regular soda drinks a day had a 50 to 60 percent higher risk of developing new-onset metabolic syndrome.
The researchers adjusted the results to take into account a number of dietary and lifestyle factors of soft drink users. They adjusted for saturated fat and trans fat intake, dietary fibre, total caloric intake, smoking and physical activity, but, as Vasan said, they "still observed a significant association of soft drink consumption and risk of developing the metabolic syndrome and multiple metabolic risk factors".
Vasan speculates that perhaps the fructose corn syrup in regular soft drinks causes weight gain, or leads to insulin resistance and diabetes, but if that were the case you would expect to see the link with regular drinks and not with diet drinks.
One theory is that the caramel in soft drinks could stimulate the development of complex sugars (high glycation end products) that result in insulin resistance and cause inflammation, as shown by some experimental studies.
The researchers said it was important to realize that these are just theories, and their study had only found a link between soft drinks and metabolic syndrome, it had not established that one causes the other.
"Soft Drink Consumption and Risk of Developing Cardiometabolic Risk Factors and the Metabolic Syndrome in Middle-Aged Adults in the Community." Ravi Dhingra, Lisa Sullivan, Paul F. Jacques, Thomas J. Wang, Caroline S. Fox, James B. Meigs, Ralph B. D'Agostino, J. Michael Gaziano and Ramachandran S. Vasan.
For more information: http://www.americanheart.org/presenter.jhtml?identifier=4756
For the abstract: http://circ.ahajournals.org/cgi/content/abstract/CIRCULATIONAHA.107.689935v1