A new US study has 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, via an increase in metabolic syndrome, a group of characteristics like excess girth, high blood pressure, and other factors that increase the chances of getting diabetes and cardiovascular problems.
The findings are published in the early online edition of Circulation, the journal of the American Heart Association.
The research is part of the large scale ongoing study known as the Framingham Heart Study (FHS), that started in 1948 and is 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).
Senior author of the study, Dr Ramachandran Vasan, who is professor of medicine at Boston University School of Medicine in Massachusetts, said that:
"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."
"In those who drink one or more soft drinks daily, there was an association of an increased risk of developing the metabolic syndrome," he added.
Other studies have linked drinking soft drinks with risk factors for heart disease, but this study suggests that diet soft drinks sweetened with artificial sweeteners are just as likely to be linked as high calorie drinks sweetened with sugar.
Dr Ravi Dhingra,an instructor in medicine at Harvard Medical School, and lead author of the study said that:
"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."
Metabolic syndrome is a group of symptoms such as excess waist circumference, high blood pressure, elevated triglycerides, low levels of high-density lipoprotein (HDL or "good" cholesterol) and high fasting glucose levels. Having three or more of the symptoms increases a person's risk of developing diabetes and cardiovascular disease.
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 per cent 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 per cent higher risk of getting metabolic syndrome for the first time (new onset).
The investigators also 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 per cent greater risk of developing new-onset obesity (defined as a body mass index or BMI of 30 kilograms per meter squared or higher).
30 per cent higher risk of developing an increased waist circumference.
25 per cent increased risk of developing high blood triglycerides or high fasting blood glucose.
32 per cent 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 (fizzy) drinks a day had a 50 to 60 per cent 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, for instance to take out the effect of food that is typically eaten at the same time as soft drinks, but this had no effect on the strength of the link. For example 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".
Speculating on their findings, Vasan said 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. "Our findings suggest this is not the case," said Vasan.
Perhaps the answer lies in the fact that drinking more liquids during a meal tends to make you eat more solids at the next meal. Whereas if you had drunk less and eaten more at the first meal, at the next meal you tend to eat less solids. Drinking more liquids does not compensate to the same extent as eating solids; you feel full at the time, but you are hungrier next time.
Another possibility could be that being used to drinking sweet drinks makes a person more likely to eat sweet things.
And another 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 realise 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. They suggested further studies be done to repeat the results and to find out what the underlying mechanisms could be, before recommendations are made about whether people should change their consumption of soft drinks.
"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.
Circulation Published online before print July 23, 2007
doi:10.1161/CIRCULATIONAHA.107.689935
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