New research linking soda consumption with an increased risk of heart disease is sparking concern among nutrition researchers and physicians who say the findings could give consumers the wrong idea about their favorite fizzy beverages.
Carbonated soft drinks, rarely the darlings of nutrition research, have been implicated by previous research in childhood obesity, diabetes and other ills -- problems compounded by the growing consumption of these beverages in recent years.
Now a new study published in the current issue of the journal Circulation suggests that drinking a soda or more per day -- even if it's a diet soda -- is associated with an increase in other risk factors for heart disease.
The study's researchers report that those who said they drank a soda or more per day had a 31 percent greater chance of developing obesity, a 30 percent increased risk for gaining inches around the waist, a 25 percent chance of developing high blood sugar levels and a 32 percent greater chance of developing lower "good" cholesterol levels.
"This study further emphasizes the importance of lifestyle measures for preventing metabolic disease," said Ramachandran Vasan, senior author of the Framingham Heart Study and one of the authors of the current study.
But Vasan, who is also a professor of medicine at Boston University School of Medicine, added that the link found in the study between diet soda and heart ills was unexpected.
"This was sort of a surprising answer, as it did not seem to make a difference whether the soda was regular or diet," Vasan said. "Especially because we know that diet soda is a zero-calorie drink."
'Killer Cola' Explanation Unlikely
Vasan and others are concerned that the results of the study could be misinterpreted by the public as evidence of a direct link between soda consumption and an increase in heart risk.
"What we have here is an association, it does not really imply causality," Vasan cautioned.
"Clearly, a diet soda should not increase risk factors," said Dr. Dean Ornish, diet book author and clinical professor of medicine at the University of California at San Francisco. "It's almost certainly an association -- i.e., people who drink soft drinks probably are more likely to lead unhealthful lifestyles in other ways."
In other words, people who drink any kind of soda, diet or otherwise, may not be eating as healthily or exercising as often as those who stay away from them.
Few, after all, would likely report drinking a tall, cold glass of milk with a Big Mac and fries, or sucking down a liter of cola after a 5K run.
"There is no reason to think that soda -- as much as I do not think it should be a part of a healthy diet -- would cause heart disease," said Dr. Darwin Deen, associate professor of clinical epidemiology at the Albert Einstein College of Medicine. "But it comes as no surprise that people who do drink soda do other heart-harming things, thus creating an association between soda drinking and [heart disease]."
Other nutrition specialists agreed.
"Think about it: Soda is usually eaten with high-calorie fast food meals and high-calorie, high-fat snacks," said Keith Ayoob, a pediatric nutritionist at Albert Einstein College of Medicine. "Know anyone who eats fruit salad with soda? I rest my case."
"This is my favorite of the explanation: Soft drinks are just a marker of this lifestyle," said Dr. George Blackburn, associate professor of nutrition at Harvard University. "The message has to be a healthy lifestyle: Eat less, eat healthy and be physically active."
Priming the Sweet Tooth
Vasan and his fellow researchers noted in the study that there may be other reasons for the apparent link between diet soda consumption and metabolic syndrome. Perhaps, he said, consuming more liquids while eating has some impact on satiety.
Another factor at play, said Andrew Flood of the University of Minnesota Division of Epidemiology and Community Health, might be the fact that many diet soda drinkers started off as regular soda drinkers -- suggesting the damage may have already been done by the time they made the switch to diet cola.
"People who like and therefore drink a lot of soda will become obese," he said. "People who don't like being obese will switch to diet soda. Classic reverse causality. But this is just guessing. From these data, we have no idea what the real answer is -- all we can do is hypothesize."
Or maybe the sweetness of cola -- whether it's natural or artificial -- has the effect of priming the taste buds for more of the same.
"Diet soda is formulated to taste just as sweet as nondiet," Deen said. "Therefore, it has the same impact on taste threshold as regular soda.
"What this means is that soda drinkers are less likely to enjoy the taste of an apple or a fresh tomato and more likely to need stronger flavors (like salt) to make their food taste good. This may be part of the explanation."
Dr. Meir Stampfer, professor of epidemiology and nutrition at the Harvard School of Public Health, supported the possibility of such a link.
"Some people have speculated that the habitual sweet taste alters other food preferences in an unhealthy direction," Stampfer said. "This seems plausible, though I do not know of solid data on that."
But as far as regular beverages are concerned, he said, the connection may be much clearer.
"The added sugar of sugared beverages surely contributes to obesity and other aspects of the metabolic syndrome," he said. "Sugared beverages are the No. 1 source of calories in America, compared with all other food items."
Kicking the Can Habit
But while the new research paves the way for some interesting debates in an already contentious battle over how bad that can of cola really is for you, there is one point on which all nutritionists seem to agree: Americans would do well to pass on the bubbly beverages just the same.
"'Don't drink soda' is a very good message for people to hear, but there are many good reasons other than this study to deliver that message," said RAND public policy expert Dr. Paul Shekelle.
Even if it doesn't prove that soda can kill you, Connie Diekman, director of university nutrition at Washington University in St. Louis and president of the American Dietetic Association, said the discussion in which the authors engage is a valuable one.
"This study doesn't provide an answer, but the authors discussion of the impact of overall diet is important," she said. "Many consumers are trying to find 'the' cause for obesity and other disease risks, when likely the answer is in your overall lifestyle -- a message that many people don't want to hear."
"As a registered dietitian," Diekman said, "this study gives me one more piece of research to support my message that healthy eating involves making the right food choices, not avoiding or blaming single foods."