The study's participants did not initially have metabolic syndrome, which is defined as the presence of at least three out of a group of five symptoms -- large waistlines, low levels of HDL, or "good," cholesterol, and higher blood pressure, blood sugar, and triglycerides. The metabolic syndrome doubles the risk of heart disease and stroke and increases the risk of developing diabetes.

Framingham residents who drank soft drinks at least once a day were 48 percent more likely to develop metabolic syndrome. Study participants who drank one or more soft drinks a day were also more likely to be obese (31 percent), have an increased waist circumference (30 percent), a low high-density lipoprotein or "good" cholesterol level (32 percent), high triglyceride counts and high blood sugar levels (25 percent).

The research's surprise came when the data showed that the increased prevalence of metabolic syndrome occurred not just in the people who drank regular soda, but in those who drank diet soda as well.

The study's authors point out that their results don't prove that diet sodas cause health problems, just that there's an association between the two, and some nutritionists say that association shouldn't be blown out of proportion.

People like to be able to point to one thing as the problem, and therefore the solution, said Dr. Brian Wansink, author of "Mindless Eating." But because this study doesn't prove that diet soda is causing metabolic syndrome or the other negative effects, there's no cause for over-reaction, he said.

"I think part of what's going on is that there's a self-selecting population that drinks diet pop," said Wansink, director of the Food and Brand Lab at Cornell University. People who drink soda, whether diet or regular, tend to have similar dietary patterns: they eat more calories, saturated fats and trans fats, exercise less, and are more sedentary.

The two markets for diet soda are people who are thin and want to stay that way, and, largely, people who have unhealthy lifestyles and want to lose weight, said Dr. Barry Popkin, a nutrition professor at the University of North Carolina at Chapel Hill, where he directs the Interdisciplinary Center for Obesity.

"You've got to realize that in this study the diet soda drinkers were not your normal Framingham person," Popkin said. "They were people who had more health conditions." Therefore, many of the people drinking diet soda were doing so because they already had health problems, and had been told to by their doctors, not because they wanted to.

The initial characteristics of the study participants show that those who drank soda at least once a day weighed more and had a higher body mass index (BMI) -- and more of them already have a BMI over 30, which is considered to be obese. They also had larger waist circumferences, which may provide a more accurate measure of the risk of weight-related health problems than BMI alone. More of the soda drinkers were diabetic, and those who drank two or more sodas a day were more likely to be smokers.

The study results accounted for fat and trans fat intake, dietary fiber consumption, smoking and physical activity and still found the association between diet and regular soda consumption and metabolic syndrome.

However, the authors acknowledged that other factors may have an effect. Popkin pointed out that when diabetics were excluded from the findings, there was only a 16-percent higher risk of health problems in the soda drinkers.

The authors of the study discussed the possibility that diet drinks may increase sugar cravings, which is a concern, Popkin said, but it hasn't been adequately studied. "I think that the big issue that's unresolved is the question of whether non-caloric sweeteners condition us to want more sweetness, so that we consume more sweet foods in general."

Both Popkin and Wansink say that what's most likely is that diet soda consumption is a marker for the general eating habits in the participants. "Sometimes when people eat something that's low-calorie or drink something that's low-calorie, it's so they can indulge in something else," said Wansink, who added that his own research supports that conclusion.

It is probably always better for people to drink more water instead of diet beverages, Wansink said, but it's important to consider the other alternatives people would choose. "What would they be drinking if they weren't drinking diet pop?"

Of those possible alternatives, the risks associated with regular soda consumption are clearer. Several well-controlled trials have shown negative health effects from consuming caloric sodas that did not occur with diet sodas, Popkin said.

Full-calorie soda has about 150 calories per can, and studies have linked soft drink consumption to diabetes and obesity in children and adolescents and high blood pressure in adults. Removing regular soft drinks from one's diet would be a positive change, Popkin and Wansink pointed out, but it isn't the only change people need to make to improve their overall health.

It would be unwise to switch back from diet to regular soda based on these results, Popkin said. "For the consumer, it'd be a very bad message," he said. "It would say 'Might as well drink regular Coke. Gonna have the same effect as Diet Coke.' Well, that's not true."

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