You may want to put down that diet soda. New research inserts a question mark after the “diet” part of your drink.

In the study, people who drank a can or more of diet soda daily showed a 34-percent higher risk of developing the metabolic syndrome: a cluster of cardiovascular disease and diabetes risk factors including elevated waist circumference and high blood pressure, blood lipids, and fasting glucose levels.

Why would that be? Study coauthor Lyn Steffen, PhD, MPH, RD, says she is as mystified as the rest of us. But she offers some possible explanations. “It could be an ingredient in the soda itself, like the artificial sweetener, which might be causing something like insulin resistance,” speculates Steffen, associate professor of epidemiology at the University of Minnesota. “Or it could be something to do with the behavior of people who consume diet soda—what other foods they’re eating and how much exercise they’re getting throughout the day.”

Her research team tracked the dietary intake and health status of 9,500 men and women, 45 to 64 years old, over nine years. They found that people who ate the most meat raised their risk of developing metabolic syndrome by about 25 percent. And those who regularly ate Western-style cuisine like refined grains and fried foods upped their risk 18 percent.

But diet soda involved the highest risk—and, Steffen notes, a recent Purdue Uni­ver­sity study suggests a possible reason. In that study, rats eating saccharin-sweetened yogurt consumed more of it, and gained more weight, than rats eating sugar-sweetened yogurt.

In Steffen’s study, most diet sodas “were likely sweetened with aspartame, not saccharin, but it could be the two work similarly,” she says. “So maybe diet soda consumers are eating more.” This isn’t the first study to link the metabolic syndrome and diet soda. However, past studies show the link with sweetened soda as well as diet versions. This study showed no such association between sweetened beverages and the syndrome.

But that’s no reason to start drinking sugary sodas, which are loaded with empty carbohydrates. Instead of reaching for soda (regular or diet), Steffen suggests trying water or green or black tea. Another good bet is skim milk. Steffen’s team found that low-fat dairy products help stave off the metabolic syndrome.

The American Heart Association published the diet soda findings online on Jan. 22, 2008, in its journal Circulation. The saccharin and weight-gain study appeared in the Feb. 2008 issue of the journal Behavioral Neuroscience.