Calorie-free diet soda may seem like a healthy option compared to sugar-laden, high-calories beverages, but a Boston University professor and researchers in Framingham have reported both diet and regular soft drinks may increase risk of heart disease and diabetes.

School of Medicine professor Dr. Ramachandran Vasan, the senior author of the Framingham Heart Study, said drinking at least one soda a day increases an adult’s risk of developing metabolic syndrome by 45 percent, compared to people who drink less than one a day. Metabolic syndrome includes risk factors that double the chance of heart disease.

The five components of metabolic syndrome are a waist circumference of at least 35 inches for women or at least 40 inches for men, high blood pressure, low levels of healthy cholesterol, high-fasting glucose levels — a method to test for diabetes after someone has not eaten for many hours — and elevated triglycerides, the chemical form of fat in the body.

“If you have three or more of these components, you’re said to have the metabolic syndrome,” Vasan said. “Having the metabolic syndrome, it approximately doubles your risk of having heart disease or stroke in the future. It is also a precursor to diabetes.”

The study received national attention throughout the summer by many major media outlets. Vasan called the study “a very interesting observation,” but said more studies must be done to replicate his findings about diet soda.

“Stay tuned for more research,” he said.

Researchers used 6,100 “person observations” of participants, with an average age of 55 years old, during the Framingham Heart Study, which was published in the July 31 issue of Circulation. When the study began, the participants did not have three or more components of metabolic syndrome, Vasan said.

The researchers were “somewhat surprised” that drinking diet soda increased adults’ risk for metabolic syndrome even though other reports have already linked diet soda consumption to heart-disease risk, Vasan said.

The increased risk could be attributed to the soda or the lifestyle and diet of the drinker, he said.

People who drink more soft drinks tend to consume more calories, more saturated and trans fats, less dietary fiber and dairy products and usually have a more sedentary lifestyle, he said.

Although the researchers tried to adjust for lifestyle factors, “one cannot really exclude the possibility that it could be about the dietary lifestyle of soda drinkers,” Vasan said.

The soda itself may also increase the risk for metabolic syndrome, he said. Because soda is liquid, it is not as filling as solid foods.

“Even though it’s a low calorie drink . . . you aren’t as satiated and end up eating more at the next meal,” he said.

Another theory is that soda — diet and regular — is sweet, and the sweetness could encourage people to eat more sweets in general.

“It could be conditioning a response causing you to eat more sweet items,” he said.

Vasan said the caramel coloring in cola-type sodas may also be to blame. Based on reactions in animals in experimental studies, the caramel coating may be a source of substances found that promote insulin resistance — a precursor to diabetes — he said.

Medical studies graduate student Alex Paiva said he drinks diet soda instead of regular soda to try to maintain his weight.

“I choose the more diet option because it’s not going to affect me as much,” he said.

Because he is not a “constant soda drinker,” Paiva said the findings of the study probably will not have an effect on his consumption of the beverage.

“I know I’m not going to be at the level where I put myself at risk,” he said.

College of Engineering freshman Patrick Henson said he drinks regular soda once or twice a week. He said the study’s findings do not faze him because there are so many other habits in American culture that pose health risks.