(Bloomberg) — Just a single soft drink a day, diet or regular, may contribute to an array of health conditions that can hurt the heart and lead to diabetes, researchers said.The results were surprising since so little soda increased the risk and because diet drinks, noteworthy for their lack of calories, had the same effect as sugary beverages, the researchers said. In the study, people who drank one or more cans a day had a 44 percent higher risk of developing the condition known as metabolic syndrome, the study found.

The study adds weight to earlier reports that drinking regular soda may lead to obesity and heart disease, said Ramachandran Vasan, a cardiologist and senior author of the paper from Boston University School of Medicine. The findings for diet drinks, about 30 percent of the $70.1 billion US soda market, must be confirmed by future research, he said.

“Having metabolic syndrome approximately doubles your risk of heart attack or stroke,” Vasan said in a telephone interview. “Drinking one soft drink a day increases that risk. This is not an association that can be good.”

The soft drink industry immediately disputed the findings.

“The study does not establish any link between soft drinks, regular or diets, and increased risk of heart disease,” said Susan Neely, president of the American Beverage Association, in a telephone interview.

The Washington-based group represents the interests of soda makers. Coca-Cola Co. spokeswoman Diana Garza Ciarlante, PepsiCo Inc. spokesman Dave DeCecco and Cadbury Schweppes Plc spokesman Greg Artkop all referred calls to the beverage association.

“With diets, it’s particularly implausible,” since they have no calories and are 99 percent water with a little flavour added, Neely said. “Diet soft drinks are terrific if you’re trying to watch your weight and want something refreshing that tastes good and has no calories.”

The researchers, from the Framingham Heart Study, have followed people in the Boston suburb for decades looking for lifestyle and other factors that increase the risk of cardiovascular disease. They analysed data from more than 6,000 people who filled out food questionnaires and were followed for an average of four years to gauge the impact of their soft drink consumption habits on the health.

Soda drinkers fared worse on the individual components of metabolic syndrome, such as high blood sugar, low levels of “good” cholesterol, abdominal fat and elevated triglycerides, the study found. Blood pressure rose slightly among those who drank the most soda, though the results could have been the result of chance, the researchers said.

“It did not really matter whether you were drinking regular soda or diet soda,” said Vasan, who called the consistency of the results striking. “This adds one more piece to the puzzle.”

The researchers said they aren’t entirely sure how diet soft drinks, with no calories, increased the risks. While soda drinkers do consume more fried foods and more calories, tend to exercise less, smoke more and eat less fibre and dairy products, the researchers took all those factors into account. Even adjusting for them, soft drinks were linked to greater risk. There are several hypotheses for how and why soda may lead to health problems, Vasan said. The high fructose corn syrup can lead to weight gain and resistance to insulin, which converts blood sugar to energy. The fact that diet sodas were implicated made the researchers look for other possibilities, he said.

The study, funded by the National Institutes of Health and the American Diabetes Association, appears in the American Heart Association’s medical journal Circulation.

People who consume a lot of liquids may not reduce their subsequent caloric intake, particularly for diet sodas, and the sweetness of both beverages may lead to cravings for other sweets, the researchers said. The caramel in both drinks may promote inflammation and insulin resistance, they said.

Americans drank an average of 814 eight-ounce servings of soda last year, up 26 percent in the past two decades, according to data compiled by industry journal Beverage Digest. That’s down from a peak of 864 servings a year in 1998. Consumers in Western Europe drank an average of 461 servings of soda in 2006.

By comparison, Americans drank an average of 349 servings of beer, 336 servings of bottled water, 312 of milk and 261 of coffee, Beverage Digest data shows.

It’s common sense that sugary soft drinks every day will lead to weight gain in some people, said Dean Ornish, a researcher and cardiologist at the University of California at San Francisco and founder of the Preventive Medicine Research Institute, in a telephone interview today. The tie to diet soda, however, defies logic, he said.

“It’s unfair to single out one particular food as being the cause of obesity, which is ludicrous,” Ornish said. “It’s not due to soft drinks or cookies or lack of exercise. It’s all of these things.”

Vasan said he wasn’t recommending that Americans curtail their soft drink habits. People need to make up their own minds about the results, as well as the impact of their other diet and exercise habits, he said.

“If you are drinking it occasionally, your risk is less than if you are drinking it every day,” he said. “Anything in moderation is what makes sense. What this study says is one per day may not be moderation.”