(Bloomberg) The typical high-fat American diet is often blamed for rising rates of obesity, diabetes and heart disease in the US. Now researchers are debating the impact of soft drinks.

A study in the journal Circulation found people who drink soda every day are 44 percent more likely to develop an array of conditions that alter metabolism and increase their health risks. It didn't matter whether the soda was a sugar-free diet version or regular.

Some doctors said the link stemmed from other habits seen in soda drinkers, including indulgence in fried foods and sedentary lifestyles. Researchers not involved in the study said the higher risk seen with diet soft drinks should invalidate all the findings. The investigators themselves said more work is needed to confirm the results before conclusions are drawn.

"It did not really matter whether you were drinking regular soda or diet soda," said Ramachandran Vasan, a cardiologist and senior author of the paper from Boston University School of Medicine, who called the consistency of the results striking. "This adds one more piece to the puzzle."

The results were unexpected since even a little soda increased the risk and because diet drinks, with their lack of calories, had the same effect as sugary beverages, the researchers said. Soda drinkers fared worse on the individual components of the condition known as metabolic syndrome, including high blood sugar, low levels of "good" cholesterol, abdominal fat and elevated triglycerides.

Diet soft drinks account for about 30 percent of the $70.1 billion U.S. soda market and are recommended by the American Heart Association for people eating at fast food restaurants.

"People studied who drank soft drinks were more likely to develop risk factors for heart disease," the association said in a statement. "It is possible that other factors could explain this relationship," since people who drink soda also consume more calories, eat more fat and get less exercise, they said. "Diet soda can be a good option to replace caloric beverages that do not contain important vitamins and minerals."

The increased risk with diet soft drinks is "particularly implausible" since they have no calories and are 99 percent water with a little flavor added, said Susan Neely, president of the American Beverage Association. "The study does not establish any link between soft drinks, regular or diets, and increased risk of heart disease."

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 industry group.

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 analyzed 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 health.

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

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 fiber 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.