People who overindulge in cola may be putting their kidneys at risk, a new study suggests.

Drinking more than two servings of cola a day more than doubled the likelihood of having chronic kidney disease, Dr. Dale P. Sandler and colleagues from the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina found. But no increased risk was seen with other carbonated beverages.

"Our study suggests that there's something about drinking cola beverages that is associated with increased risk for chronic kidney disease," Sandler told Reuters Health. "We don't believe that it's caffeine, and we don't believe it's the sugar."

The likely culprit, she and her colleagues say, is phosphoric acid, which gives cola its characteristic tangy taste while acting as a preservative; in other beverages, citric acid is used. Exposure to high levels of phosphates has been linked to acute and long-term kidney damage, Sandler points out, as well as kidney stones.

People with kidney disease are advised to avoid colas and high-phosphate foods, such as meat, the researchers note in their report in the journal Epidemiology.

Sandler and her team asked 465 people who had recently been diagnosed with kidney disease and 467 healthy individuals matched by age, sex and race about the types of beverages they drank and how often they drank them.

People who drank two or more servings of cola daily were at 2.3-fold greater risk of kidney disease, the researchers found, and the risk was the same for both diet and sugar-sweetened cola. But people who drank two or more non-cola carbonated beverages daily were not at increased risk.

Sandler said other researchers should repeat the findings before sweeping recommendations can be made on cola consumption. Nevertheless, she added, "moderation is always a good thing."