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"Among women, cola beverages were associated with lower bone mineral density," said lead researcher Katherine Tucker, director of the Epidemiology and Dietary Assessment Program at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University.

There was a pretty clear dose-response, Tucker added. "Women who drink cola daily had lower bone mineral density than those who drink it only once a week," she said. "If you are worried about osteoporosis, it is probably a good idea to switch to another beverage or to limit your cola to occasional use."

The report was published in the October issue of the American Journal of Clinical Nutrition.

About 55 percent of Americans, mostly women, are at risk for developing osteoporosis, according to the National Osteoporosis Foundation.

In the study, Tucker's team collected data on more than 2,500 participants in the Framingham Osteoporosis Study, averaging just below 60 years of age. The researchers looked at bone mineral density at three different hip sites, as well as the spine.

They found that in women, drinking cola was associated with lower bone mineral density at all three hip sites, regardless of age, menopause, total calcium and vitamin D intake, or smoking or drinking alcohol. Women reported drinking an average of five carbonated drinks a week, four of which were cola.

Bone density among women who drank cola daily was almost 4 percent less, compared with women who didn't drink cola, Tucker said. "This is quite significant when you are talking about the density of the skeleton," she said.

Cola intake was not associated with lower bone mineral density in men. The findings were similar for diet cola, but weaker for decaffeinated cola, the researchers reported.

The reason for cola's effect on bone density may have to do with caffeine, Tucker said. "Caffeine is known to be associated with the risk of lower bone mineral density," she said. "But we found the same thing with decaffeinated colas."

Another explanation may have to do with phosphoric acid in cola, which can cause leeching of calcium from bones to help neutralize the acid, Tucker said.

One expert agrees that women should reduce the amount of cola they drink.

"I would expect this finding," said Dr. Mone Zaidi, director of the Mount Sinai Bone Program at Mount Sinai School of Medicine, in New York City. "It's probably a caffeine-related problem."

Women should limit their caffeine intake, Zaidi said. "Caffeine interferes with calcium absorption, which results in less bone formation," he said.

This can be a problem for younger women who never develop peak bone density, Zaidi noted. "Younger women who have a lot of coke will not form bone to an extent their peers would; so, years later, in menopause, they are going to be disadvantaged," he said.