Women who regularly drink cola could be increasing their risk of osteoporosis, US researchers believe.
Low bone density puts people at risk of osteoporosis
Their study of 2,500 people revealed drinking cola was linked with low bone mineral density in women regardless of their age or calcium intake.
But the work published in the American Journal of Clinical Nutrition found no such link in men.
Over a million UK women have been diagnosed with osteoporosis, although many more may have the condition.
Osteoporosis literally means "porous bones" and makes it more likely for them to fracture as they lose their density.
Professor Katherine Tucker, director of the Epidemiology and Dietary Assessment Program at Tufts University in Boston, and colleagues compared information from dietary questionnaires with bone mineral density measurements at the spine and three different hip sites.
Men reported drinking an average of five cola drinks a week, and women reported consuming four cola drinks a week.
Cola consumption was linked to lower bone mineral density in all three hip sites in women, although not in the spine, but was not associated with bone mineral density in men.
There was no association with bone mineral density loss for women with other carbonated drinks, however.
Past work suggests that consumption of cola may be detrimental to bone health because it replaces milk in the diet, but in the current study the women who drank higher amounts of cola did not drink less milk than others.
However, calcium intake from all sources, including non-dairy sources such as dark leafy greens or beans, was lower for women who drank the most cola.
An ingredient of cola called phosphoric acid might be responsible for the link, but the mechanism is not fully understood.
Professor Tucker said: "We think that particularly in doses in cola and especially when it's repeated daily, phosphoric acid creates an acidic environment in the blood. Calcium is then taken out of the bones to balance that out. But that view is controversial."
She advised: "Women, particularly those concerned about osteoporosis should limit their intake of cola to occasional."
A spokesperson for the National Osteoporosis Society said: "We know that phosphoric acid seems to play a role in bone health and that excessive amounts may lower bone density although there have not been any studies that show exactly why this is.
"What's interesting about this study is that most of the women did seem to be getting a good intake of calcium from other food sources, yet their bone density was affected by drinking as little as four cans of colas a week, which isn't much."
She said more studies were needed.
Dr Peter Stott, a GP in Tadworth, Surrey and member of the NOS scientific advisory board said young women should think about protecting their bones for the future by eating a healthy diet and getting plenty of exercise.
A spokesperson for the British Soft Drinks Association said the most important preventative measures for osteoporosis were consuming adequate amounts of calcium-rich foods, taking regular weight-bearing exercise, stopping smoking, and reducing consumption of alcohol.
He added: "The scientific evidence does not suggest that phosphate, used in the form of phosphoric acid in some carbonated drinks, has a detrimental effect on bone health. In any case, soft drinks provide only 3 per cent of total phosphorus intake from dietary sources, the two main sources being cereals and cereal products, and milk and milk products."