People who are prone to kidney stones should limit their caffeine intake, new research suggests.

When investigators gave people with a history of kidney stones a dose of caffeine equivalent to that found in two cups of coffee, they began to excrete more calcium in their urine, putting them at increased risk of forming kidney stones.

Study author Dr. Linda K. Massey told Reuters Health that stone suffers should limit themselves to less than two cups of coffee per day or a comparable amount of caffeine from other sources. And that limit of two cups means, literally, 16 ounces-not two enormous mugs of coffee, which can contain much more caffeine, she said.

Massey, who is based at Washington State University in Spokane, explained that a spike in urinary calcium increases the risk of stones because calcium is an important ingredient in kidney stones - so the more there is, the more likely there will be stones.

Previous research has shown that people who do not tend to form kidney stones also excrete more calcium in their urine after consuming caffeine.

To investigate whether the same thing happens in people prone to stones, Massey and Dr. Roger A. L. Sutton at the University of British Columbia in Vancouver, asked 39 people with kidney stones and nine who never had stones to drink caffeine added to water after 14 hours of fasting. The researchers tested their urine 2 hours before and after they consumed caffeine.

As the investigators report in the Journal of Urology, after caffeine, the stone-formers showed an increase in calcium, sodium, magnesium and citrate in their urine. The same pattern also occurred in the people with no history of kidney stones.

In an interview, Massey explained that more calcium and sodium in urine likely increases the risk of kidney stones, while increases in magnesium and citrate appear to protect people from kidney stones.

However, after plugging these changes into a formula that predicts kidney stone risk, Massey said, “The increase in magnesium and citrate did not offset the increase in calcium.”

SOURCE: The Journal of Urology, August 2004.