People who are prone to kidney stones should limit their caffeine intake to less than two cups of coffee per day, new research suggests. And that limit of two cups means 16 ounces, not two enormous mugs of coffee, which can contain much more caffeine.
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 calcium-oxalate kidney stones.
A spike in urinary calcium increases the risk of stones because calcium is an important ingredient in kidney stones; 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, researchers from Washington, US and 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.
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.
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