By Anthony L. Komaroff, M.D.
Harvard Health Letters
QUESTION: Two of my friends suffer from kidney stones. Each was advised to give up tea. I drink a lot of tea. Am I in danger of getting kidney stones?
ANSWER: Your kidneys get rid of excess minerals -- calcium and potassium, for example -- from the bloodstream. Various compounds in the urine prevent the minerals and other substances from sticking together, but sometimes they still manage to find each other and clump together. The result is a kidney stone.
About 75 percent of all kidney stones are made of calcium and oxalate, a substance found mainly in plant foods. About half the time, such stones form because of high levels of calcium in the urine, which is probably an inherited tendency.
If your friends had calcium oxalate stones, I must say I'm a little puzzled by the advice they were given. Coffee and tea contain oxalate, but there's now a lot of evidence that increased fluid intake, particularly in the form of coffee or tea, decreases the risk of calcium oxalate stones. It's not clear why. Caffeine is a diuretic, and by encouraging urination, it may make urine more dilute so stones are less likely to form.
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