About 75% if all kidney stones area made of calcium oxalate, a substance found mainly in plant foods. About half the time, such stones from because of high levels of calcium in the urine which, experts, is probably an inherited tendency. Coffee and tea contain oxalate, but there’s now a lot of evidence that increased fluid intake, particularly in the from 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. Food rich in phytate (a phosphorus containing compound found in plants) or potassium seem to reduce the risk of calcium oxalate stones. Contrary to what you might think calcium rich diets also protect against rather than promote calcium oxalate stones, though the same can’t be said for calcium pills. Not drinking enough fluids and eating too much salt, protein (particularly from meat), and sugar increase the risk of kidney stones.