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What Are Kidney Stones?

Kidney stones are created when certain substances in urine — including calcium and uric acid — crystallize and the crystals clump together. Usually, they form in the center of the kidney, where urine collects before flowing into the ureter, the tube that leads to the bladder. Small stones are able to pass out of the body in the urine and often go completely unnoticed. But larger stones irritate and stretch the ureter as they move toward the bladder, causing excruciating pain and blocking the flow of urine. Rarely, a stone can be as large as a golf ball, in which case it remains lodged in the kidney, creating a more serious condition.

Why some people form kidney stones and others don't is not always clear. They are more common in young and middle-aged adults than in the elderly, and they are much more prevalent in men than in women. People living in hot climates are sometimes prone to kidney stones because their bodies become dehydrated, concentrating the minerals in their urine. Medical evidence suggests that drinking too few fluids can cause stones, as can prolonged bed rest or pregnancy.

In 90% of cases, the cause remains unknown. The condition appears to run in families, and people who suffer from gout, inflammatory bowel diseases, and chronic urinary tract infections also seem to be predisposed to it. In genetically susceptible people, eating foods that are themselves rich in oxalate — such as chocolate, grapes, rhubarb, tea, spinach, and strawberries — may also promote stones.

Chronic use of antacids containing calcium has also been linked to kidney stones. Dietary deficiencies, especially of vitamin B-6 and magnesium, and excessive amounts of vitamin D also may be factors in the formation of stones. An imbalance of these vitamins and minerals can increase the amount of calcium oxalate in the urine; when the levels become too high, the calcium oxalate does not dissolve, and crystals may begin to form.