Diet can help prevent kidney stones

Q: I recently passed a kidney stone, and it was really painful. My doctor gave me medicine. Is there anything else I can do to prevent another one?

A: Besides the medicine, your diet can influence your risk of getting another kidney stone.

Kidney stones are hard chemical deposits that form inside the kidneys. They can be as small as grains of sand and pass painlessly out of the body in urine. But they also can be the size of a pea, a marble or even larger.

If one of those stones gets into the ureters - the tubes that connect each kidney to the bladder - it can cause intense pain and bleeding. This may require medication and/or hospitalization to have the stone removed or broken into fragments.

Kidney stones form if too many minerals are excreted into the urine. Low fluid volume, caused by dehydration, is another cause.

Most kidney stones are made of calcium oxalate. This is a combination of calcium and oxalic acid. Stones also may be made of other substances. Your doctor can have your stone analyzed to see what it was made of.

People who have already had a kidney stone have a high risk of forming another one. Your doctor should tailor your dietary recommendations based on what your kidney stone is made of.

These are the dietary changes that often are recommended:

• Drink plenty of fluids to keep your urine diluted, so minerals won't accumulate into stones. Drink at least 10 glasses of fluid a day, half of which should be water.

• It may seem paradoxical, but eating calcium-rich foods actually reduces your risk of stones. That's because absorbing lots of calcium in your gut reduces how much oxalic acid you absorb. Calcium doesn't form stones without oxalic acid. Good sources of calcium include milk, cheese, yogurt, oranges and broccoli.

• On the other hand, calcium pills can raise your risk for kidney stones because they increase the calcium in your blood and urine without decreasing oxalic acid. So avoid calcium pills.

• Don't eat too much of foods that are rich in oxalic acid. These include beets, spinach, chard, rhubarb, tea, coffee, cola, chocolate and nuts.

• Keep your sodium intake under 1,500 mg a day. A low-sodium diet reduces the amount of calcium the kidney puts into the urine.

• Avoid or limit red meat. Animal protein increases the tendency of stones to form in the urine.

• Most plant-based foods protect against kidney stones. High-potassium fruits such as tomatoes and bananas are best. Citrus fruits - except grapefruit - are another good choice.

• Obesity increases the risk of stones, so try to shed excess pounds through a combination of fewer calories and regular exercise.

No one who has suffered the pain of a first kidney stone wants to experience a second stone.

Fortunately, you can do a lot to reduce your risk.

• Dr. Komaroff is a physician and professor at Harvard Medical School. For questions, go to AskDoctorK.com.