By Marc O'Meara, R.D., L.D.N., C.D.E.
Brigham and Women's Hospital
Kidney stones are hard, crystal-like concentrations of minerals in the urine. Stones that form in the kidney are called nephrolithiasis; stones that form in the urinary tract are called urolithiasis. The most common type of kidney stone is the calcium oxalate stone. It makes up about 80% of all kidney stones. Five percent of people will develop kidney stones at some point in their lives. Men have kidney stones about four times more often than females.
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What Causes Kidney Stones?
Kidney stones form when a person doesn't drink enough fluid to dilute the urine. This increases the chance that chemicals or excess minerals will combine to form stones. A calcium oxalate stone forms when there is an excess of calcium in the urine.
The risk of developing kidney stones goes up after age 40 and increases as you get older. Individuals who have had more than one kidney stone and those with a family history of kidney stones are more likely to develop kidney stones. More than 50% of people with an inherited condition called hypercalciuria form kidney stones. Their bodies absorb extra calcium in food, which results in a high calcium level in the urine.
As a stone passes through the urinary tract, it can cause severe pain in the abdomen and groin. The pain can lead to nausea and vomiting. If the stone damages the lining of the urinary tract, there may be blood in the urine (hematuria).
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Treating Kidney Stones
The most important treatment for passing stones is drinking enough fluid. This will help flush the stone through the urinary tract. The goal is to drink 8 to 12 cups (64 to 96 ounces total) each day. Pain medication, if needed, can help while trying to pass a kidney stone. Medications, such as nifedipine, may help speed up the process of passing the stone. If the stone is too large to pass through the urinary tract, your doctor may recommend treatment with shock waves (lithotripsy) to break it up or a minor surgical procedure to either break it up or remove it.
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Preventing Kidney Stones
You can help prevent kidney stone formation by avoiding high levels of calcium and uric acid in urine. Here's how:
- Drink plenty of fluids. This will help prevent dehydration and flush substances that form stones out of the kidneys and urinary tract. Drink 8 to 12 cups (64 to 96 ounces) a day. Drinking alcohol only affects stone formation by causing dehydration since it has a diuretic effect. Alcohol can be enjoyed in moderation if the drinker is careful to maintain hydration. Studies have shown that coffee may help reduce the incidence of stone formation, despite having caffeine, which is a natural diuretic and may increase urinary calcium excretion. Most teas only have small amounts of oxalate and are generally safe. Black tea has more oxalate than green tea and herbal teas, but one or two cups per day should not be a problem.
- Eat 1,000 to 1,200 milligrams of calcium a day. This is important because in the gut, calcium binds oxalate, a bitter component found in some plants. This helps decrease the amount of oxalate that is absorbed. When less oxalate is absorbed, less is excreted in the urine and fewer calcium oxalate stones can form. Oxalate promotes stone formation in the urine about 15 times more frequently than calcium. Some good sources of calcium include low-fat or fat-free dairy products, such as milk, yogurt, cottage cheese and cheese. Other good sources of calcium include calcium-fortified orange juice, calcium-fortified soy milk, tofu, broccoli, sardines, collard greens, almonds and black beans.
- Limit oxalate intake to 40 to 50 milligrams a day. It's not clear how much oxalate from food the body absorbs, or even how accurately we can measure oxalate levels in food. Nevertheless, avoid the following foods, some of which have more than 10 milligrams of oxalates per 3.5 ounce serving: Most types of beans, nuts, dark green leafy vegetables, berries, chocolate, rhubarb stalks, most soy products, star fruit and black pepper (over 1 teaspoon). It's especially important that individuals whose urine contains a high level of urinary oxalate avoid these foods.
- Limit animal protein (dairy products, eggs, meats, poultry and fish) intake since it can increase the acidity of the blood. The body metabolizes animal protein by pulling calcium from the bones to buffer the acidity. The excess calcium eventually ends up in the urine at a higher concentration, which can make it easier for oxalate stones to form.
- Limit sodium intake. Excess sodium promotes calcium excretion so limit intake to 2,400 milligrams daily. If supplementing with vitamin C, do not exceed 2,000 millig rams daily, because your body can convert extra vitamin C into oxalate.
- Speak with your doctor about whether these medications or supplements may be an option for you:
- Thiazide diuretics can lower urine calcium excretion.
- Cholestyramine (2 to 4 grams with each meal) reduces oxalate excretion in the urine by binding oxalates in the gut.
- Calcium citrate supplements (500 milligrams) can bind oxalate in the gut and hinder its absorption if taken with a meal that includes a high-oxalate food.
- Potassium citrate supplements increase the urinary citrate level and make the urine less acidic. This helps reduce calcium oxalate stone formation.
If you've already had a kidney stone, a 24-hour urine test (to show if you excrete too much oxalate) and an analysis of your stone's chemical composition can help your doctor determine the best approach to preventing a recurrence.
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Marc O'Meara, R.D., L.D.N., C.D.E. is a senior nutritionist at the Brigham and Women's Hospital and the Roxbury Heart Center, and also works in the lipid clinic at Children's Hospital Boston. He graduated from the University of Vermont in 1991 with a bachelor of science in dietetics. He completed his dietetic internship at Beth Israel Deaconess Medical Center in 1992.