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Kidney stones

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of kidney stones.

Alternative Names

Calcium stones; Nephrolithiasis

Prevention

All individuals who have experienced kidney stones should take some specific preventive measures to prevent recurrence. The following are some general observations:

• The most important dietary recommendations for reducing the risk for calcium stones are increasing fluid intake, restricting sodium, and reducing protein intake.
• A lower risk for calcium stones is also associated with higher potassium intake.
• A high calcium diet does not appear to increase the risk for kidney stones as long as it also contains plenty of fluids and dietary potassium and phosphate. (Increasing calcium alone may pose a modest risk for stones.)
• Patients should try to correct any dietary habits that cause acidic or alkaline imbalances in the urine that promote stone formation.

Because kidney stone types may require specific dietary changes, patients should work with their doctors to develop an individualized plan. It should be stressed that nutritional considerations are very important in preventing recurrence, and patients should be vigilant in complying with the proper diet.

Fluids (Water, Juice, and Other Beverages)

Good voiding habits, particularly frequent urination, are important. Therefore, of all the preventive recommendations, drinking enough fluid is the most important guideline for people with any type of kidney stones.

• In general, patients with calcium or uric acid stones should drink at least 10 full glasses of fluid each day (at least half should be water). This includes one with each meal and drinking fluids at night, even if it means getting up from sleep. Fluid intake should produce at least two and a half quarts of urine each day.
• More water (over a gallon, or 16 8-ounce cups, every day) is needed to prevent cystine stones, and it must be drunk at regular intervals throughout the night and day.

In all cases, more fluid is needed after exertion and during times of stress. If fluid intake is sufficient, the urine should be pale and almost watery, not dark and yellow.

Water. Although water is best, it may vary depending on its source. Variations in water itself may have different impacts. One study reported that drinking hard tap water increased urinary calcium concentration by 50% compared to soft bottled water. On the other hand, mineral water containing both calcium and magnesium may reduce several risk factors for both calcium and uric acid stone formation.

Juices and Specific Effects. Other beverages have various positive or negative effects, depending on the type of stone:

• Lemon Juice. Drinking one-half cup of pure lemon juice (enough to make eight glasses of lemonade) every day raises citrate levels in the urine, which might protect against calcium stones. (While orange juice also increases citrate levels, it does not lower calcium and it raises oxalate levels. It is not recommended.)
• Cranberry and Apple Juice. Apple and cranberry juice contain oxalates, and both have been associated with a higher risk for risk for calcium oxalate stones. Cranberry juice has properties that may increase the risk for both calcium oxalate and uric acid stones. On the other hand, cranberry juice helps prevent urinary tract infections and so may be helpful for reducing the risk for struvite and brushite stones. (These stones are far less common, however.)
• Blackcurrant Juice. In one study, blackcurrant juice reduced acidity and was associated with protection against uric acid stones.
• Grapefruit Juice. A number of studies have found a risk for stones from drinking grapefruit juice. In one study, just one 8-ounce cup of grapefruit juice per day increased the risk for forming stones by 44%.

Other Beverages and Their Effects on Stone Formation.

• Soft Drinks. Cola can severely reduce citrate in the urine and should be avoided. Many soft drinks contain phosphoric acid, which increases the risk for stones. Some research shows that drinking one quart (less than three 12-ounce cans) of soda per week may increase a person's risk of developing stones by 15%.
• Alcohol. Wine may be protective against kidney stones. A study conducted in Finland, however, suggests that the risk of developing stones decreases with beer consumption. It should be noted that beer is high in oxalates. Beer and other alcoholic beverages also contain purines, which may increase the specific risk for the less common uric acid stones in susceptible people. Binge drinking, in any case, increases uric acid and the risk for stones
• Coffee and Tea. Some research as reported a lower risk for stones with tea and both regular and decaffeinated coffee.

Low-Salt and Low-Protein Diets

In a long-term 2002 study of men with calcium oxalate stones and high levels of urinary calcium, a low-sodium, low-protein diet containing normal levels of calcium dramatically reduced the recurrence of stones compared to a diet that was simply low in calcium.

Salt Restriction. Because salt intake increases the amount of calcium in urine, patients with calcium stones should restrict their sodium intake. Sodium may also elevate levels of urate, the crystalline substance that can trigger formation of recurrent calcium oxalate stones. Although the relative contribution of sodium restriction in this and other studies has not been confirmed, some researchers believe that restricting sodium along with increasing fluid intake is the most important dietary measure for preventing stones.

Protein Restriction. Protein increases uric acid, calcium, and oxalates in the urine and reduces citrate. Diets high in protein, particularly meat protein, have been consistently associated with kidney stones. (Meat protein has a higher sulfur content and generates more acid than vegetable protein.) A 2002 trial of those following a high-protein, low-carbohydrate diet, popularized in such weight-loss regimens as the Atkins diet, for example, found dramatically increased levels of urinary uric acid and calcium after just several weeks. These effects put patients at higher risk not just of kidney stones but possibly of osteoporosis as well. According to Swiss studies, about a third of people at risk for calcium stones may have a sensitivity to meat proteins that cause mild hyperoxaluria.

Whether restricting meat protein alone has any protective value without restricting sodium as well is unknown. Most studies to date have found no difference in stone development between people with low and normal meat protein diets over four years. A 2000 study reported that only dramatic reductions in meat protein had any preventive effect against stone recurrence.

Although the precise role of dietary protein in kidney stones needs further elucidation, it is reasonable for everyone to consume meat protein in moderation. People with struvite stones, who need to reduce phosphates in their diets, should also cut down on proteins.

Role of Calcium

Calcium from Foods. Dietary calcium recommendations for kidney stone prevention need to be determined on an individual basis. A doctor will suggest calcium guidelines based on a patient’s age, gender, body size, and type of stone. Most studies indicate that dietary calcium (found in milk, yogurt, and cheese) protects against many types of calcium oxalate stones. Large studies of both men and women found that those with the highest intake of calcium from foods had a much lower risk for stones than those who had little calcium in their diets. A diet containing a normal amount of calcium, but reduced amounts of animal protein and salt, may protect against stones better than a low-calcium regimen. However, calcium metabolism changes as people age. Some studies suggest that a high calcium intake protects against kidney stones in men younger than age 60, but not in older men.

Dietary calcium may actually bind the oxalate in foods, preventing it from being absorbed into the blood and excreted into the urine. In a normal healthy diet, dairy products supply almost 80% of the daily calcium requirement. For people have calcium stones associated with resorption (the breakdown of bone that releases calcium into the bloodstream), limiting calcium intake could cause further bone loss.

Calcium Supplements. Evidence on calcium supplements is mixed, although in general many studies suggest that they reduce oxalate levels and so help prevent calcium oxalate stones. One study suggested that taking 500 mg of calcium supplements a day regularly may "reprogram" the intestines to absorb less calcium and so be protective. Experts generally advise that calcium supplementation within dosage recommendations, approximately 1,200 mg per day, remains safe. In one study, however, women who took calcium supplements had a 20% higher risk for stones. Research indicates that dosages of calcium above 2,000 mg per day are clearly associated with the formation of stones. Some experts speculate that this higher risk may occur because supplements are often taken in the morning, either without food or with breakfast, which is typically low in oxalates. Taking supplements with later meals may not incur the same risk.

Calcium Restriction in Certain Cases. Some calcium stone patients may need to restrict calcium, such as those whose stones are caused by genetic defects in which the intestine over-absorbs calcium. More studies are needed to define this group precisely.

Fiber-Rich Foods and Their Compounds

Fiber may be beneficial for people with kidney stones. In addition, some fiber-rich foods may contain compounds that help protect against kidney stones. A wide variety of high-fiber plant foods contain a compound called phytate (also called inositol hexaphosphate, InsP6, or IP6), which appears to help prevent crystallization of calcium salts, both oxalate and phosphate. Phytate is found in legumes and wheat and rice bran. (Soybeans are also rich in phytate but they are also very high in oxalates, so the overall effects of soy on kidney stones are not clear.)

Purine Restriction in People at Risk for Uric Acid Stones

A high intake of purines can increase the amount of uric acid in the urine, so those at risk for uric acid stones should reduce their intake of foods that contain purines. They include beer and other alcoholic beverages, anchovies, sardines, yeast, organ meats (e.g., liver, kidneys), legumes (e.g., dried beans, peas, and soybeans), mushrooms, spinach, asparagus, cauliflower, and poultry.

Oxalate Restriction in Hyperoxaluria

Most people with calcium oxalate stones should not avoid oxalate-rich foods unless the doctor specifically recommends a restrictive diet. Oxalate binds with calcium in the intestine, which may actually reduce calcium absorption. Some studies, in fact, indicate that eating foods containing oxalates and calcium together may reduce the risk of stones. Most of the foods that contain oxalates are very important for good health. Restricting oxalates may be particularly harmful in people with bowel disorders marked by malabsorption.

• Foods high in oxalic acid include beets, soy, beet tops, black tea, chenopodium, chocolate, cocoa, dried figs, ground pepper, lamb, lime peel, nuts, parsley, poppy seeds, purslane, rhubarb, sorrel, spinach, and Swiss chard.
• Foods containing moderate amounts of oxalates include beans (green and wax), blackberries, blueberries, carrots, celery, coffee (roasted), concord grapes, currants, dandelion greens, endive, gooseberries, lemon peel, okra, green onions, oranges, green peppers, black raspberries, strawberries, and sweet potatoes.

Role of Fats

Certain fats may play a beneficial or harmful role in specific cases of kidney stones.

Restricted Fats in Patients with Stones Associated with Bowel Disease. Patients who have stones associated with short-bowel syndrome should restrict their intake of fat as well oxalates. In such cases, calcium may bind to unabsorbed fat instead of to oxalates, which increase oxalate levels.

Fish Oil. Omega-3 fatty acids, which are found in oily fish like mackerel, salmon, and albacore tuna, have many health benefits but the most current evidence suggests they do not help prevent kidney stones. A 2005 study of over 200,000 adults found that increased omega-3 fatty acid intake did not reduce kidney stone risk.

Role of Vitamins

Vitamin B6. Vitamin B6, or pyridoxine, is used to treat people with primary hyperoxaluria, a severe inherited disorder. Patients should not try to self-medicate with vitamin B6. Very high doses (500 to 2,000 mg daily over long periods) can cause nerve damage with loss of balance and numbness in the feet and hands. Food sources of vitamin B6 include meats, oily fish, poultry, whole grains, dried fortified cereals, soybeans, avocados, baked potatoes with skins, watermelon, plantains, bananas, peanuts, and brewer's yeast.

Vitamin C . Ascorbic acid (vitamin C) may convert to tiny insoluble crystals called oxalates. People with hyperoxaluria (too much oxalate in the urine) should avoid vitamin C supplements. Even for men with normal oxalate levels, higher consumption of vitamin C (more than 1000 mg a day) may increase kidney stone risk.

Role of Minerals

Magnesium and potassium may help reduce the risk for kidney stones in men.

Stress Management Techniques

Because of an association between stress and kidney stones, relaxation and stress management techniques may also be beneficial.

Preventing Recurrence

Dietary Considerations. People with kidney stones appear to be more sensitive to certain foods than people who do not form kidney stones and need to make specific changes in their diet. They should work with their doctors to develop a dietary plan that fits their individual situation. Drinking plenty of fluids is important for preventing recurrence of any kidney stone.

Indications for Drug Treatments. If dietary measures fail then drug treatments may be helpful. A number of drugs are available to prevent recurrences of calcium oxalate and other stones. Allopurinol, thiazide, potassium citrate, and potassium-magnesium citrate have all been shown to inhibit stone formation. In addition, drug treatments can sometimes also help prevent other complications related to stones, such as osteoporosis.

Correcting Underlying Conditions Known to Cause Kidney Stones. It is also important to treat and correct, if possible, any underlying disorder that may be causing stones to form. Such disorders include distal renal tubular acidosis, hyperthyroidism, sarcoidosis, and certain cancers. To prevent calcium stones that form in hyperparathyroid patients, a surgeon may remove the affected parathyroid gland (located in the neck). In most cases, only one of the glands is enlarged. Removing it ends the patient's problem with kidney stones.

• Review Date: 5/22/2006
• Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital

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