What dietary factors and lifestyle measures are used for prevention of kidney stones?
Patients should take some specific preventive measures to prevent kidney stones. The most important 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 potassium intake. Dietary calcium itself does not appear to increase the risk for kidney stones. Patients should try to correct any acidic or alkaline imbalance in the urine so that such stone-forming substances are more likely to dissolve. Try to avoid stone recurrence using dietary changes; if such measures fail then drug treatments may be helpful.
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 ten full glasses of fluid each day (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 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. Although water is best, it may vary depending on its source. Other beverages have various positive or negative effects: 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. 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 therefore not recommended.) Cola beverages can severely reduce citrate in the urine and should be avoided. Many soft drinks contain phosphoric acid, which increase the risk for stones. Some research shows that drinking one quart per week may increase a person's risk of developing stones by 15%. 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%. Fruit juices, particularly apple and cranberry juice, contain oxalates, although it is unclear whether oxalate-containing foods pose a risk for calcium oxalate stones. A 2000 study reported significantly higher oxalate levels in the urine of patients taking cranberry supplements, indicated that people at risk for calcium stones should avoid cranberry products. Cranberry juice helps prevent urinary tract infections, however, and so may be helpful for reducing the risk for struvite stones, although these stones are rare. A 1999 study conducted in Finland suggests that the risk of developing stones decreases with beer consumption. This study found no effects from coffee, tea, or wine, but other research has reported a lower risk with these beverages. 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. No one, of course, is suggesting excessive alcohol intake for prevention of kidney stones.
Because salt intake increases the amount of calcium in urine, patients with calcium stones should restrict their sodium intake. Some researchers believe that restricting sodium along with increasing fluid intake is the most important dietary measure for preventing stones.
Calcium from Foods. It has been fairly well established that dietary calcium (such as in dairy products) is actually protective against many cases 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. 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. It should be further noted that many people have calcium stones associated with resorption (the breakdown of bone that releases calcium into the blood stream). Limiting calcium intake in such people could actually promote further bone loss.
Calcium Supplements. Evidence on calcium supplements are mixed, although in general many 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 1200 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 2000 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 who have supersaturation of calcium in the urine and who are not at risk for bone loss may need to restrict calcium, but more studies are needed to define this group precisely. Certainly, children with hypercalciuria should not restrict calcium, since this could harm bone growth.
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:
Phytate. A wide variety of high-fiber plant foods, such as wheat or rice bran and soybeans, contain a compound called phytate (inositol hexaphosphate). It appears to help prevent crystallization of calcium salts, both oxalate and phosphate. (It is sold as dietary supplements, usually called IP6 as well. It should be noted that phytate itself can inhibit absorption of certain important minerals, including zinc.) It should be noted that soy is very high in oxalates, so the overall effects of soy on kidney stones are not clear.
Gahat. Gahat is a lentil-like bean that has been used for centuries in Nepal, Northern India, and Pakistan to treat kidney stones. A US study attempted to identify changes in the urine that might account for these reports by having subjects consume gahat three times daily for two days. Results found no changes in calcium, citrate, phosphate, sodium, or oxalate, although there was an increase in urine volume, and an increased excretion of uric acid and magnesium. More study is needed to learn if Gahat has any benefit.
Some evidence suggests that omega-3 fatty acids, which are found in certain oily fish, may have properties that reduce the risk for calcium stones. Such fatty acids reduce factors that produce inflammation and which may increase the risk for stone production. Omega-3 fatty acids can be obtained in supplements, but their efficacy is not known.
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.) 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 has any protective value is unknown. Studies to date have found no difference in stone development between people with low and normal meat protein diets over four years. A 2001 study, for example, found no difference in stone formation in two groups of patients who consumed beef or plant proteins in equal amounts. A 2000 study reported that only dramatic reductions in meat protein had any preventive effect against stone recurrence. It is reasonable, in any case, 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.
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 (eg, liver, kidneys), legumes (eg, 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 physician 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 malabsorption.
Foods high in oxalic acid include beets, soy, beet tops, black tea, chenopodium, chocolate, cocoa, dried figs, ground pepper, lamb quarters, lime peel, nuts, parsley, poppy seeds, purslane, rhubarb, sorrel, spinach, and Swiss chard. Foods containing moderate 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.
Vitamins in Hyperoxaluria
Ascorbic acid (vitamin C) may convert to oxalates, and people with hyperoxaluria should avoid vitamin C supplements. (A high intake of vitamin C does not appear to increase the risk of stone formation in people with no risk factors for stones.) Vitamin B6, or pyridoxine, is used to treat people with hyperoxaluria when dietary reduction of oxalates and calcium supplements is not effective in preventing stones. It is particularly beneficial for the inherited disorder, type I primary hyperoxaluria. 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. Citrate salts may also be important for people with excessive levels of oxalates.