Kidney stones can form when substances in the urine—such as calcium, oxalate, and phosphorus—become highly concentrated. The body uses food for energy and tissue repair. After the body uses what it needs, waste products in the bloodstream are carried to the kidneys and excreted as urine. Diet is one of several factors that can promote or inhibit kidney stone formation. Certain foods may promote stone formation in people who are susceptible, but scientists do not believe that eating any specific food causes stones to form in people who are not susceptible. Other factors that affect kidney stone formation include genes, environment, body weight, and fluid intake.
What are the types of kidney stones?
Four major types of kidney stones can form:
• Calcium stones are the most common type of kidney stone and occur in two major forms: calcium oxalate and calcium phosphate. Calcium oxalate stones are more common. Calcium oxalate stone formation may be caused by high calcium and high oxalate excretion. Calcium phosphate stones are caused by the combination of high urine calcium and alkaline urine, meaning the urine has a high pH.
• Uric acid stones form when the urine is persistently acidic. A diet rich in purines—substances found in animal protein such as meats, fish, and shellfish—may increase uric acid in urine. If uric acid becomes concentrated in the urine, it can settle and form a stone by itself or along with calcium.
• Struvite stones result from kidney infections. Eliminating infected stones from the urinary tract and staying infection-free can prevent more struvite stones.
• Cystine stones result from a genetic disorder that causes cystine to leak through the kidneys and into the urine, forming crystals that tend to accumulate into stones.
People can help prevent kidney stones by making changes in fluid intake and, depending on the type of kidney stone, changes in consumption of sodium, animal protein, calcium, and oxalate.
Drinking enough fluids each day is the best way to help prevent most types of kidney stones. Health care providers recommend that a person drink 2 to 3 liters of fluid a day. People with cystine stones may need to drink even more. Though water is best, other fluids may also help prevent kidney stones, such as citrus drinks.
People who have had a kidney stone should drink enough water and other fluids to produce at least 2 liters of urine a day. People who have had cystine stones may need to drink even more. The amount of fluid each person needs to drink depends on the weather and the person’s activity level—people who work or exercise in hot weather need more fluid to replace the fluid they lose through sweat. A 24-hour urine collection may be used to determine the volume of urine produced during a day. If the volume of urine produced is too low, the person can be advised to increase fluid intake. Drinking enough fluid is the most important thing a person can do to prevent kidney stones.
Sodium, often from salt, causes the kidneys to excrete more calcium into the urine. High concentrations of calcium in the urine combine with oxalate and phosphorus to form stones. Reducing sodium intake is preferred to reducing calcium intake.
The U.S. recommended dietary allowance (RDA) of sodium is 2,300 milligrams (mg), but Americans’ intake averages 3,400 mg, according to the U.S. Department of Agriculture.1 The risk of kidney stones increases with increased daily sodium consumption. People who form calcium oxalate or calcium phosphate stones should limit their intake to the U.S. RDA level, even if they take medications to prevent kidney stones.
Learning the sodium content of foods can help people control their sodium intake. Food labels provide information about sodium and other nutrients. Keeping a sodium diary can help a person limit sodium intake to 2,300 mg. When eating out, people should ask about the sodium content of the foods they order.
Some foods have such large amounts of sodium that a single serving provides a major portion of the RDA. Foods that contain high levels of sodium include
• hot dogs
• canned soups and vegetables
• processed frozen foods
• luncheon meats
• fast food
People who are trying to limit their sodium intake should check labels for ingredients and hidden sodium, such as
• monosodium glutamate, or MSG
• sodium bicarbonate, the chemical name for baking soda
• baking powder, which contains sodium bicarbonate and other chemicals
• disodium phosphate
• sodium alginate
• sodium nitrate or nitrite
Meats and other animal protein—such as eggs and fish—contain purines, which break down into uric acid in the urine. Foods especially rich in purines include organ meats, such as liver. People who form uric acid stones should limit their meat consumption to 6 ounces each day.
Animal protein may also raise the risk of calcium stones by increasing the excretion of calcium and reducing the excretion of citrate into the urine. Citrate prevents kidney stones, but the acid in animal protein reduces the citrate in urine.
Calcium from food does not increase the risk of calcium oxalate stones. Calcium in the digestive tract binds to oxalate from food and keeps it from entering the blood, and then the urinary tract, where it can form stones. People who form calcium oxalate stones should include 800 mg of calcium in their diet every day, not only for kidney stone prevention but also to maintain bone density. A cup of low-fat milk contains 300 mg of calcium. Other dairy products such as yogurt are also high in calcium. For people who have lactose intolerance and must avoid dairy products, orange juice fortified with calcium or dairy with reduced lactose content may be alternatives. Calcium supplements may increase the risk of calcium oxalate stones if they are not taken with food.
Some of the oxalate in urine is made by the body. However, eating certain foods with high levels of oxalate can increase the amount of oxalate in the urine, where it combines with calcium to form calcium oxalate stones. Foods that have been shown to increase the amount of oxalate in urine include
• wheat bran
Avoiding these foods may help reduce the amount of oxalate in the urine.
A dietitian can help a person plan meals that lower the risk of forming stones based on the type of stone the person formed in the past. A person with a history of kidney stones may want to talk to a dietitian who specializes in kidney stone prevention or nutrition for people with kidney problems.
A dietitian can also help overweight people plan meals to help them lose weight. Studies have shown that being overweight increases the risk of kidney stones, particularly uric acid stones. Diets that are low in carbohydrates have been shown to further increase the risk of uric acid stones and should be avoided.
• Kidney stones can form when substances in the urine—such as calcium, oxalate, and phosphorus—become highly concentrated. Diet is one of several factors that can promote or inhibit kidney stone formation.
• Four major types of kidney stones can form: calcium stones, uric acid stones, struvite stones, and cystine stones.
• Drinking enough fluid is the most important thing a person can do to prevent kidney stones.
• People who have had a kidney stone should drink enough water and other fluids to make at least 2 liters of urine a day.
• Sodium, often from salt, causes the kidneys to excrete more calcium into the urine. High concentrations of calcium in the urine combine with oxalate and phosphorus to form stones. Reducing sodium intake is preferred to reducing calcium intake.
• Meats and other animal protein—such as eggs and fish—contain purines, which break down into uric acid in the urine.
• Calcium from food does not increase the risk of calcium oxalate stones. Calcium in the digestive tract binds to oxalate from food and keeps it from entering the blood, and then the urinary tract, where it can form stones.
• A dietitian can help a person plan meals that lower the risk of forming stones based on the type of stone the person formed in the past.
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