14 FAQ on diet and kidney stones


Here are 14 questions and answers on the role of diet in the development and prevention of kidney stones.

How does diet affect the risk of developing kidney stones?
Diet is one of several factors that can promote or inhibit kidney stone formation. Other factors include heredity, environment, weight, and fluid intake. 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. Certain foods create wastes that may form crystals in the urinary tract. In some people, the crystals grow into stones. For people who have had a kidney stone, preventing another will be a priority. In addition to dietary changes, a person may need medicine to prevent kidney stones. The first step in preventing kidney stones is to learn what kind of stones a person's body typically makes.

How much fluid should a person drink to prevent stone formation?
The amount of fluid a person needs to drink depends on the weather and the person's activity level. People who have had a kidney stone should drink enough water and other fluids to produce at least 2 quarts of urine a day. Some doctors have their patients collect urine for 24 hours so the volume can be measured. The doctor can then advise the patient about increasing fluid intake, if necessary. People who work or exercise in hot weather need more fluid to replace the fluid they lose through sweat. Drinking enough water helps keep urine diluted and flushes away materials that might form stones and is the most important thing a person can do to prevent kidney stones.

A person at risk for cystine stones should drink enough water each day to dilute the concentration of cystine that escapes into the urine, which may be difficult. More than a gallon of water may be needed every 24 hours.

What fluids protect against kidney stone formation?
Water is an inexpensive and calorie-free protection against kidney stones.

Some studies suggest citrus drinks like lemonade and orange juice protect against stones because they contain citrate, which stops crystals from growing into stones. But no large-scale trials have been conducted to confirm these findings. While citrus drinks may be helpful in preventing calcium oxalate stones and uric acid stones, they might be harmful for people who form calcium phosphate stones.

Coffee and tea can add to a person's total fluid intake and have been shown to reduce the risk of stone formation, but they do contain oxalate. Moderate intake of beer and wine may also protect against stone formation.

What fluids should be avoided?
Grapefruit juice and dark colas have been found to increase the risk of stone formation and should be avoided by people who are prone to calcium oxalate stone formation. Although cranberry juice is often promoted as useful for preventing urinary tract infections, it contains oxalate and may be harmful to stone formers.

How does salt in the diet affect kidney stone formation?
Salt is made up of sodium and chloride. The sodium in salt, when excreted by the kidneys, causes more calcium to be excreted into the urine. High concentrations of calcium in the urine combine with oxalate and phosphorus to form stones. Reducing salt intake is preferred to reducing calcium intake.

The U.S. recommended daily allowance (RDA) of sodium is 2,400 milligrams (mg), but Americans' intake averages 3,300 mg, according to the National Heart, Lung, and Blood Institute. The risk of kidney stones increases with increased daily sodium consumption. Limiting salt to the U.S. RDA goal of 2,400 mg is an important step for people who form calcium oxalate or calcium phosphate stones. People taking medications-such as hydrochlorothiazide, chlorthalidone, or indapamide-to prevent stones still need to limit salt intake.

How can a person limit sodium intake?
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 2,400 mg. When eating out, people should ask about the sodium content of foods they order.

Some foods have such large amounts of sodium that a single serving provides a major portion of the daily allowance. 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 (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

How does animal protein in the diet affect kidney stone formation?
Meats and other animal proteins-such as eggs and fish-contain purines, which break down into uric acid in the urine. Foods that are 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.

Nondairy animal proteins may also increase 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.

How does calcium in the diet affect kidney stone formation?
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 yoghurt 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. Some studies indicate that calcium supplements increase the risk of calcium oxalate stone formation. Researchers theorize that calcium must be taken at the same time as dietary oxalate to protect against stone formation. Calcium supplements taken with meals may have the same protective effect as dietary calcium.

How does oxalate in the diet affect kidney stone formation?
Some of the oxalate in urine is made by the body. But 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.

How can a person limit oxalate in the urine?
Many foods and beverages contain oxalate, but only a few have been shown to increase the amount of oxalate in urine:

  • spinach
  • rhubarb
  • nuts
  • wheat bran

Avoiding these foods may help reduce the amount of oxalate in the urine. Eating foods containing calcium also reduces oxalate in the urine. Calcium binds oxalate in the digestive tract so it is not excreted into the urine.

How does alcohol affect kidney stone formation?
Although drinking alcohol may promote purine production in the body, studies have not shown more stones in people who drink beer and wine. People should have no more than two drinks-two 12-ounce servings of beer or two 5-ounce servings of wine or two 1.5-ounce servings of hard liquor-a day.

How do supplements affect kidney stone formation?
Supplements containing vitamin C or D may contribute to stone formation. Vitamin C is ascorbate and can be turned into oxalate by the body. Doctors recommend no more than 500 milligrams each day for people who have had kidney stones. Calcium supplements should be taken with meals so the calcium can bind with the oxalate in food. A person who has a tendency to form kidney stones should consult a doctor or dietitian before taking large doses of vitamins or minerals.

How does a person's weight affect kidney stone formation?
Studies have shown that being overweight increases the risk of uric acid and calcium kidney stones. Scientists don't know whether losing weight by itself can reduce the risk of kidney stones. Maintaining a healthy weight through healthy food choices and exercise may help reduce the risk of kidney stones.

Can a dietician help a person prevent kidney stones?
Yes. After a doctor has completed an evaluation and determined the causes of a person's stones, a dietician can help a person plan meals that lower the risk of forming stones. People who form stones can ask their doctor or nurse to help them find a dietician who specializes in kidney stone prevention or renal, or kidney, nutrition.

Read more on 5 types of kidney stones.

(Source: National Institutes of Health, NIDDK, February 2013)

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