Your kidneys play a critical role in health by helping to eliminate waste products that are created by the process of digestion and the breakdown of muscle tissue. These waste materials are carried through the bloodstream to the kidneys, where impurities are cleaned out of the blood and converted into urine. Without this cleansing process, toxic wastes would build up and cause extensive damage to your body.
Sometimes, substances in the urine can cause crystals to form that build up on the inner surfaces of the kidneys. These solid masses are called 'kidney stones'. Normally, the urine contains compounds that prevent crystals from forming but these chemical inhibitors don't seem to work effectively for everyone.
Kidney stones can vary considerably in size. Some are as small as a grain of sand but others can grow to the size of a golf ball. Tiny stones are usually passed out of the body along with your urine, causing little or no pain. However, when larger stones are washed out of the kidney, they may become stuck in the bladder or another part of the urinary tract. These stones block the flow of urine and can cause excruciating pain.
There are four main types of kidney stones:
- Calcium oxalate or calcium phosphate stones are the most common type. These chemicals are a part of the normal diet and contribute to bone and muscle tissues
- Struvite stones usually forms after a urinary tract infection
- Uric acid stones can occur when there is too much acid in the urine
- Cysteine stones are quite rare and usually are caused by an inherited disease
The exact cause of kidney stones remains unknown. A genetic predisposition and diet and lifestyle factors appear to play a role. Recently, scientists have identified tiny bacteria called nanobacteria inside kidney stones that are thought to trigger stone formation.
- Extreme pain in the small of the back or lower abdomen - the pain may come and go, lasting for minutes or hours at a time
- Blood in the urine
- Nausea and vomiting
- Fever and chills
- Burning during urination
- An urge to urinate frequently
- Cloudy or foul smelling urine
Who's at risk?
It is estimated that one out of 10 Canadians will have a kidney stone at some point in their life. Those more susceptible to stone formation include:
- Men - men suffer from kidney stones 3 to 4 times more often than women
- People with a family history of kidney disease
- Middle aged adults
- People who live in hot climates and don't drink enough fluids
- People who have recurrent urinary tract infections, kidney disorders and metabolic disorders
Dietary strategies for preventing kidney stones
In as many as 50 percent of cases, kidney stones will recur. If you have kidneys stones the following dietary modifications can minimize crystal formation and help prevent a future attack.
Water & Other Beverages. Drinking more fluids helps flush away substances that can cause crystals to form in the kidneys. When the urine is less dilute, it becomes more concentrated with chemicals that can crystallize into stones. Drink three litres (12 cups) of water in divided doses throughout the day. In hot weather, drink an additional 0.5 to 1 litre (2 to 4 cups) to make up for lost fluid from sweating.
Certain beverages may lower the risk of kidney stone recurrences. Coffee, decaffeinated coffee, tea, beer and wine have been shown to prevent stone formation presumably by increasing the flow of a more dilute urine. Grapefruit juice and apple juice increased the risk of stones for reasons unknown. Lemonade made from 4 ounces of reconstituted lemon juice may be beneficial for some people with calcium kidney stones.
Oxalate rich foods. Oxalate, or oxalic acid, is a compound found naturally in plant foods that can crystallize with calcium in the urine. If you have calcium oxalate kidney stones, restricting your intake of oxalate containing foods is advised, especially if a urine test reveals you excrete high amounts oxalate. Although oxalate is present in many foods, it is not always available to the body and therefore may not contribute much oxalate to the urine. Research has found eight foods to increase oxalate excretion: spinach, rhubarb, beets, nuts, chocolate, tea, wheat bran and strawberries.
The following foods contain moderate or high amounts of oxalate and should be limited:
- Beverages: Draft beer, chocolate beverage mixes, cocoa, instant tea, instant coffee
- Breads & Cereals: Grits (white corn), wheat bran, wheat germ, whole-wheat flour
- Fruits: Berries, Concord grapes, red currants, Damson plums, lemon, lime, orange peel, rhubarb, tangerines
- Vegetables: Beets, celery, eggplant, endive, escarole, leeks, parsley, spinach, summer squash, sweet potatoes, Swiss chard, wax beans
- Legumes: Dried peas, beans and lentils, baked beans with tomato sauce, nuts, nut butters, peanut butter, tofu
- Desserts: Fruitcake, desserts containing fruits listed above
- Other: Chocolate, cocoa, carob powder
Calcium-rich foods. If you have calcium containing kidney stones it is not necessary to limit your intake of calcium rich foods. In fact, restriction of dietary calcium is not recommended Studies have found that eating high calcium foods is associated with a lower risk of stone formation in men and women. One study even found that drinking two servings of milk or a calcium fortified orange juice per day did not increase the of calcium kidney stones in susceptible individuals.
Researchers have learned that consuming enough calcium helps to decrease the amount of oxalate that is excreted in the urine. Calcium in foods binds to oxalate in the intestine preventing its absorption. Many calcium rich foods also contain potassium and magnesium, two nutrients that might help prevent kidney stones.
Ensure that you are meeting your daily calcium requirements of 1000 to 1200 milligrams. Some individuals with calcium kidney stones may absorb too much calcium from their intestine. Specific tests will reveal this and your doctor may advise that you limit your calcium intake to 800 milligrams daily. This means consuming no more than two milk product servings per day (e.g. 1 cup milk, yogurt, calcium fortified beverage or 45 grams of cheese).
Protein-rich foods. Overeating protein foods like meat, poultry, eggs and dairy products can increase the amount of calcium excreted in the urine. A handful of studies have shown that a moderately low protein diet reduces the amount of calcium, sodium, phosphates and uric acid in the urine, components known to cause kidney stones. Substitute animal protein with legumes and soy foods more often since vegetable proteins may reduce the risk of kidney stones.
Sodium. A high salt diet causes more calcium to be excreted in the urine and makes the urine more acidic, two factors that increase the likelihood of calcium salts crystallizing to form kidney stones. If your calcium kidney stones are caused because you excrete too much calcium in your urine, limit your sodium intake to no more than 2400 milligrams per day.
To help cut back on sodium, avoid the saltshaker at the table, minimize the use of salt in cooking, and buy commercial food products that are low in added salt.
Vitamins and Minerals
Vitamin C. In high doses, some vitamin C is converted to oxalate in the body. Indeed, studies have shown oxalate excretion to increase with vitamin C supplementation. For this reason, people with recurrent calcium oxalate kidney stones are often advised to consume no more than 100 milligrams of vitamin C per day.
However, recent studies suggest that restricting vitamin C intake may be unwarranted. A large Harvard study conducted among 85,500 women reveled that vitamin C intake was not associated with the risk of kidney stones. Another American study did not find any link between levels of vitamin C in the blood and kidney stone formation and kidney stone development among adult men and women.
Because it is still not known if vitamin C supplements actually cause kidney stones to recur, take vitamin C supplements only under the advice of your doctor or dietitian.
Calcium supplements. If you have calcium kidney stones, it may be wise to avoid calcium supplements. Instead, you should aim to meet your calcium needs through foods such as milk, yogurt, and calcium-fortified beverages. A large Harvard study found that women who took calcium supplements had a higher risk of stone formation compared to those who did not use supplements. However, the women in the study took their calcium supplements apart from a meal, or with a meal low in oxalate content, which may affect how much oxalate ends up in the urine. A much smaller study did not find an increased risk of kidney stones in postmenopausal women who took 750 milligrams of calcium carbonate daily.
Because calcium reduces the absorption of oxalate, both the type of calcium supplement and timing of ingestion may be important factors.
The Kidney Foundation of Canada
National Kidney Foundation
National Kidney and Urologic Diseases Information Clearinghouse
A service of The National Institute of Diabetes and Digestive and Kidney Diseases
The above excerpt is from "Leslie Beck's Nutrition Encyclopedia" (Penguin Canada, 2001/2003), available at bookstores across the country. The following is copyrighted and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or means - electronic, mechanical, photocopying, or likewise.
All research on this web site is the property of Leslie Beck Nutrition Consulting Inc. and is protected by copyright. Keep in mind that research on these matters continues daily and is subject to change. The information presented is not intended as a substitute for medical treatment. It is intended to provide ongoing support of your healthy lifestyle practices.