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A display of fruits and vegetables in the new grocery section of the new Wal-mart Supercentre in Stouffville, Ontario, Canada, opening Wednesday November 08, 2006. (Deborah Baic/Deborah Baic /The Globe and Mail)
A display of fruits and vegetables in the new grocery section of the new Wal-mart Supercentre in Stouffville, Ontario, Canada, opening Wednesday November 08, 2006. (Deborah Baic/Deborah Baic /The Globe and Mail)

Leslie Beck's Food For Thought

Be kind to your kidneys by adopting the DASH diet Add to ...

It's diet advice that's been proven to lower blood pressure: Eat more fruit, vegetables, beans and low-fat dairy while curbing sweets and red meat.

Now, according to a study from Harvard Medical School, it's advice that can also defend against kidney stones.

Kidney stones are formed when excessive amounts of certain chemicals in the urine - such as calcium, oxalate, uric acid and cystine - form crystals that build up on the inner surfaces of the kidneys. Stones can range in size from a grain of sand to a golf ball.

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Most kidney stones (70 to 80 per cent) are made of calcium oxalate. Oxalate is a type of salt found in certain foods such as nuts, spinach, rhubarb, parsley, tea and chocolate.

One in 10 Canadians will have a kidney stone at some point in their life, men more often than women. After a first attack, a person has about a 50 per cent chance of forming another stone within five years.

In the study, published last week in the online edition of the Clinical Journal of the American Society of Nephrology, researchers set out to see if a DASH-style diet had potent kidney-stone-fighting properties.

The Dietary Approaches to Stop Hypertension diet, which lowers blood pressure, is high in fruits, vegetables, beans, nuts, low-fat dairy and whole grains, and low in refined sugars and red and processed meats.

The investigators studied 24-hour urine samples of 3,426 men and women with and without a history of kidney stones. (The 24-hour test can determine if urine contains too many stone-forming chemicals or too few stone-inhibiting substances.) They also assessed the subjects' diets and scored them according to how closely their diets matched the DASH guidelines.

People who followed a DASH-style diet excreted more urine than those who did not, despite similar fluid intakes. (Kidney stones are more likely to form when there is a decrease in urine volume.) It's speculated that higher urine volumes were a result of the higher water content of foods in the DASH diet.

The urine of the DASH-diet consumers also contained a higher concentration of citrate, an important inhibitor of calcium stones.

Despite a substantially greater calcium intake, individuals with high DASH scores had the same or only marginally higher amount of calcium in their urine. A high intake of potassium from fruits, vegetables and dairy is thought to reduce the amount of calcium making its way to the urine.

As well, the higher oxalate content of the DASH diet did not lead to large increases of oxalate in the urine. Although it's present in many foods, only a fraction of dietary oxalate is available for absorption.

Calcium also binds to oxalate in the intestinal tract, reducing the amount of oxalate that gets absorbed and makes its way to the urine.

The following strategies can reduce the likelihood of kidney stones reappear:

Adopt the DASH diet

To consume a diet with kidney-stone-fighting properties, aim for two to three daily servings of low-fat dairy products, four to five servings of vegetables and four to five servings of fruit.

Choose 100 per cent whole grain foods instead of their refined (white) counterparts. Include nuts and beans in your diet four times a week.

Drink enough fluids

Adequate fluid intake helps flush away substances that can cause crystals to form in the kidneys. If you've already had a kidney stone, drink 12 cups (three litres) of water in divided doses throughout the day. In hot weather, drink an additional two to four cups to make up for fluid lost through sweating.

Limit oxalate-rich foods

Although foods don't contribute much oxalate to the urine, studies do show that spinach, rhubarb, nuts, chocolate, tea, wheat bran and strawberries increase oxalate excretion the most. These should be avoided if you're at risk for calcium oxalate kidney stones.

Meet calcium needs

For years, the standard prescription for calcium-containing kidney stones has been a low-calcium diet. However, restricting dietary calcium is no longer recommended. Many studies have found that eating a calcium-rich diet - such as the DASH diet - is associated with a lower risk of kidney-stone formation.

Adults aged 19 to 50 need 1,000 milligrams of calcium a day, the equivalent of three servings of milk, yogurt or calcium-enriched beverage. Older adults should consume 1,200 to 1,500 milligrams of the mineral per day.

If you don't get all your calcium from food, take a supplement. Since calcium reduces the absorption of oxalate from foods, take your calcium supplement with rather than apart from meals. Speak to your dietitian or doctor about supplementing safely.

Don't push protein

Overeating protein foods such as meat, poultry, fish and eggs can increase the amount of calcium excreted in the urine.

Keep your meat and poultry portions to three ounces (90 grams). Substitute animal protein with vegetable protein such as beans and nuts, since these may reduce the risk of kidney stones.

Control your weight

Studies suggest that being overweight or obese increases the risk of kidney stones. Excess weight is linked with altered acidity of the urine and higher uric acid levels in the blood, two factors that can trigger stone formation.

Reduce vitamin C

Large amounts of vitamin C can increase the risk of calcium oxalate since the vitamin is converted to oxalate in the body.

If you have had a calcium oxalate kidney stone, don't take high-dose (500 to 1,000-milligram) vitamin C supplements. Instead, focus on getting your vitamin C from foods such as citrus fruit, kiwi, mango, cantaloupe, broccoli, Brussels sprouts, cauliflower and bell peppers.

Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com.

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