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Low-Calcium Diet Won't Help Kidney Stones

Study: Limiting meat and salt stops them from coming back

WEDNESDAY, Jan. 9, 2002 (HealthDayNews) -- The longstanding advice to cut calcium from your diet to prevent kidney stones from recurring is based on rocky scientific evidence, a new study says.

A meal plan with normal amounts of the mineral, but that's modest in meat and salt, is roughly twice as effective as a low-calcium diet at preventing kidney stones from recurring in men with a history of the painful accretions, Italian researchers say.

In an about-face from earlier thinking, experts say the findings should assign the low-calcium prescription to the scientific outhouse. Although kidney stones are largely composed of calcium, denying the body the mineral appears in fact to promote stones and deprives the skeleton of a critical building block.

"I think by putting someone with kidney stones on a low-calcium diet you're taking a great risk that this will lead to a depletion of their bone mineral," says Dr. David Bushinsky, a kidney expert at the University of Rochester in New York. "It's a sure way to promote osteoporosis," especially in older women, he says.

Bushinsky, author of an editorial accompanying the article, which appears in tomorrow's issue of the New England Journal of Medicine, says stone patients don't have to radically change their lifestyle to reduce their risk of future problems.

"No one has to give up anything, they just have to do things in moderation," he says.

Kidney stones are hard clusters of calcium and a compound called oxalate. They manage to avoid being flushed out of the kidney in urine, possibly by clinging to cells in the ducts that funnel the fluid.

The affliction was so much a part of ancient medicine that the Hippocratic Oath mentions it by name. About 13 percent of Americans will endure at least one kidney stone in their lifetime. Once one appears, the chance of having another in the next five or so years is about 50 percent.

Despite millennia of experience with stones, doctors haven't mastered their treatment. Indeed, prevention -- drinking plenty of water, avoiding urinary infections, and proper bathroom habits -- remains the best way to deal with them.

Because of their heavy calcium content, doctors previously thought that restricting dietary intake of the mineral might avoid new accretions. However, mounting evidence has shown the approach causes more harm than good.

In the latest study, a team led by Dr. Loris Borghi of the University of Parma pitted a low-calcium diet of 400 milligrams a day against a normal-calcium (1,200 milligrams a day), low animal protein, low-salt diet in 120 men with a history of kidney stones. Animal protein promotes the excretion of calcium in urine through a series of metabolic steps.

Both groups of men were also instructed to drink two to three liters of water a day.

After five years of follow-up, 23 of the 60 men on the low-calcium diet had developed new stones, compared with just 12 of the 60 who didn't cut back on it.

Both groups saw sharp falloffs in urine levels of calcium. What's more, while the men on the low-calcium diet showed an increase in their excretion of oxalate, those on the normal calcium diet had a decrease in the compound.

Calcium ties up oxalate in the gut, rendering it harmless. So rising oxalate in urine signals that too much of the substance is getting into the kidneys.

"Our study suggests that a diet characterized by normal calcium, low animal protein, and low salt levels is more effective than the traditional low-calcium diet for the prevention of recurrent stones" in men with elevated urine calcium, the researchers write. "We speculate that this type of diet will be of the greatest value when it is started early in the course of the disease."

Dr. Umberto Maggiore, a University of Parma stone specialist and study co-author, admits that keeping Americans away from meat and salt might be harder than it is in Italy.

"However, if appropriately encouraged by their doctors, even the American patient should be able to restrict meat intake in favor of dairy foods, while having ample provision of vegetables," Maggiore says.

What To Do

Both Maggiore and Bushinsky say there's no reason to think that the Italian findings wouldn't apply to women, since the physiology of kidney stones is the same for both sexes. And while Bushinsky adds that asking patients to drink up to three liters of water a day -- to keep their urine output at about two liters -- is a tall order, "I think it's a matter of will."

To find out more about kidney stones, try the National Institute of Diabetes and Digestive and Kidney Diseases or the Urology Forum.

SOURCES: Interviews with Umberto Maggiore, M.D., kidney specialist, Department of Clinical Sciences, University of Parma, Italy; David Bushinsky, M.D., professor of medicine, University of Rochester School of Medicine and Dentistry, chief of nephrology, Strong Memorial Hospital, Rochester, N.Y.; Jan. 10, 2002, New England Journal of Medicine
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