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Study - Low-salt, low-protein diet can prevent kidney stones

Thursday, January 10, 2002

A diet low in salt and meat can dramatically reduce the risk of kidney stones, according to an Italian study that could spell the end for the low-calcium diet that doctors have been recommending for years.

About 10 percent of the U.S. population -- and a much larger percentage of men, who make up 80 percent of sufferers -- will have at least one kidney stone sometime in their lives, and it can be excruciatingly painful. Most stones can be excreted, but in about 15 percent of cases, surgery or shockwave treatment to pulverize the stones is needed.

Many doctors have told such patients to cut down on calcium because most kidney stones are made of a calcium compound. But recent studies have suggested that such a diet might not prevent kidney stones after all and may even promote them -- along with osteoporosis.

The new study "dispelled a myth that a low-calcium diet is important in preventing kidney stones," said Dr. David A. Bushinsky, a kidney specialist at the University of Rochester.

William Keane, president of the National Kidney Foundation, said the new diet "will become the gold standard."

The University of Parma study assigned either a low-calcium diet or a diet low in salt and extremely low in protein to men who had had at least one kidney stone. Sixty men were assigned to each diet.

Twenty-three men on the low-calcium diet had another kidney stone within five years, compared with 12 on the low-salt, low-protein diet, Dr. Loris Borghi wrote in the New England Journal of Medicine.

A co-author, Dr. Umberto Maggiore, explained that people on a low-calcium diet excrete less calcium in their urine but more of the other substance -- oxalate -- that combines with calcium to form most kidney stones.

In addition, salt and one protein common in meat lead to more calcium in the urine, which in turn contributes to the formation of kidney stones, Bushinsky said.

He said he has not prescribed a low-calcium diet in a decade because of evidence that it leaches calcium from the bones, making them weaker. Women were not included in the Italian study because of the danger of osteoporosis.

The low-salt, low-protein diet allowed 2,900 milligrams of salt per day; 2,400 is the maximum recommended by the American Heart Association. But people were allowed only about three-quarters of an ounce of meat per day, with just over an ounce of cheese or other dairy protein.

The American Foundation for Urologic Disease does not mention a low-calcium diet on its Web page about kidney stones, but does say a doctor may suggest a low-meat, low-salt diet. The National Kidney Foundation's Web page says a doctor may suggest either diet, depending on test results.

Borghi's study is the first direct comparison of the two diets, said Dr. Julie R. Ingelfinger, the journal's deputy editor. She wrote that the diet has been proved effective only for men in Italy's Parma region, but is worth trying "for anyone who has had a stone or who has witnessed the suffering of a friend or family member with a stone."

Maggiore said the diet would be equally effective in any country, but he noted that it might be harder to get Americans to stick to it because meat makes up such a great part of the U.S. diet.

Health care for kidney stones added up to $1.8 billion in 1993, according to the National Institute of Diabetes and Digestive and Kidney Diseases. Drinking 10 to 12 big glasses of water during the day can keep stones from forming and push out those smaller than a pea.


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