Wednesday January 9

Low-Protein Diet Cuts Risk of Kidney Stones

By Suzanne Rostler

NEW YORK (Reuters Health) - Men who develop kidney stones are often told to limit their dietary intake of calcium. But new research suggests that restricting animal protein and salt may be a better way to prevent stones from recurring.

In the study, 20% of men on a low-animal protein, low-salt diet that contained normal amounts of calcium had recurrent stones after 5 years, compared with 38% of men on a low-calcium diet.

The results point to a new dietary approach to manage kidney stones, according to Dr. Loris Borghi from the University of Parma in Italy and colleagues. Reducing calcium intake, they explain, has not been shown to reduce the risk of recurrent stones over the long term and may cause calcium deficiency, which can weaken the bones.

``Restricted intake of animal protein and salt, combined with a normal calcium intake, provides greater protection than the traditional low-calcium diet,'' Borghi and colleagues report.

``We speculate that this type of diet will be of greatest value when it is started early in the course of the disease,'' the authors write in the January 10th issue of The New England Journal of Medicine (news - web sites).

A diet that restricts animal protein and salt is thought to reduce the urinary excretion of oxalate, a compound that combines with calcium and other compounds to form the deposits commonly known as kidney stones, the researchers explain. Low-calcium diets, on the other hand, may reduce urinary excretion of calcium but they also cause levels of oxalate in urine to rise, research has shown.

To investigate the long-term effects of these two approaches, the researchers had a group of men follow a diet that included no milk, yogurt or cheese. Another group followed a diet that was low in animal protein and salt but contained normal amounts of calcium--about 1,200 milligrams a day. All of the men were advised to limit oxalate-containing foods such as walnuts, spinach, rhubarb, parsley and chocolate, and to consume 2 to 3 liters of water daily.

After 5 years, men on the diet restricting animal protein and salt were significantly less likely to have recurrent stones, the report indicates. Urinary levels of oxalate increased in the men on the low-calcium diet but decreased in men on the other diet. Urinary calcium levels declined in both groups.

The findings are not surprising, Dr. David A. Bushinsky from the University of Rochester in New York noted in an interview with Reuters Health. Bushinsky wrote an editorial that accompanied the study.

``However, until the current study, no one had ever compared (a low-animal protein, low-salt) diet to one low in calcium. Based on the results of this carefully done study, we now have experimental evidence as to the preferred approach in men,'' he said.

Bushinsky suggests that future studies investigate the individual effects of calcium, animal protein and salt on kidney stones, and determine the effect of these diets in women, for whom adequate calcium intake is more important.

Kidney stones, which are often extremely painful to excrete, cost billions of dollars each year in medical care and lost productivity. Roughly 10% of Americans develop a kidney stone at some point in their lives.

While the stones can sometimes be prevented by drinking plenty of water, avoiding infections and emptying the bladder frequently, medication to reduce the excretion of calcium in urine may be prescribed. These drugs may increase cholesterol and can cause bloating and other uncomfortable side effects.

SOURCE: The New England Journal of Medicine 2002;346:74-75, 77-84, 124-

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