Patients with idiopathic hypercalciuria and recurrent calcium oxalate stones are often advised to limit their intake of calcium. But a recent study in the New England Journal of Medicine suggests that a more effective diet may be one that is adequate in calcium, but low in animal protein and sodium.

Comparing diets

Researchers in Italy conducted a five-year randomized trial comparing two diets in 120 men with recurrent calcium oxalate stones and hypercalciuria. Sixty men were instructed to eat a diet low in calcium (400 mg/day), by strictly limiting their intake of calcium-containing foods, such as milk, yogurt, and cheese. The other 60 men were instructed to eat a diet adequate in calcium (1,200 milligrams per day), but low in animal protein (52 g/day) and salt (3 grams).

All men were instructed to drink at least 2-3 liters of water per day and limit their consumption of oxalate-containing foods, such as rhubarb, spinach, parsley, and chocolate.

Twenty-four-hour urine samples were collected at baseline, one week after randomization, and at yearly intervals during the five years of study. The primary outcome measure was time to first recurrence of a symptomatic renal stone or the presence of stones identified on radiography.

At the end of five years, 12 of the men in the normal-calcium, low-protein, low-salt group had experienced recurrences, compared with 23 in the low-calcium group (RR=0.49, 95% CI 0.24-0.98; P=0.04).

Dietary compliance was not measured with dietary recalls or food frequency questionnaires, but was reflected, say the researchers, in the urinary variables. Urinary excretion of sodium, urea nitrogen, and sulfate remained unchanged in the men following the low-calcium diet, but decreased in the men following the normal-calcium, low-protein, low-salt diet.

How the diet may work

The researchers believe the results may be attributable to the different diets' effects on urinary oxalate excretion. They say that while a low-calcium diet may decrease urinary calcium excretion, it may actually cause an increase in oxalate excretion through increased intestinal absorption, resulting from the low level of calcium available to form a complex with oxalate in the intestinal lumen.

Conversely, in the normal-calcium, low-protein, low-salt diet, more calcium may be available in the intestinal lumen to form a complex with oxalate, thereby reducing its absorption.

Further research necessary

While these results are intriguing, further studies are needed to confirm them. Also, the current study included only men, so it is unclear if the results would have been the same in women.

In the meantime, however, recommending a diet that limits protein and salt may be useful for patients with recurrent calcium oxalate stones and hypercalciuria, particularly if low-calcium diets have failed in the past. The researchers believe that this diet will be most effective when started early in the course of disease.