New dietary regimen prevents kidney stones better than low-calcium diet

By Karla Gale

NEW YORK (Reuters Health) – A diet low in animal protein and sodium, but with normal calcium levels, is more effective in preventing recurrence of calcium oxalate kidney stones than a diet low in calcium, according to a report in the January 10th issue of The New England Journal of Medicine.

In an Italian study, 120 men were randomly assigned to one of the two diets. All subjects had idiopathic hypercalciuria and a history of at least two episodes of calcium oxalate stones. Analysis of outcomes was based on intention to treat.

Dr. Umberto Maggiore and colleagues, of the University of Parma, devised the low-calcium diet so that it restricted milk, yogurt, and cheese to a calcium intake of approximately 10 mmol per day. The other diet limited animal protein to 52 g per day and sodium chloride to 50 mmol per day. Subjects in both groups were advised to drink 2 to 3 liters of water per day and to avoid consuming large amounts of oxalate-rich food.

During the following 5 years, 23 subjects on the low-calcium diet and 12 on the low-protein, low-salt diet experienced stone recurrence. The relative risk for recurrence on the new diet was 0.49 compared with the low-calcium diet. The difference became significant more toward the end of the study, presumably because of "early recurrences in the highest-risk patients, regardless of the diet to which they were assigned," the investigators write.

Urinary calcium excretion decreased with both diets. However, the low-protein, low-salt diet caused a decrease in oxalate excretion, while the low-calcium diet caused an increase. The higher level of calcium available in the intestinal lumen complexes with oxalate, the investigators suggest, reducing its absorption. They also believe the endogenous synthesis of oxalate may be decreased because protein intake is lower.

Dr. Maggiore told Reuters Health, "We now offer this [low-protein, low-salt] diet as the first line of treatment for our patients." He and his associates believe such a diet would be of greatest value if started early in the course of the disease.

In the future, Dr. Maggiore and his colleagues hope to "address whether this diet is beneficial in the prevention of osteoporosis in women" he added.

In an editorial, Dr. David A. Bushinsky, of the University of Rochester School of Medicine and Dentistry in New York advises that future studies should evaluate the independent roles that dietary calcium, animal protein, and salt play in the recurrence of stones. Nonetheless, based on the findings of this "carefully controlled study," as well as those from previous studies, he concludes: "Physicians should no longer prescribe a low-calcium diet to prevent recurrent nephrolithiasis in patients with idiopathic hypercalciuria."