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Abstracts & Commentary
Synopsis: Restricted intake of salt and protein in conjunction with a normal calcium intake provides greater protection from recurrent calcium oxalate stones than a low-calcium diet.
Sources: Borghi L, et al. N Engl J Med. 2002;346:77-84; Bushinsky DA. N Engl J Med. 2002;346:124-125.
Borghi and colleagues point out that physicians usually prescribe a low-calcium diet for patients with idiopathic hypercalciuria and calcium oxalate stone formation. They note that there are no long-term studies on the efficacy of this approach.
Their study was a 5-year randomized trial comparing the effect of 2 diets in 120 men with recurrent calcium oxalate stones and hypercalciuria. Sixty men were assigned to a diet containing a normal amount of calcium (30 mmol/d) but reduced amounts of animal protein (52 g/d) and salt (50 mmol of sodium chloride/d); the other 60 men were assigned to the traditional low-calcium diet, which contained 10 mmol of calcium per day.
After 5 years, 12 of the 60 men on the normal-calcium, low-animal protein, low-salt diet and 23 of the 60 men on the low-calcium diet had had relapses, the unadjusted relative risk of a recurrence for the group on the first diet, as compared with the group on the second diet, was 0.49 (P = 0.04). During follow-up, urinary calcium levels dropped significantly in both groups by approximately 170 mg/d. However, urinary oxalate excretion increased in the men on the low-calcium diet, by average of 5.4 mg per day, but decreased in those on the normal calcium, low animal protein, low salt diet by an average of 7.2 mg per day.
In men with recurrent calcium oxalate stones and hypercalciuria, restricted intake of animal protein and salt, combined with a normal calcium intake, provides greater protection than the traditional low calcium diet.
Comment by Ralph R. Hall, MD, FACP
It is surprising to read that there are still many physicians who recommend a low-calcium diet for this disorder.
The paper by Curhan and colleagues in 19931 with a convincing editorial by Lemann2 contained enough data to convince most physicians that a low-calcium diet was potentially harmful. Low-calcium diets in a group of patients who had been losing excessive amounts of calcium for most of their lives was certainly exposing them to serious osteoporosis. Curhan et al also documented the need for increased fluid intake.
Bushinsky, the author of the excellent editorial commenting on the paper by Borghi et al, also presented evidence that low-calcium diets should not be used.3 He reviews the evidence that kidney stones are a significant problem resulting in an estimated 1.32 million visits to physicians in 1995. In approximately 70% of the cases, the stones are composed of calcium oxalate. In his editorial, Bushinsky succinctly reviewed Lemann’s data indicating a low-calcium diet may indeed increase the production of calcium oxalate stones.
There is strong experimental evidence that animal protein and salt increase calcium excretion.4 Borghi et al also advised their patients to avoid large amounts of oxalate-rich foods (eg, walnuts, spinach, rhubarb, parsley, and chocolate.)
As Bushinsky notes in his editorial, "It is not known whether these results are valid for women." Further, he suggests that future studies should address the independent role of these 3 dietary components. There is strong experimental evidence, but no long-term studies to verify the long-term results of treatment.
It also would be helpful to know how simply prescribing small doses of inexpensive thiazide diuretics might compare to these diets. Prospective, controlled clinical trials have demonstrated the effectiveness of low-dose treatment with thiazides.5 This approach must be associated with a restricted sodium intake and monitoring of the serum potassium.
1. Curhan GC, et al. N Engl J Med. 1993;328:833-838.
2. Lemann J Jr. N Engl J Med. 1993;328:880-882.
3. Bushinsky DA. J Am Soc Nephrol. 1998;9:917-924.
4. Assimos DG, Holmes Ross P. Urol Clin North Am. 2000;27:255-268.
5. Scholz D, et al. J Urol. 1982;128:903-908.
Dr. Hall, Emeritus Professor of Medicine, University of Missouri-Kansas City School of Medicine, is Associate Editor of Internal Medicine Alert.