Correspondence

Calcium and Kidney Stones

N Engl J Med 1993; 329:508-509August 12, 1993

Article

To the Editor:

From their study of dietary calcium and other nutrients (March 25 issue),1 Curhan et al. concluded that “a high dietary calcium intake decreases the risk of symptomatic kidney stones.” This statement has resulted in much confusion and consternation among patients with kidney stones and their doctors. The news media and the public seem to have overinterpreted this conclusion as being applicable to all patients with kidney stones, which is clearly not the message intended by the authors.

Nephrolithiasis is not a single disease; rather, there are diverse stone types and diverse causes of calcium oxalate stones in particular. The men in whom stones developed during this study represent a somewhat skewed population2. For example, there were unusually large numbers of men with inflammatory bowel disease (6.7 percent) or uric acid stones (23.1 percent), whereas there were few with hyperparathyroidism (0.6 percent). Stones are known to form in patients with inflammatory bowel disease because of enteric hyperoxaluria; treatment of these patients with a high-calcium diet is standard practice3. In contrast, many patients with calcium oxalate stones have absorptive hypercalciuria; this large subgroup should be treated with a reduced calcium intake2. Other patients may need to avoid excessive dietary intake of sodium or purine. It is as incorrect to advise a high-calcium diet for all patients with kidney stones as it is to prescribe a low-calcium diet indiscriminately. Each patient deserves a metabolic evaluation to determine the causes and risks of stone formation in his or her particular case.

Curhan et al. found a reduction in stone occurrence with increased dietary calcium intake; this decrease was evident primarily from the first to the second quintile group (shown in their Table 4), with little further decline in the occurrence of stones at higher calcium intakes. Thus, there is little evidence that a high calcium intake prevents stones. Rather, a more accurate conclusion would be that a low calcium intake (less than 605 mg per day) increases the risk of kidney stones. This low calcium intake would certainly be expected to increase the risk of hyperoxaluria and stones in the relatively large group of patients with inflammatory bowel disease. It cannot be determined from the data presented by Curhan et al. whether it also increases risk in other subgroups of patients prone to stone formation.

William J. Burtis, M.D., Ph.D.
Arthur E. Broadus, M.D., Ph.D.
Karl L. Insogna, M.D.
Yale University, New Haven, CT 06510

3 References

1
Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993;328:833-838
Full Text | Web of Science | Medline

2
Insogna KL, Broadus AE. Nephrolithiasis. In: Felig P, Baxter JD, Broadus AE, Frohman LA, eds. Endocrinology and metabolism. New York: McGraw-Hill, 1987:1500-77.

3
Coe FL, Parks JH, Asplin JR. The pathogenesis and treatment of kidney stones. N Engl J Med 1992;327:1141-1152
Full Text | Web of Science | Medline

To the Editor:

It is hard to reconcile two important findings in the study by Curhan et al.: that a reduced risk of kidney stones was associated with increased dietary calcium intake, provided primarily by milk, cottage cheese or ricotta cheese, and yogurt, and that an increased risk of kidney stones was associated with an increased intake of animal protein. After all, milk, cheese, and yogurt are animal-protein foods.

Rachel Stern, R.D.
44 Paulin Blvd., Leonia, NJ 07605

Author/Editor Response

The authors reply:

To the Editor: We agree with Drs. Burtis, Broadus, and Insogna that there are diverse causes of calcium oxalate stones and that all patients with kidney stones should have at least a limited metabolic evaluation. For those with calcium oxalate stones, we believe that dietary calcium restriction is not indicated and may be harmful -- in terms of both stone recurrence and bone density1. Well-designed studies demonstrating a benefit of calcium restriction on rates of stone recurrence (not just changes in urine composition) should be conducted before calcium restriction is recommended for certain subgroups. In addition to the role of calcium, the importance of other dietary factors should not be ignored.

We believe that the patients we studied are representative of men with a first kidney stone. The apparent differences between our results and others' may reflect the fact that most patients in those studies had recurrent stones and were treated at referral centers. The low reported prevalence of hyperparathyroidism in our cohort may be either a more representative figure than that from a referral center or an underestimate; most patients in whom a first stone forms receive little evaluation as to the cause. In 505 men with new stones, only 26 percent had blood tests to look for specific causes and only 43 percent had a 24-hour urine collection. Although calcium restriction may lower urinary calcium excretion in patients with absorptive hypercalciuria, there are no controlled studies demonstrating a beneficial effect on stone-recurrence rates. Furthermore, Dr. Lemann, in his editorial accompanying our article,2 suggested that a high-calcium diet should decrease calcium oxalate supersaturation even in patients with absorptive hypercalciuria.

The greatest reduction in risk occurred between the first and second quintile groups for calcium intake, but there was a suggestion of some further benefit in the higher quintile groups. Excluding the 34 patients with self-reported inflammatory bowel disease had no effect on the results; thus, the findings appear to apply to other persons prone to stone formation.

Ms. Stern is correct in pointing out that dairy products contain animal protein, but they also contain substantial amounts of calcium and potassium. For example, an 8-oz glass of skim milk contains 8.4 g of animal protein, 308 mg of calcium, and 402 mg of potassium. Since the intake of calcium and potassium was inversely associated with risk, the overall effect of these foods is to decrease the risk of stone formation.

Gary C. Curhan, M.D.
Walter C. Willett, M.D.
Meir J. Stampfer, M.D.
Harvard School of Public Health, Boston, MA 02115

2 References