Too much calcium may not be to blame for recurrent kidney stones

About the study

Between June 1993 and December 1994, researchers at the University of Parma (Italy) enrolled 120 men from the city of Parma into this study. All of the men had unexplained high levels of calcium in their urine and recurrent kidney stones, but were not receiving treatment for kidney stones or eating a restricted diet of any kind. Men who had medical conditions known to be associated with kidney stones were excluded from this study, because the researchers were looking for ways to reduce kidney stones in men whose stones had no known cause.

The men were randomly assigned to one of two diets. They were instructed to eat their normal diets with the following exceptions:

• Low-calcium diet – only 400 mg of calcium per day
• Normal-calcium, low-animal-protein and low-salt diet – 1200 mg of calcium per day, 52 g of animal protein per day, 2900 mg of salt (sodium chloride) per day, and limited oxalate-rich foods
• Both diets – two to three liters of water per day and moderate consumption of wine, beer, carbonated beverages and coffee

Urine samples were collected from the men at the start of the study, one week later, and yearly for five years thereafter. The urine samples were analyzed for sodium content, urea content (a measure of total protein intake), sulfate content (a measure of animal-protein intake), calcium content, and oxalate content. (Most kidney stones are composed of calcium oxalate.)

After five years of follow-up, the researchers looked at which men experienced kidney stones over the five years and which diet they were eating.

The findings

At the end of the study, 12 of the 60 men on the normal-calcium, low-animal-protein, low-salt diet had a recurrence of kidney stones, compared with 23 of the 60 men on the low-calcium diet who had a recurrence. Men on the normal-calcium, low-animal-protein, low-salt diet were 50% less likely to have a recurrence of kidney stones than men on the low-calcium diet.

Over the course of the study, urinary calcium levels dropped in both groups, but urinary oxalate excretion increased in men on the low-calcium diet and decreased in men on the normal-calcium, low-animal-protein, low-salt diet. The authors propose that this increase in oxalate excretion may be an important factor in the production of kidney stones.

There are limitations to this study that should be considered along with the results. First, the study only included men, which means these results may not apply to women. Second, we don't know how closely the men followed their assigned diets over the five years of the study, though the yearly urine tests indicate that the men complied with the diets fairly well.

How does this affect you?

These findings, coupled with those of several other studies, question the effectiveness of the low-calcium diet in preventing recurrent kidney stones. In his editorial accompanying the NEJM article, Dr. David Bushinsky proposes that physicians should no longer prescribe a low-calcium diet to prevent recurrent kidney stones with no known cause.

Should you try the low-animal-protein, low-salt diet used in this study if you have recurrent kidney stones? Considering that kidney stones are extremely painful and are likely to recur, this diet may well be worth trying. With 15% of total calories from protein, 33% from fat, and 52% from carbohydrates, this diet is not particularly extreme or restrictive. If you have a history of kidney stones that have no known cause, consider asking your doctor about this diet.

Future studies to determine the safety and effectiveness of this diet in women are needed. It holds particular promise for women with kidney stones because it does not restrict calcium intake—a factor that puts women at risk for osteoporosis.