The belief that a low-calcium diet can help prevent kidney stones is based on little scientific evidence, according to a new study.
A meal plan with normal amounts of the mineral, but with modest levels of meat and salt, is roughly twice as effective as a low-calcium diet at preventing kidney stones from recurring in men with a history of the painful accretions, Italian researchers say.
Although kidney stones are largely composed of calcium, denying the body the mineral appears in fact to promote stones and deprives the skeleton of a critical building block.
Kidney stones are hard clusters of calcium and a compound called oxalate. They manage to avoid being flushed out of the kidney in urine, possibly by clinging to cells in the ducts that funnel the fluid.
In the latest study, published in the New England Journal of Medicine, a team led by Dr. Loris Borghi of the University of Parma compared a low-calcium diet of 400 milligrams a day with a normal-calcium (1,200 milligrams a day), low animal protein, low-salt diet in 120 men with a history of kidney stones. Animal protein promotes the excretion of calcium in urine through a series of metabolic steps.
Both groups of men were also instructed to drink two to three litres of water a day.
After five years, 23 of the 60 men on the low-calcium diet had developed new stones, compared with just 12 of the 60 who maintained the normal balanced diet. Both groups saw sharp reductions in urine levels of calcium, but while the men on the low-calcium diet showed an increase in their excretion of oxalate, those on the normal calcium diet had a decrease in the compound.
Calcium keeps oxalate in the gut, rendering it harmless, so an increase in oxalate in urine signals that too much of the substance is getting into the kidneys.
"Our study suggests that a diet characterised by normal calcium, low animal protein, and low salt levels is more effective than the traditional low-calcium diet for the prevention of recurrent stones in men with elevated urine calcium,"the researchers said. "We speculate that this type of diet will be of the greatest value when it is started early in the course of the disease."