Calcium and kidney stones - recent studies dispel the notion that a diet high in calcium contributes to the formation of kidney stones - Brief Article

Harvard Health Letter, June, 1997

It was long thought that a diet high in calcium contributed to the development of kidney stones because up to 80% of the deposits contain calcium. The stones, which can cause excruciating pain, often become lodged in the kidneys or in the ureters, the tubes that carry urine to the bladder.

In 1993, scientists were surprised to learn that previous assumptions about the mineral were wrong. A large study found that men who ate the most dietary calcium were actually the least likely to develop stones. Now, a similar investigation of women shows comparable results.

When Harvard researchers examined the diets of 91,731 women over a 12-year period they found that those who ate the most dairy products decreased their risk of developing kidney stones by 35%. However, women who took calcium supplements were 20% more likely to form the stones.

Scientists suspect the differences may be related to calcium's interaction with oxalate, an acid found in peanuts, chocolate, spinach, and other foods. Like calcium, oxalate is a major constituent of stones; but when calcium and oxalates are eaten together, they combine in the intestine and are excreted in the stool. Because supplements are often taken between meals, there is no available oxalate in the gut for the calcium to bind to.

Researchers don't know whether dietary calcium prevents kidney stones because dairy products are generally eaten with other foods or if there is some substance in milk products that is protective. Because kidney stones are uncommon in women, there is no reason for most of them to stop taking supplements. But those who have had kidney stones in the past should discuss the use of supplements with their doctor. (Annals of Internal Medicine, April 1, 1997, pp. 497-504.)