Kidney stones: don't curb the calcium.

It starts off innocuously, as a microscopic crystal floating in the tubules of the kidney, Over time, though, such invisible particles can grow layer upon layer, ultimately forming painful kidney stones that afflict one in nine people in the United States at some point in their lives. Because most kidney stones are made of calcium, physicians often recommend that patients who have already suffered from stones reduce their calcium intake. But a new study of more than 45,000 men challenges the traditional thinking, suggesting that low-calcium diets actually raise the risk of developing kidney stones.

In the March 25 NEW ENGLAND JOURNAL OF MEDICINE, a research team from the Harvard School of Public Health reports that men who ate a diet rich in calcium faced a 34 percent lower risk of developing kidney stones than did men who consumed a restricted calcium diet.

"This goes against everything we had been taught," says kidney specialist Gary C. Curhan, who led the calcium investigation. Curhan and his colleagues took their data from the Health Professionals Follow-up Study, a long-term investigation of diet and illness among male health care professionals age 40 through 75.

While it seems counterintuitive that consuming calcium could protect against calcium-based kidney stones, Curhan thinks that a chemical called oxalate may explain the apparent puzzle, Oxalate is present in many foods, and it combines with calcium to form the insoluble crystals that make up most kidney stones.

Curhan suggests that a normal diet might provide enough calcium to tie up oxalate in the intestines, creating crystals that pass directly out of the body without harm. But a low-calcium diet would allow more oxalate to enter the bloodstream and eventually reach the kidneys, where it can form the crystals that lead to kidney stones.

That doesn't mean people should head to the local market and raid the dairy case with abandon. "The important thing we're trying to emphasize is not that adding calcium is necessarily going to help you but that for someone who has already had a stone, restricting calcium is not the right thing to do," says Curhan. He suggests people consume the recommended 800 milligrams per day.

He stresses that increasing fluid in-take substantially cuts the risk of stones. The new study also hints that potassium may protect against developing stones: Men who ate a potassium-rich diet faced a 50 percent lower risk than did those who consumed the least amount of potassium.