The findings counter the popular myth that too much calcium can cause kidney stones although there is some evidence that supplements of the mineral can raise the risk of stones in an older population.
More than 1 million people a year get kidney stones in the US and the numbers are growing. Some experts have linked this increase to the popularity of high protein diets like Atkins, causing an imbalance of acidic and alkaline foods in the diet. Higher acid excretion can lead to kidney stones.
Greater intakes of dietary calcium and potassium have already been shown to reduce the risk of kidney stone formation in older men and women but there is no similar data for younger women, said the team from Harvard Medical School and the Brigham and Womens' Hospital in Boston.
Data from food frequency questionnaires from more than 96,000 women aged 27-44 years old shows that a higher dietary calcium intake is associated with a reduced risk of kidney stones, they report in Archives of Internal Medicine (164:885-891).
Women in the highest quintile of dietary calcium intake had a 27 per cent lower risk of kidney stones compared with women in the lowest quintile. Dietary levels of phytate, a salt that contains magnesium and calcium, also appeared to cut risk of stone formation, and further than dietary calcium (by 37 per cent).
Supplemental calcium intake was not associated with risk of stone formation, while animal protein and fluid intake lowered the risk of stones and sucrose raised the risk by more than 30 per cent.
The authors concluded that "routine restriction of dietary calcium in patients who have had a kidney stone is no longer justified". They add that phytate, found in cereal and some bread, may be "a new, important, and safe addition to our options for stone prevention".