In a large new analysis, men and women who consumed the most dietary calcium from foods had about 20% lower risk of developing kidney stones than peers who consumed the least calcium.
"This is another piece of data to suggest that there's no role for dietary calcium restriction for kidney stones," said lead study author Dr Eric Taylor, a kidney specialist at Maine Medical Center in Portland.
Although most stones that form in the kidneys are made of calcium oxalate, people should not be afraid of consuming calcium in foods, Taylor said.
On the contrary, past studies have found that eating calcium-rich foods - though not taking calcium supplements - seems to ward off stone formation.
Since those studies focused almost exclusively on dairy products, there was lingering doubt about whether some other component of milk might be responsible for the effect. So Taylor and his colleagues set out to see whether the results held true for calcium from non-dairy foods.
They analysed data from three large studies that followed more than one million men and women, sometimes for decades, and included periodic detailed food questionnaires.
The researchers divided those participants into five smaller groups, based on how much calcium they consumed from dairy and other sources over as long as 20 years.
Only participants who were generally healthy and had never suffered from a kidney stone before dietary data collection began were included.
For both dairy and non-dairy sources of calcium, people who consumed the most calcium were least likely to develop painful, symptomatic kidney stones - overall, their risk was 77% of that seen among people who ate the least calcium, Taylor's team reports in the Journal of Urology.
Family history of kidney stones
Looking just at dairy foods, the researchers found that people who consumed the least calcium, about 150 milligrams per day or the equivalent of half a glass of skim milk, had a 30% higher chance of developing a kidney stone than the group that got the most, between 800 and 900 milligrams per day or about three glasses of milk.
For non-dairy foods, people who consumed 250 milligrams of calcium a day - the amount in eight ounces of cooked kale or two sardines, for example - were twice as likely to develop a stone as people who got 450 milligrams daily.
Rates of kidney stones in the US translate to the average person having a 6% chance of developing a stone during a lifetime. So, in the new study, low calcium consumption doubled that risk to 12%. That difference isn't significant for most people, unless they have already had a kidney stone and know they are at risk for another, Taylor said.
"If they don't have a history of kidney stones, or some risk factor, then don't worry about it," said Dr Lynda Frassetto, a kidney specialist at the University of California, San Francisco, who was not involved in the study.
People with a family history of stones should consult their doctors for a
risk assessment, but shouldn't start changing their diets on their own, she
said. Because most kidney stones are made of calcium oxalate, it might seem
counterintuitive to consume more calcium and end up with less in your kidneys,
but there is theory about why it works, Frassetto said.
The real culprit is oxalate, not calcium. Oxalate is found in many foods including fruits, vegetables, nuts and chocolate, and calcium binds to it avidly.
The more calcium in the digestive tract, she said, the more oxalate it can bind and take out of the body before the oxalate is absorbed into the bloodstream and ends up in the kidneys and bile duct.
Many patients don't understand this relationship and cut back on calcium anyway, which actually increases their risk, according to Dr Mathew Sorensen, a urologist at the University of Washington in Seattle.
"I see patients every week, and most have cut back on their calcium intake," Sorensen said.
"It is one of the most important myths in the stones world that needs to be corrected."
Really excessive calcium levels aren't a good idea either, Sorensen said. He
recommends patients keep levels moderate, around 1000 to 1200 milligrammes per