Researchers found that among more than 78,000 U.S. women in their 50s and 70s, the risk of developing first-time kidney stones declined as calcium or fluid intake climbed. On the other hand, the odds went up with a higher sodium intake.

Kidney stones develop when the urine contains more crystal-forming substances -- like calcium, uric acid and a compound called oxalate -- than can be diluted by the available fluid.

People prone to developing kidney stones have long been told to boost their fluid intake; that helps dilute the substances that can lead to stones. Cutting down on salt can help because too much sodium boosts calcium levels in the urine.

Since most kidney stones contain calcium, it was once thought that cutting down on calcium could help, but studies in recent years have suggested that dairy foods may actually be protective -- though calcium supplements may not be wise for "stone formers."

These latest findings, reported in the Journal of Urology, confirm the conventional wisdom.

Doctors have been particularly concerned about the calcium question in older women, since they are at increased risk of thinning bones and fractures, said Dr. Mathew Sorensen, a urologist at the University of Washington School of Medicine in Seattle who led the study.

"When people form stones," he said, "many naturally think they should reduce their calcium intake."

However, it is "good general advice" to stick with your usual dairy intake, according to Sorensen. And if you take calcium supplements, ask your doctor about whether you should stop.

The findings are based on 78,293 U.S. women who were followed for an average of eight years. During that time, 2.5% -- one in 40 -- were diagnosed with kidney stones for the first time.

Among the 20% of women who ate the most calcium at the outset, the odds of developing kidney stones was 28% lower, versus the 20% of women who got the least calcium. On average, women were eating about 800mg of calcium per day, according to surveys.

Similarly, the group who drank the most fluid had a 20% lower risk of kidney stones than women with lowest intakes. Average water intake was about 1.5L per day.

More sodium, meanwhile, meant a higher risk. The one-fifth of women with the saltiest diets were 61% more likely to develop stones than the fifth with the lowest sodium intake.

"It's really important to reduce your salt intake," Sorensen said. "And the American diet is filled to the brim with salt."

Some of the biggest sources of sodium, he noted, include fast food, processed meats and canned foods. So, people prone to kidney stones need to become good label-readers and be careful when eating out.

As for calcium, Sorensen said it's important for everyone -- but particularly older women -- to be aware that cutting out dairy foods may actually contribute to kidney stones.

Calcium supplements are different, and it may be because they provide a large, isolated dose of the mineral.

People prone to stones should be "cautious" about calcium supplements, Sorensen said. However, if a woman is on calcium to protect her bones, she should talk to her doctor about whether she can stick with it.

"For any woman who needs to take a calcium supplement," Sorensen said, "I would recommend taking it with a meal." That may help mitigate any effect of the calcium on stone formation.

In general, experts recommend that women older than 50 get 1,200mg of calcium per day. In reality, few do. Of women in this study, for instance, 80% got less than the recommended amount of calcium.

When it comes to fluids, Sorensen said it is hard to make a specific recommendation.

"I usually tell patients, if you're a stone-former, whatever you've been getting is not enough," Sorensen said.

 From the March 30, 2012, Prepared Foods’ Daily Update