Prevent the formation of painful kidney stones

Since at least 1936, people who have had a calcium-containing kidney stone have been advised to cut back on the calcium in their diets. Even people related to stone formers often cut back on calcium for fear of suffering from the "family curse." On the face of it, the recommendation makes sense. Calcium-containing kidney stones develop in those who are susceptible to them when the urine becomes so saturated with calcium that calcium-rich crystals start collecting on the kidney wall and eventually form a pebble-sized mass. When small pieces or "stones" break away, they can block urine flow and also travel jagged-edged through the urinary tract, both of which contribute to excruciating pain.

But a study involving more than 90,000 women followed by a team of Harvard researchers for 12 years suggests that cutting back on dietary calcium may do more harm than good. The women who ate the most calcium over the years -- an average of about 1,100 milligrams daily, or more than their 800-milligram Recommended Dietary Allowance -- were least likely to develop kidney stones. On the other hand, women who consumed fewer than 500 milligrams of calcium daily were at a relatively high risk of suffering stones.

The new findings lend weight to an earlier Harvard study of men -- who are 4 times as likely to develop kidney stones as women. The men who consumed the most calcium were the least likely to develop a stone.

Calcium may confer protection because it binds with a substance called oxalate -- an "ingredient" in 3 out of 4 kidney stones. Found in foods such as spinach, strawberries, and nuts, oxalate can prove particularly problematic. In vulnerable people, it takes only a little bit of excess oxalate in the urine to start forming a kidney stone. Yet when oxalate is consumed along with calcium, the calcium "bonds" with it before it gets into urine and carries it out of the body via the feces.

The oxalate connection may explain a twist in the women's study: those who took calcium pills were more likely to suffer kidney stones than those who didn't. Most of the supplement takers took their calcium pills between meals, and the researchers suspect that the extra calcium didn't have the opportunity to bind with oxalate from food.
If you've already had a kidney stone...

While the new study provides some insights into prevention of first-time kidney stones, it doesn't address whether people who have already suffered a calcium-containing stone should follow the age-old advice to cut back on calcium. The answer, according to most experts, is that across-the-board calcium restrictions for people who have had a calcium stone are passe.

One reason is that low-calcium diets pose a hazard, particularly for people whose stones are the result of a physiologic glitch that causes them to excrete abnormally high levels of calcium in their urine no matter what amount they eat. It appears that when such people cut back on calcium, the mineral is pulled from their bones even more aggressively and excreted in urine. Over time, they may be at high risk of kidney stones and suffer from weakened bones that fracture easily.

Another reason low-calcium diets are being phased out is that medications called thiazides have been shown to be preferable for decreasing the calcium excreted in urine, thereby lowering the risk of kidney stones. Thiazides also appear to shunt dietary calcium to the bones, where the mineral is needed.

To be sure, a very small percentage of people who have suffered kidney stones do benefit from calcium restriction. But these people can be identified only through a series of expensive urine analyses. Before accepting a "prescription" for a low-calcium diet, patients should make sure their physicians have run the appropriate tests.

EATING TO AVOID KIDNEY STONES

If you're concerned about suffering from kidney stones, or have already had one, here are a few dietary steps you can take to keep your future risk low.

Meet your calcium needs as much as possible through foods rather than supplements. If you do use calcium supplements, take them with or right after meals. The calcium will keep out of the urine substances in certain foods called oxalates, which can form stones in much smaller amounts than calcium.

Drink 12 to 16 cups of fluid, preferably water, throughout the day. That will allow the kidneys to make about 2 1/2 quarts of urine, which is dilute enough to help decrease the likehood that calcium will "clump together" and form deposits. In hot weather, add another couple of cups to account for loss of fluid through sweating.

Cut back on salt. A diet high in salt can increase the amount of calcium that is drawn into the urine.

Alert your physicians if you have had kidney stones and are taking vitamin C, vitamin D, fish liver oil, or vitamin/mineral supplements containing any of those nutrients. They may increase the risk of stone formation in some people.

Last updated Jan 2/07