Preventing Kidney Stones

By Donald R. Davis, Ph.D.

In the United States about one in seven men and one in fifteen women will be affected by kidney stones during their lives. Nearly all kidney stones grow from substances normally present in urine, caused by lack of other substances that prevent stone formation. Most stones pass spontaneously, sometimes without notice, but they can be extremely painful and debilitating. Fortunately, those that don’t pass by themselves can be removed by various means, though at significant cost and risk. Recurrences are common, but much has been learned about prevention. Here I highlight key findings, some of which are poorly known.

Topping the list of successful measures [for prevention] is drinking plenty of fluid, preferably water.

About 80 percent of kidney stones are composed of calcium oxalate, sometimes with phosphate. The calcium and phosphate come from diet, but the oxalate usually comes about equally from diet and internal synthesis. Everyone’s urine is capable of forming calcium oxalate stones, but stone initiation and growth normally are prevented by inhibitors that interfere with calcium’s ability to combine with oxalate. The known inhibitors include citrate, phytate, and magnesium.

Citrate Inhibitor. Normally we excrete about 500 to 800 mg of citrate daily. It comes mostly from internal synthesis, with usually little from diet. Deficient excretion is estimated to cause about one-third of calcium-containing stones. Low excretion results primarily from excessively acid urine, which causes citrate to be metabolized by kidneys. The main remedy is to increase urine pH to the desirable range of 5.5 to 6.0. However, dietary or supplemental citrate can help. Potassium citrate is a prescription drug widely used to increase urine pH and citrate. Unfortunately, it tends to cause gastrointestinal irritation, despite divided doses of 6 to 12 slow-release tablets daily. Side effects, inconvenience, and cost cause many patients to abandon this drug, which needs to be taken for life if more natural remedies are not used.

The most natural ways to increase urine pH and citrate are by reducing meat consumption or by balancing its acidifying effect with alkali from increased intakes of vegetables, fruits, and whole grains. Orange juice, or preferably the whole fruit, seems especially promising as a source of both alkali and citrate. Citrate in lemons seems less beneficial, and grapefruit unexpectedly increased stone risk in some studies. Also, there are available nutritional supplements of calcium citrate and magnesium citrate that increase urine pH and citrate with far fewer side effects than potassium citrate.

Phytate Inhibitor. Phytate is the major form of phosphate in plant foods. It is especially rich in whole grains, beans, and nuts. Phytate is best known for reducing intestinal absorption of some minerals. But its mineral-binding abilities are beneficial too, including preventing cancer and calcium stones. Some phytate is absorbed intact and excreted in urine. There, it seems to function like citrate. A Spanish study found that urinary phytate averaged about 40 percent lower in stone formers than in healthy controls. In a large study of U.S. nurses, there was a 37 percent reduced risk of kidney stones in those who consumed relatively large amounts of phytate.

Magnesium Inhibitor. Magnesium in urine reduces the ability of oxalate to combine with calcium. Further, dietary or supplemental magnesium (with meals) hinders intestinal absorption of oxalate and sometimes reduces excessive absorption of calcium. Magnesium-rich foods and supplements also help raise urine pH and citrate. Several studies with magnesium supplements (as the oxide or hydroxide) have found at least some decrease in stone formation. Magnesium citrate has the advantages of superior solubility and its citrate content. Large doses of magnesium are used as a laxative. Even modest supplements sometimes have a similar effect that can be countered by combining calcium with magnesium.

Other Measures. Besides getting enough stone inhibitors, several other measures have been used to prevent kidney stones. Some work well, but others are questionable or wrong. Topping the list of successful measures is drinking plenty of fluid, preferably water. Fluid dilutes the stone-forming substances in urine. Doctors commonly recommend sufficient fluids to produce two quarts of urine per day.

Another measure is to get plenty of dietary calcium. Until recently, doctors recommended the opposite, reasoning that dietary calcium increases urinary calcium and, thus, the tendency to make calcium stones. We now know that dietary calcium, or supplemental calcium taken with meals, has another, more powerful effect. It combines with dietary oxalate in the intestines, thereby blocking absorption of oxalate that would otherwise appear in urine. Three large studies of men and women have all found that stone risk decreases as dietary calcium increases (mostly from dairy sources). However, calcium supplements probably do not help, or may increase stone risk if they are not taken with meals.

Some recommend restricting oxalate-containing foods such as spinach, other greens, carrots, green beans, sweet potatoes, peanuts, and chocolate. However, the amounts in individual foods are quite variable in amount and absorbability. So, tables of oxalate in foods have low relevance. Further, the many oxalate-containing foods have multiple virtues, including stone-preventing qualities. The better approach seems to be minimizing oxalate absorption with dietary and supplemental calcium and magnesium (with meals) and creating urine conditions unfavorable for stone formation. Those who make excessive oxalate may be able to reduce internal synthesis with vitamin B6 supplements of 10 to 100 mg per day.

Claims that vitamin C supplements should be avoided are based on flawed experiments or speculation. Large studies in men and women have found no evidence of harm from supplements of over 1.5 grams per day.

More information is available in a video presentation by the author, “Preventing The #2 Worst Pain: Kidney Stones,” available for viewing in the Center’s Mabee Library or for purchasing in the Gift of Health.